Retinal pigment epithelial transport mechanisms and their contributions to the electroretinogram

Imaging the eye as a window to brain health: frontier approaches and future directions

Recent years have seen significant advances in diagnostic testing of central nervous system (CNS) function and disease. However, there remain challenges in developing a comprehensive suite of non- or minimally invasive assays of neural health and disease progression. Due to the direct connection with the CNS, structural changes in the neural retina, retinal vasculature and morphological changes in retinal immune cells can occur in parallel with disease conditions in the brain. The retina can also, uniquely, be assessed directly and non-invasively. For these reasons, the retina may prove to be an important "window" for revealing and understanding brain disease. In this review, we discuss the gross anatomy of the eye, focusing on the sensory and non-sensory cells of the retina, especially microglia, that lend themselves to diagnosing brain disease by imaging the retina. We include a history of ocular imaging to describe the different imaging approaches undertaken in the past and outline current and emerging technologies including retinal autofluorescence imaging, Raman spectroscopy, and artificial intelligence image analysis. These new technologies show promising potential for retinal imaging to be used as a tool for the diagnosis of brain disorders such as Alzheimer's disease and others and the assessment of treatment success.

Neuron cilia constrain glial regulators to microdomains around distal neurons

Each glia interacts with multiple neurons, but the fundamental logic of whether it interacts with all equally remains unclear. We find that a single sense-organ glia modulates different contacting neurons distinctly. To do so, it partitions regulatory cues into molecular microdomains at specific neuron contact-sites, at its delimited apical membrane. For one glial cue, K/Cl transporter KCC-3, microdomain-localization occurs through a two-step, neuron-dependent process. First, KCC-3 shuttles to glial apical membranes. Second, some contacting neuron cilia repel it, rendering it microdomain-localized around one distal neuron-ending. KCC-3 localization tracks animal aging, and while apical localization is sufficient for contacting neuron function, microdomain-restriction is required for distal neuron properties. Finally, we find the glia regulates its microdomains largely independently. Together, this uncovers that glia modulate cross-modal sensor processing by compartmentalizing regulatory cues into microdomains. Glia across species contact multiple neurons and localize disease-relevant cues like KCC-3. Thus, analogous compartmentalization may broadly drive how glia regulate information processing across neural circuits.

Osmotic gradients and transretinal water flow-a quantitative elemental microanalytical study of frozen hydrated chick eyes

Optical clarity and efficient phototransduction are necessary for optimal vision, however, how the associated processes of osmoregulation and continuous fluid drainage across the whole eye are achieved remains relatively unexplored. Hence, we have employed elemental microanalysis of planed surfaces of light-adapted bulk frozen-hydrated chick eyes to determine the unique intracellular elemental localization, compositions, and hydration states that contribute to maintaining osmotic gradients and water flow from the vitreous, across the retina, retinal pigment epithelium (RPE), to choroid and sclera. As expected, the greatest difference in resultant osmotic concentration gradients, [calculated using the combined concentrations of sodium (Na) and potassium (K)] and tissue hydration [oxygen-defined water concentration], occurs in the outer retina and, in particular, in the RPE where the apical and basal membranes are characterized by numerous bioenergetically active, osmoregulating ion transport mechanisms, aquaporins, and chloride (Cl) channels. Our results also demonstrate that the high intracellular Na+ and K+ concentrations in the apical region of the RPE are partially derived from the melanosomes. The inclusion of the ubiquitous osmolyte taurine to the calculation of the osmotic gradients suggests a more gradual increase in the osmotic transport of water from the vitreous into the ganglion cell layer across the inner retina to the outer segments of the photoreceptor/apical RPE region where the water gradient increases rapidly towards the basal membrane. Thus transretinal water is likely to cross the apical membrane from the retina into the RPE cells down the Na+ and K+ derived osmotic concentration gradient and leave the RPE for the choroid across the basal membrane down the Cl- derived osmotic concentration gradient that is sustained by the well-described bioenergetically active RPE ion transporters and channels.

Fast and slow light-induced changes in murine outer retina optical coherence tomography: complementary high spatial resolution functional biomarkers

Fast (seconds) and slow (minutes to hours) optical coherence tomography (OCT) responses to light stimulation have been developed to probe outer retinal function with higher spatial resolution than the classical full-field electroretinogram (ERG). However, the relationships between functional information revealed by OCT and ERG are largely unexplored. In this study, we directly compared the fast and slow OCT responses with the ERG. Fast responses [i.e. the optoretinogram (ORG)] are dominated by reflectance changes in the outer segment (OS) and the inner segment ellipsoid zone (ISez). The ORG OS response has faster kinetics and a higher light sensitivity than the ISez response, and both differ significantly with ERG parameters. Sildenafil-inhibition of phototransduction reduced the ORG light sensitivity, suggesting a complete phototransduction pathway is needed for ORG responses. Slower OCT responses were dominated by light-induced changes in the external limiting membrane to retinal pigment epithelium (ELM-RPE) thickness and photoreceptor-tip hyporeflective band (HB) magnitudes, with the biggest changes occurring after prolonged light stimulation. Mice with high (129S6/ev) vs. low (C57BL/6 J) ATP(adenosine triphosphate) synthesis efficiency show similar fast ORG, but dissimilar slow OCT responses. We propose that the ORG reflects passive physiology, such as water movement from photoreceptors, in response to the photocurrent response (measurable by ERG), whereas the slow OCT responses measure mitochondria-driven physiology in the outer retina, such as dark-provoked water removal from the subretinal space.

Macular Hole Surgery in Dome-Shaped Maculopathy

Purpose

To present a successful case of macular hole surgery in a patient with dome-shaped maculopathy (DSM).

Observations

Patient with DSM had successful closure of a full-thickness macular hole with surgery. There was persistent subretinal fluid after the surgery, which showed some response to a combination of a diuretic and a nonsteroidal anti-inflammatory drug (NSAID).

Conclusions and Importance

Full-thickness macular hole in DSM may be effectively managed with modern surgical techniques.

AI-based monitoring of retinal fluid in disease activity and under therapy

Retinal fluid as the major biomarker in exudative macular disease is accurately visualized by high-resolution three-dimensional optical coherence tomography (OCT), which is used world-wide as a diagnostic gold standard largely replacing clinical examination. Artificial intelligence (AI) with its capability to objectively identify, localize and quantify fluid introduces fully automated tools into OCT imaging for personalized disease management. Deep learning performance has already proven superior to human experts, including physicians and certified readers, in terms of accuracy and speed. Reproducible measurement of retinal fluid relies on precise AI-based segmentation methods that assign a label to each OCT voxel denoting its fluid type such as intraretinal fluid (IRF) and subretinal fluid (SRF) or pigment epithelial detachment (PED) and its location within the central 1-, 3- and 6-mm macular area. Such reliable analysis is most relevant to reflect differences in pathophysiological mechanisms and impacts on retinal function, and the dynamics of fluid resolution during therapy with different regimens and substances. Yet, an in-depth understanding of the mode of action of supervised and unsupervised learning, the functionality of a convolutional neural net (CNN) and various network architectures is needed. Greater insight regarding adequate methods for performance, validation assessment, and device- and scanning-pattern-dependent variations is necessary to empower ophthalmologists to become qualified AI users. Fluid/function correlation can lead to a better definition of valid fluid variables relevant for optimal outcomes on an individual and a population level. AI-based fluid analysis opens the way for precision medicine in real-world practice of the leading retinal diseases of modern times.

Functional regulation of an outer retina hyporeflective band on optical coherence tomography images

Human and animal retinal optical coherence tomography (OCT) images show a hyporeflective band (HB) between the photoreceptor tip and retinal pigment epithelium layers whose mechanisms are unclear. In mice, HB magnitude and the external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness appear to be dependent on light exposure, which is known to alter photoreceptor mitochondria respiration. Here, we test the hypothesis that these two OCT biomarkers are linked to metabolic activity of the retina. Acetazolamide, which acidifies the subretinal space, had no significant impact on HB magnitude but produced ELM-RPE thinning. Mitochondrial stimulation with 2,4-dinitrophenol reduced both HB magnitude and ELM-RPE thickness in parallel, and also reduced F-actin expression in the same retinal region, but without altering ERG responses. For mice strains with relatively lower (C57BL/6J) or higher (129S6/ev) rod mitochondrial efficacy, light-induced changes in HB magnitude and ELM-RPE thickness were correlated. Humans, analyzed from published data captured with a different protocol, showed a similar light-dark change pattern in HB magnitude as in the mice. Our results indicate that mitochondrial respiration underlies changes in HB magnitude upstream of the pH-sensitive ELM-RPE thickness response. These two distinct OCT biomarkers could be useful indices for non-invasively evaluating photoreceptor mitochondrial metabolic activity.

Tsg101 Is Necessary for the Establishment and Maintenance of Mouse Retinal Pigment Epithelial Cell Polarity

The retinal pigment epithelium (RPE) forms a monolayer sheet separating the retina and choroid in vertebrate eyes. The polarized nature of RPE is maintained by distributing membrane proteins differentially along apico-basal axis. We found the distributions of these proteins differ in embryonic, post-natal, and mature mouse RPE, suggesting developmental regulation of protein trafficking. Thus, we deleted tumor susceptibility gene 101 (Tsg101), a key component of endosomal sorting complexes required for transport (ESCRT), in embryonic and mature RPE to determine whether ESCRT-mediated endocytic protein trafficking correlated with the establishment and maintenance of RPE polarity. Loss of Tsg101 severely disturbed the polarity of RPE, which forms irregular aggregates exhibiting non-polarized distribution of cell adhesion proteins and activation of epidermal growth factor receptor signaling. These findings suggest that ESCRT-mediated protein trafficking is essential for the development and maintenance of RPE cell polarity.

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.

RNA-seq and GSEA identifies suppression of ligand-gated chloride efflux channels as the major gene pathway contributing to form deprivation myopia

Currently there is no consensus regarding the aetiology of the excessive ocular volume that characterizes high myopia. Thus, we aimed to test whether the gene pathways identified by gene set enrichment analysis of RNA-seq transcriptomics refutes the predictions of the Retinal Ion Driven Efflux (RIDE) hypothesis when applied to the induction of form-deprivation myopia (FDM) and subsequent recovery (post-occluder removal). We found that the induction of profound FDM led to significant suppression in the ligand-gated chloride ion channel transport pathway via suppression of glycine, GABA_A and GABA_C ionotropic receptors. Post-occluder removal for short term recovery from FDM of 6 h and 24 h, induced significant upregulation of the gene families linked to cone receptor phototransduction, mitochondrial energy, and complement pathways. These findings support a model of form deprivation myopia as a Cl⁻ ion driven adaptive fluid response to the modulation of the visual signal cascade by form deprivation that in turn affects the resultant ionic environment of the outer and inner retinal tissues, axial and vitreal elongation as predicted by the RIDE model. Occluder removal and return to normal light conditions led to return to more normal upregulation of phototransduction, slowed growth rate, refractive recovery and apparent return towards physiological homeostasis.

An x-ray microanalytical method for measuring in vivo element and water concentrations, relating to osmoregulation, in cells and tissues of the posterior eye

Osmoregulation is critical for cell and tissue survival yet there are relatively few methods available to determine osmotic gradients from water and elemental concentration either in single cells or across multiple cellular layers of tissue. X-ray microanalysis of frozen-hydrated preparations in a scanning electron microscope is one such powerful, sensitive, nondestructive technique. Here we use x-ray microanalysis to quantitatively analyse intracellular element concentrations and oxygen concentrations, as a proxy for water concentrations, in selected individual cells of the posterior eye. Using frozen-hydrated preparations of the retinal complex of chicken eyes, it is shown that structural preservation is sufficient to identify cell layers and individual cells. The quantitative analysis of selected areas in the photoreceptor layer, inner nuclear layer and ganglion cell layer, where specific cell types were known to be present, provided measurements of intracellular element concentrations comparable with the analysis of individual cells. It is also shown that in the cells of the retinal pigment epithelium and outer photoreceptor segments elemental analyses were reasonably consistent at the cellular level in different depth levels of the same sample. Comparison of oxygen concentrations, as a proxy for water concentration, at two accelerating voltages (15 and 5 kV) indicated that at 15 kV oxygen concentration was largely derived from intracellular water. Water concentrations could be calculated and concentrations of diffusible elements (Na, K) could be defined in mmol/L. From the latter it is possible to calculate osmotic concentrations of individual cells and osmotic gradients across the tissue. LAY DESCRIPTION: Understanding many cellular processes, in both healthy and diseased states, depends on knowing how the water content of cells and their surrounding fluids is controlled. The transport of water is generally down its concentration gradient or against the osmotic concentration gradient defined by solutes such as sodium, potassium and chloride dissolved in the water. We have refined a microanalytical method, that detects the x-rays emitted from specific elements when they are bombarded by electrons in a scanning electron microscope, to apply it to the analysis of the retina of the eye. The method facilitates the measurement of the elemental composition, water and osmotic concentration gradients of cells and tissues in the eye, that may be involved in the development of myopia, or short sightedness, a condition that afflicts many people including some 80 - 90% of children in Asia.

Sphingolipids as critical players in retinal physiology and pathology

Sphingolipids have emerged as bioactive lipids involved in the regulation of many physiological and pathological processes. In the retina, they have been established to participate in numerous processes, such as neuronal survival and death, proliferation and migration of neuronal and vascular cells, inflammation, and neovascularization. Dysregulation of sphingolipids is therefore crucial in the onset and progression of retinal diseases. This review examines the involvement of sphingolipids in retinal physiology and diseases. Ceramide (Cer) has emerged as a common mediator of inflammation and death of neuronal and retinal pigment epithelium cells in animal models of retinopathies such as glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa. Sphingosine-1-phosphate (S1P) has opposite roles, preventing photoreceptor and ganglion cell degeneration but also promoting inflammation, fibrosis, and neovascularization in AMD, glaucoma, and pro-fibrotic disorders. Alterations in Cer, S1P, and ceramide 1-phosphate may also contribute to uveitis. Notably, use of inhibitors that either prevent Cer increase or modulate S1P signaling, such as Myriocin, desipramine, and Fingolimod (FTY720), preserves neuronal viability and retinal function. These findings underscore the relevance of alterations in the sphingolipid metabolic network in the etiology of multiple retinopathies and highlight the potential of modulating their metabolism for the design of novel therapeutic approaches.

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.

No flow through the vitreous humor: How strong is the evidence?

When analyzing vitreal drug delivery, or the pharmacological effects of drugs on intraocular pressure, or when interpreting outflow facility measurements, it is generally accepted that the fluid in the vitreous humor is stagnant. It is accepted that for all practical purposes, the aqueous fluid exits the eye via anterior pathways only, and so there is negligible if any posteriorly directed flow of aqueous through the vitreous humor. This assumption is largely based on the interpretation of experimental data from key sources including Maurice (1957), Moseley (1984), Gaul and Brubaker (1986), Maurice (1987) and Araie et al. (1991). However, there is strong independent evidence suggesting there is a substantial fluid flow across the retinal pigment epithelium from key sources including Cantrill and Pederson (1984), Chihara and Nao-i, Tsuboi (1985), Dahrouj et al. (2014), Smith and Gardiner (2017) and Smith et al. (2019). The conflicting evidence creates a conundrum-how can both interpretations be true? This leads us to re-evaluate the evidence. We demonstrate that the data believed to be supporting no aqueous flow through the vitreous are in fact compatible with a significant normal aqueous flow. We identify strong and independent lines of evidence supporting fluid flow across the RPE, including our new outflow model for the eye. On balance it appears the current evidence favors the view that there is normally a significant aqueous flow across the RPE in vivo. This finding suggests that past and future analyses of outflow facility, interpretations of some drug distributions and the interpretation of some drug effects on eye tissues, may need to be revised.

Fluid and solute transport across the retinal pigment epithelium: a theoretical model

The retina is composed of two main layers-the neuroretina and the retinal pigment epithelium (RPE)-that are separated by a potential gap termed the sub-retinal space (SRS). Accumulation of fluid in the SRS may result in a retinal detachment. A key function of the RPE is to prevent fluid accumulation in the SRS by actively pumping fluid from this space to the choroid. We have developed a mathematical model of this process that incorporates the transport of seven chemical species: Na⁺, K⁺, Cl^-, HCO3-, H⁺, CO₂ and H₂CO₃. This allows us to estimate solute and water fluxes and to understand the role of the different membrane ion channels. We have performed a global sensitivity analysis using the extended Fourier amplitude sensitivity test to investigate the relative importance of parameters in generating the model outputs. The model predicts that flow across the RPE is driven by an osmotic gradient in the cleft gap between adjacent cells. Moreover, the model estimates how water flux is modified in response to inhibition of membrane ion channels and carbonic anhydrase (CA). It provides a possible explanation for how CA inhibitors, which are used clinically to prevent fluid accumulation in the SRS, may be acting.

Melatonin and the control of intraocular pressure

Melatonin is not only synthesized by the pineal gland but by several ocular structures. This natural indoleamine is of great importance for regulating several eye processes, among which pressure homeostasis is included. Glaucoma, the most prevalent eye disease, also known as the silent thief of vision, is a multifactorial pathology that is associated to age and, often, to intraocular hypertension (IOP). Indeed IOP is the only modifiable risk factor and as such medications are available to control it; however, novel medications are sought to minimize undesirable side effects. Melatonin and analogues decrease IOP in both normotensive and hypertensive eyes. Melatonin activates its cognate membrane receptors, MT₁ and MT₂, which are present in numerous ocular tissues, including the aqueous-humor-producing ciliary processes. Melatonin receptors belong to the superfamily of G-protein-coupled receptors and their activation would lead to different signalling pathways depending on the tissue. This review describes the molecular mechanisms underlying differential functionalities that are attributed to melatonin receptors. Accordingly, the current work highlights the important role of melatonin and its analogues in the healthy and in the glaucomatous eyes, with special attention to the control of intraocular pressure.

Lessons learned from quantitative fundus autofluorescence

Quantitative fundus autofluorescence (qAF) is an approach that is built on a confocal scanning laser platform and used to measure the intensity of the inherent autofluorescence of retina elicited by short-wavelength (488 nm) excitation. Being non-invasive, qAF does not interrupt tissue architecture, thus allowing for structural correlations. The spectral features, cellular origin and topographic distribution of the natural autofluorescence of the fundus indicate that it is emitted from retinaldehyde-adducts that form in photoreceptor cells and accumulate, under most conditions, in retinal pigment epithelial cells. The distributions and intensities of fundus autofluorescence deviate from normal in many retinal disorders and it is widely recognized that these changing patterns can aid in the diagnosis and monitoring of retinal disease. The standardized protocol employed by qAF involves the normalization of fundus grey levels to a fluorescent reference installed in the imaging instrument. Together with corrections for magnification and anterior media absorption, this approach facilitates comparisons with serial images and images acquired within groups of patients. Here we provide a comprehensive summary of the principles and practice of qAF and we highlight recent efforts to elucidate retinal disease processes by combining qAF with multi-modal imaging.

A new paradigm for delivering personalised care: integrating genetics with surgical interventions in BEST1 mutations

BACKGROUND

The availability and reduced cost of genotyping has improved gene susceptibility testing and our scientific understanding of disease pathophysiology. Whilst several personalised translational models exist within medical frameworks, genetic-based surgical therapy is a translational application not widely used in surgical specialties.

METHOD

We present a clinical series of five patients with genetically confirmed bestrophinopathy and malignant glaucoma (MG). Patients were followed up for 12 months or more after receiving surgical intervention to manage refractory intraocular pressure (IOP) resistant to medical treatment.

FINDINGS

Patients with BEST1 gene mutations are at higher risk of MG after filtration surgery. A multi-disciplinary approach after four patients experienced poor outcomes concluded that traditional first-line glaucoma surgery was not sufficient to prevent visual loss. A fifth patient presenting with the identified at-risk phenotype underwent primary pars plana vitrectomy, with pars plana Baerveldt tube insertion, successfully preventing MG and had no glaucoma progression after 5 years.

INTERPRETATION

We provide proof-of-principle that genetic analysis can be used to inform the selection of surgical therapy to improve outcomes. In this case, a refinement of current surgical methods to avoid MG. Although challenges remain, personalised surgery has the potential to improve clinical outcomes beyond the scope of current surgical practice.

Nanomedicine-based Curcumin Approach Improved ROS Damage in Best Dystrophy-specific Induced Pluripotent Stem Cells

Best dystrophy (BD), also termed best vitelliform macular dystrophy (BVMD), is a juvenile-onset form of macular degeneration and can cause central visual loss. Unfortunately, there is no clear definite therapy for BD or improving the visual function on this progressive disease. The human induced pluripotent stem cell (iPSC) system has been recently applied as an effective tool for genetic consultation and chemical drug screening. In this study, we developed patient-specific induced pluripotent stem cells (BD-iPSCs) from BD patient-derived dental pulp stromal cells and then differentiated BD-iPSCs into retinal pigment epithelial cells (BD-RPEs). BD-RPEs were used as an expandable platform for in vitro candidate drug screening. Compared with unaffected sibling-derived iPSC-derived RPE cells (Ctrl-RPEs), BD-RPEs exhibited typical RPE-specific markers with a lower expression of the tight junction protein ZO-1 and Bestrophin-1 (BEST1), as well as reduced phagocytic capabilities. Notably, among all candidate drugs, curcumin was the most effective for upregulating both the BEST1 and ZO-1 genes in BD-RPEs. Using the iPSC-based drug-screening platform, we further found that curcumin can significantly improve the mRNA expression levels of Best gene in BD-iPSC-derived RPEs. Importantly, we demonstrated that curcumin-loaded PLGA nanoparticles (Cur-NPs) were efficiently internalized by BD-RPEs. The Cur-NPs-based controlled release formulation further increased the expression of ZO-1 and Bestrophin-1, and promoted the function of phagocytosis and voltage-dependent calcium channels in BD-iPSC-derived RPEs. We further demonstrated that Cur-NPs enhanced the expression of antioxidant enzymes with a decrease in intracellular ROS production and hydrogen peroxide-induced oxidative stress. Collectively, these data supported that Cur-NPs provide a potential cytoprotective effect by regulating the anti-oxidative abilities of degenerated RPEs. In addition, the application of patient-specific iPSCs provides an effective platform for drug screening and personalized medicine in incurable diseases.

Glia-Neuron Interactions in Caenorhabditis elegans

Glia are abundant components of animal nervous systems. Recognized 170 years ago, concerted attempts to understand these cells began only recently. From these investigations glia, once considered passive filler material in the brain, have emerged as active players in neuron development and activity. Glia are essential for nervous system function, and their disruption leads to disease. The nematode Caenorhabditis elegans possesses glial types similar to vertebrate glia, based on molecular, morphological, and functional criteria, and has become a powerful model in which to study glia and their neuronal interactions. Facile genetic and transgenic methods in this animal allow the discovery of genes required for glial functions, and effects of glia at single synapses can be monitored by tracking neuron shape, physiology, or animal behavior. Here, we review recent progress in understanding glia-neuron interactions in C. elegans. We highlight similarities with glia in other animals, and suggest conserved emerging principles of glial function.

Temporal whole field sawtooth flicker without a spatial component elicits a myopic shift following optical defocus irrespective of waveform direction in chicks

Purpose

Myopia (short-sightedness) is the commonest visual disorder and greatest risk factor for sight threatening secondary pathologies. Myopia and hyperopia can be induced in animal models by rearing with optical lens defocus of opposite sign. The degree of refractive compensation to lens-induced defocus in chicks has been shown to be modified by directionally drifting sawtooth spatio-temporal luminance diamond plaids, with Fast-ON sawtooth spatio-temporal luminance profiles inhibiting the myopic shift in response to negative lenses, and Fast-OFF profiles inhibiting the hyperopic shift in response to positive lenses. What is unknown is whether similar sign-of-defocus dependent results produced by spatio-temporal modulation of sawtooth patterns could be achieved by rearing chicks under whole field low temporal frequency sawtooth luminance profiles at 1 or 4 Hz without a spatial component, or whether such stimuli would indiscriminately elicit a myopic shift such as that previously shown with symmetrical (or near-symmetrical) low frequency flicker across a range of species.

Methods

Hatchling chicks (n = 166) were reared from days five to nine under one of three defocus conditions (No Lens, +10D lens, or -10D lens) and five light conditions (No Flicker, 1 Hz Fast-ON/Slow-OFF sawtooth flicker, 4 Hz Fast-ON/Slow-OFF sawtooth flicker, 1 Hz Fast-OFF/Slow-ON sawtooth flicker, or 4Hz Fast-OFF/Slow-ON sawtooth flicker). The sawtooth flicker was produced by light emitting diodes (white LEDs, 1.2 -183 Lux), and had no measurable dark phase. Biometrics (refraction and ocular axial dimensions) were measured on day nine.

Results

Both 1 Hz and 4 Hz Fast-ON and Fast-OFF sawtooth flicker induced an increase in vitreous chamber depth that was greater in the presence of negative compared to positive lens defocus. Both sawtooth profiles at both temporal frequencies inhibited the hyperopic shift in response to +10D lenses, whilst full myopic compensation (or over-compensation) in response to -10D lenses was observed.

Conclusions

Whole field low temporal frequency Fast-ON and Fast-OFF sawtooth flicker induces a generalized myopic shift, similar to that previously shown for symmetrical sine-wave and square-wave flicker. Our findings highlight that temporal modulation of retinal ON/OFF pathways per se (without a spatial component) is insufficient to produce strong sign-of-defocus dependent effect.

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.

Adult-Onset Vitelliform Macular Dystrophy caused by BEST1 p.Ile38Ser Mutation is a Mild Form of Best Vitelliform Macular Dystrophy

Adult-onset vitelliform macular dystrophy (AVMD) is a common and benign macular degeneration which can be caused by BEST1 mutation. Here, we investigated the clinical characteristics associated with a newly identified BEST1 mutation, p.Ile38Ser and confirmed the associated physiological functional defects. The 51-year-old patient presented bilateral small subretinal yellow deposits. Consistent with AVMD, the corresponding lesions showed hyperautofluorescence, late staining in fluorescein angiography, and subretinal hyper-reflective materials in spectral-domain optical coherence tomography. Genetic analysis demonstrated that the patient presented with a heterozygous p.Ile38Ser BEST1 mutation. Surface biotinylation and patch clamp experiments were performed in transfected HEK293T cells. Although, the identified BEST1 mutant maintains normal membrane expression, p.Ile38Ser mutant showed significantly smaller currents than wild type (WT). However, it showed larger currents than other BEST1 mutants, p.Trp93Cys, causing autosomal dominant best vitelliform macular dystrophy (BVMD), and p.Ala195Val, causing autosomal recessive bestrophinopathy (ARB). The cells expressing both WT and each BEST1 mutant showed that the functional defect of p.Ile38ser was milder than that of p.Trp93Cys, whereas combination of p.Ala195Val with WT showed good current. We identified and described the phenotype and in vitro functions of a novel BEST1 mutation causing AVMD. AVMD induced by p.Ile38Ser BEST1 mutation is a mild form of BVMD.

A Glial K/Cl Transporter Controls Neuronal Receptive Ending Shape by Chloride Inhibition of an rGC

Neurons receive input from the outside world or from other neurons through neuronal receptive endings (NREs). Glia envelop NREs to create specialized microenvironments; however, glial functions at these sites are poorly understood. Here, we report a molecular mechanism by which glia control NRE shape and associated animal behavior. The C. elegans AMsh glial cell ensheathes the NREs of 12 neurons, including the thermosensory neuron AFD. KCC-3, a K/Cl transporter, localizes specifically to a glial microdomain surrounding AFD receptive ending microvilli, where it regulates K(+) and Cl(-) levels. We find that Cl(-) ions function as direct inhibitors of an NRE-localized receptor-guanylyl-cyclase, GCY-8, which synthesizes cyclic guanosine monophosphate (cGMP). High cGMP mediates the effects of glial KCC-3 on AFD shape by antagonizing the actin regulator WSP-1/NWASP. Components of this pathway are broadly expressed throughout the nervous system, suggesting that ionic regulation of the NRE microenvironment may be a conserved mechanism by which glia control neuron shape and function.

An asymmetric outer retinal response to drifting sawtooth gratings

Electroretinogram (ERG) studies have demonstrated that the retinal response to temporally modulated fast-ON and fast-OFF sawtooth flicker is asymmetric. The response to spatiotemporal sawtooth stimuli has not yet been investigated. Perceptually, such drifting gratings or diamond plaids shaded in a sawtooth pattern appear brighter when movement produces fast-OFF relative to fast-ON luminance profiles. The neural origins of this illusion remain unclear (although a retinal basis has been suggested). Thus we presented toad eyecups with sequential epochs of sawtooth, sine-wave, and square-wave gratings drifting horizontally across the retina at temporal frequencies of 2.5-20 Hz. All ERGs revealed a sustained direct-current (DC) transtissue potential during drift and a peak at drift offset. The amplitudes of both phenomena increased with temporal frequency. Consistent with the human perceptual experience of sawtooth gratings, the sustained DC potential effect was greater for fast-OFF cf. fast-ON sawtooth. Modeling suggested that the dependence of temporal luminance contrast on stimulus device frame rate contributed to the temporal frequency effects but could not explain the divergence in response amplitudes for the two sawtooth profiles. The difference between fast-ON and fast-OFF sawtooth profiles also remained following pharmacological suppression of postreceptoral activity with tetrodotoxin (TTX), 2-amino-4-phosphonobutric acid (APB), and 2,3 cis-piperidine dicarboxylic acid (PDA). Our results indicate that the DC potential difference originates from asymmetries in the photoreceptoral response to fast-ON and fast-OFF sawtooth profiles, thus pointing to an outer retinal origin for the motion-induced drifting sawtooth brightness illusion.

Barrier properties of cultured retinal pigment epithelium

The principal function of an epithelium is to form a dynamic barrier that regulates movement between body compartments. Each epithelium is specialized with barrier functions that are specific for the tissues it serves. The apical surface commonly faces a lumen, but the retinal pigment epithelium (RPE) appears to be unique by a facing solid tissue, the sensory retina. Nonetheless, there exists a thin (subretinal) space that can become fluid filled during pathology. RPE separates the subretinal space from the blood supply of the outer retina, thereby forming the outer blood-retinal barrier. The intricate interaction between the RPE and sensory retina presents challenges for learning how accurately culture models reflect native behavior. The challenge is heightened by findings that detail the variation of RPE barrier proteins both among species and at different stages of the life cycle. Among the striking differences is the expression of claudin family members. Claudins are the tight junction proteins that regulate ion diffusion across the spaces that lie between the cells of a monolayer. Claudin expression by RPE varies with species and life-stage, which implies functional differences among commonly used animal models. Investigators have turned to transcriptomics to supplement functional studies when comparing native and cultured tissue. The most detailed studies of the outer blood-retinal barrier have focused on human RPE with transcriptome and functional studies reported for human fetal, adult, and stem-cell derived RPE.

Ouabain inhibition of Na/K-ATPase across the retina prevents signed refractive compensation to lens-induced defocus, but not default ocular growth in young chicks

Purpose: The relevance of retinal integrity and energy pathways to ocular growth and induction of refractive errors has seldom been investigated. Thus, we used ouabain to target the channels that are essential for the maintenance of membrane potentials in cells, sodium potassium ATPase (Na/K-ATPase), to examine refractive compensation and ocular growth in response to lens-induced defocus in the chick.

Methods:  A single intravitreal injection of 1 mM ouabain in dimethyl sulfoxide (DMSO) carrier or DMSO alone was followed by monocular defocus with positive or negative 10 D lens (or no lens) from post-hatching days 5-9 under 12/12 hr light/dark conditions. Biometry and dark-adapted flash and electroretinography (ERG) were conducted on day 9, followed by immunohistological analyses.

Results: Ouabain inhibited differential ocular growth and refractive compensation to signed defocus compared to DMSO. By 4-days post-ouabain injection all components of the typical ERG responses to light had been eliminated, and widespread histological damage was apparent, though some ‘default state’ ocular growth was measurable. Immunohistochemistry demonstrated reduction in the specialized water channel Aquaporin 4 (AQP4) expression and increased evidence of caspase 3 expression (a cell death associated protein) in ouabain-treated eyes compared with DMSO alone.

Conclusion: The current study demonstrates that blockade of photoreceptor and inner retinal responses to light onset and offset by ouabain inhibits differential refractive compensation to optical blur, but does not prevent ocular growth.

Characterization of the R162W Kir7.1 mutation associated with snowflake vitreoretinopathy

KCNJ13 encodes Kir7.1, an inwardly rectifying K(+) channel that is expressed in multiple ion-transporting epithelia. A mutation in KCNJ13 resulting in an arginine-to-tryptophan change at residue 162 (R162W) of Kir7.1 was associated with snowflake vitreoretinal degeneration, an inherited autosomal-dominant disease characterized by vitreous degeneration and mild retinal degeneration. We used the Xenopus laevis oocyte expression system to assess the functional properties of the R162W (mutant) Kir7.1 channel and determine how wild-type (WT) Kir7.1 is affected by the presence of the mutant subunit. Recordings obtained via the two-electrode voltage-clamp technique revealed that injection of oocytes with mutant Kir7.1 cRNA resulted in currents and cation selectivity that were indistinguishable from those in water-injected oocytes, suggesting that the mutant protein does not form functional channels in the plasma membrane. Coinjection of oocytes with equal amounts of mutant and WT Kir7.1 cRNAs resulted in inward K(+) and Rb(+) currents with amplitudes that were ∼17% of those in oocytes injected with WT Kir7.1 cRNA alone, demonstrating a dominant-negative effect of the mutant subunit. Similar to oocytes injected with WT Kir7.1 cRNA alone, coinjected oocytes exhibited inwardly rectifying Rb(+) currents that were more than seven times larger than K(+) currents, indicating that mutant subunits did not alter Kir7.1 channel selectivity. Immunostaining of Xenopus oocytes or Madin-Darby canine kidney cells expressing mutant or WT Kir7.1 demonstrated distribution of both proteins primarily in the plasma membrane. Our data suggest that the R162W mutation suppresses Kir7.1 channel activity, possibly by negatively impacting gating by membrane phosphadidylinositol 4,5-bisphosphate.

Comparative effects of posterior eye cup tissues from myopic and hyperopic chick eyes on cultured scleral fibroblasts

The role of individual ocular tissues in mediating changes to the sclera during myopia development is unclear. The aim of this study was to examine the effects of retina, RPE and choroidal tissues from myopic and hyperopic chick eyes on the DNA and glycosaminoglycan (GAG) content in cultures of chick scleral fibroblasts. Primary cultures of fibroblastic cells expressing vimentin and α-smooth muscle actin were established in serum-supplemented growth medium from 8-day-old normal chick sclera. The fibroblasts were subsequently co-cultured with posterior eye cup tissue (full thickness containing retina, RPE and choroid) obtained from untreated eyes and eyes wearing translucent diffusers (form-deprivation myopia, FDM) or -15D lenses (lens-induced myopia, LIM) for 3 days (post-hatch day 5-8) (n = 6 per treatment group). The effect of tissues (full thickness and individual retina, RPE, and choroid layers) from -15D (LIM) versus +15D (lens-induced hyperopia, LIH) treated eyes was also determined. Refraction changes in the direction predicted by the visual treatments were confirmed by retinoscopy prior to tissue collection. Glycosaminoglycan (GAG) and DNA content of the scleral fibroblast cultures were measured using GAG and PicoGreen assays. There was no significant difference in the effect of full thickness tissue from either FDM or LIM treated eyes on DNA and GAG content of scleral fibroblasts (DNA 8.9 ± 2.6 μg and 8.4 ± 1.1 μg, p = 0.12; GAG 11.2 ± 0.6 μg and 10.1 ± 1.0 μg, p = 0.34). Retina from LIM eyes did not alter fibroblast DNA or GAG content compared to retina from LIH eyes (DNA 27.2 ± 1.7 μg versus 23.2 ± 1.5 μg, p = 0.21; GAG 28.1 ± 1.7 μg versus. 28.7 ± 1.2 μg, p = 0.46). Similarly, the choroid from LIH and LIM eyes did not produce a differential effect on DNA content (DNA LIM 46.9 ± 6.4 versus LIH 51.5 ± 4.7 μg, p = 0.31). In contrast, scleral fibroblast DNA was greater in co-culture with RPE from LIM eyes than the empty basket and DNA content less for co-culture with RPE from LIH eyes (LIM: 72.4 ± 6.3 μg versus empty basket: 46.03 ± 1.0 μg; p = 0.0005 and LIH: 27.9 ± 2.3 μg versus empty basket: 46.03 ± 1.0 μg; p = 0.0004). GAG content was lower with RPE from LIM eyes (LIM: 27.7 ± 0.9 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.021) and was higher with RPE from LIH eyes (LIH: 33.7 ± 1.9 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.010). Choroid from LIM eyes induce a relative increase in scleral GAG content e.g. (LIM: 32.5 ± 0.7 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.0004) while, choroid from LIH eyes induced a relative decrease in scleral GAG content (LIH: 18.9 ± 1.2 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.0034). GAG content of cells in co-culture with choroid from LIM versus LIH treated eyes was significantly different (32.5 ± 0.7 μg versus 18.9 ± 1.2 μg respectively, p = 0.0002). In conclusion, these experiments provide an evidence for a directional growth signal that is present (and remains) in the ex-vivo RPE/choroid, but that does not remain in the ex-vivo retina. The identity of this factor(s) that can modify scleral cell DNA and GAG content requires further research.

Electrical signaling in control of ocular cell behaviors

Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

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.

Missense mutations in a retinal pigment epithelium protein, bestrophin-1, cause retinitis pigmentosa

Bestrophin-1 is preferentially expressed at the basolateral membrane of the retinal pigmented epithelium (RPE) of the retina. Mutations in the BEST1 gene cause the retinal dystrophies vitelliform macular dystrophy, autosomal-dominant vitreochoroidopathy, and autosomal-recessive bestrophinopathy. Here, we describe four missense mutations in bestrophin-1, three that we believe are previously unreported, in patients diagnosed with autosomal-dominant and -recessive forms of retinitis pigmentosa (RP). The physiological function of bestrophin-1 remains poorly understood although its heterologous expression induces a Cl--specific current. We tested the effect of RP-causing variants on Cl- channel activity and cellular localization of bestrophin-1. Two (p.L140V and p.I205T) produced significantly decreased chloride-selective whole-cell currents in comparison to those of wild-type protein. In a model system of a polarized epithelium, two of three mutations (p.L140V and p.D228N) caused mislocalization of bestrophin-1 from the basolateral membrane to the cytoplasm. Mutations in bestrophin-1 are increasingly recognized as an important cause of inherited retinal dystrophy.

Dysregulation of human bestrophin-1 by ceramide-induced dephosphorylation

Best vitelliform macular dystrophy is an inherited autosomal dominant, juvenile onset form of macular degeneration caused by mutations in a chloride ion channel, human bestrophin-1 (hBest1). Mutations in Best1 have also been linked to several other forms of retinopathy. In addition to mutations, hBest1 dysfunction might come about by disruption of other processes that regulate Best1 function. Here we show that hBest1 chloride channel activity is regulated by ceramide and phosphorylation. We have identified a protein kinase C (PKC) phosphorylation site (serine 358) in hBest1 that is important for sustained channel function. Channel activity is maintained by PKC activators, protein phosphatase inhibitors, or pseudo-phosphorylation by substitution of glutamic acid for serine 358. When ceramide levels are elevated by exogenous addition of ceramide to the bath, by addition of bacterial sphingomyelinase, or by hypertonic stress, S358 is rapidly dephosphorylated. The dephosphorylation is mediated by protein phosphatase 2A. Hypertonic stress-induced dephosphorylation is blocked by a dihydroceramide, an inactive form of ceramide, and manumycin, an inhibitor of neutral sphingomyelinase. Our results support a model in which ceramide accumulation during early stages of retinopathy inhibits hBest1 function, leading to abnormal fluid transport across the retina, and enhanced inflammation.

Human disease-causing mutations disrupt an N-C-terminal interaction and channel function of bestrophin 1

Mutations in the human bestrophin 1 (hBest1) chloride channel cause Best vitelliform macular dystrophy. Although mutations in its transmembrane domains were found to alter biophysical properties of the channel, the mechanism for disease-causing mutations in its N and C termini remains elusive. We hypothesized that these mutations lead to channel dysfunction through disruption of an N-C-terminal interaction. Here, we present data demonstrating that hBest1 N and C termini indeed interact both in vivo and in vitro. In addition, using a spectrum-based fluorescence resonance energy transfer method, we showed that functional hBest1 channels in the plasma membrane were multimers. Disease-causing mutations in the N terminus (R19C, R25C, and K30C) and the C terminus (G299E, D301N, and D312N) caused channel dysfunction and disruption of the N-C interaction. Consistent with the functional and biochemical results, mutants D301N and D312N clearly reduced fluorescence resonance energy transfer signal, indicating that the N-C interaction was indeed perturbed. These results suggest that hBest1 functions as a multimer in the plasma membrane, and disruption of the N-C interaction by mutations leads to hBest1 channel dysfunction.

Potassium channel and NKCC cotransporter involvement in ocular refractive control mechanisms

Myopia affects well over 30% of adult humans globally. However, the underlying physiological mechanism is little understood. This study tested the hypothesis that ocular growth and refractive compensation to optical defocus can be controlled by manipulation of potassium and chloride ion-driven transretinal fluid movements to the choroid. Chicks were raised with +/-10D or zero power optical defocus rendering the focal plane of the eye in front of, behind, or at the level of the retinal photoreceptors respectively. Intravitreal injections of barium chloride, a non-specific inhibitor of potassium channels in the retina and RPE or bumetanide, a selective inhibitor of the sodium-potassium-chloride cotransporter were made, targeting fluid control mechanisms. Comparison of refractive compensation to 5 mM Ba(2+) and 10(-5) M bumetanide compared with control saline injected eyes shows significant change for both positive and negative lens defocus for Ba(2+) but significant change only for negative lens defocus with bumetanide (Rx(SAL)(-10D) = -8.6 +/- .9 D; Rx(Ba2+)(-10D) = -2.9 +/- .9 D; Rx(Bum)(-10D) = -2.9 +/- .9 D; Rx(SAL)(+10D) = +8.2 +/- .9 D; Rx(Ba2+)(+10D) = +2.8 +/- 1.3 D; Rx(Bum)(+10D) = +8.0 +/- .7 D). Vitreous chamber depths showed a main effect for drug conditions with less depth change in response to defocus shown for Ba(2+) relative to Saline, while bumetanide injected eyes showed a trend to increased depth without a significant interaction with applied defocus. The results indicate that both K channels and the NKCC cotransporter play a role in refractive compensation with NKCC blockade showing far more specificity for negative, compared with positive, lens defocus. Probable sites of action relevant to refractive control include the apical retinal pigment epithelium membrane and the photoreceptor/ON bipolar synapse. The similarities between the biometric effects of NKCC inhibition and biometric reports of the blockade of the retinal ON response, suggest a possible common mechanism. The selective inhibition of refractive compensation to negative lens in chick by loop diuretics such as bumetanide suggests that these drugs may be effective in the therapeutic management of human myopia.

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.

Bestrophin Cl- channels are highly permeable to HCO3-

Bestrophin-1 (Best1) is a Cl(-) channel that is linked to various retinopathies in both humans and dogs. Dysfunction of the Best1 Cl(-) channel has been proposed to cause retinopathy because of altered Cl(-) transport across the retinal pigment epithelium (RPE). In addition to Cl(-), many Cl(-) channels also transport HCO3(-). Because HCO3(-) is physiologically important in pH regulation and in fluid and ion transport across the RPE, we measured the permeability and conductance of bestrophins to HCO3(-) relative to Cl(-). Four human bestrophin homologs (hBest1, hBest2, hBest3, and hBest4) and mouse Best2 (mBest2) were expressed in HEK cells, and the relative HCO3(-) permeability (P HCO3/PCl) and conductance (G HCO3/GCl) were determined. P HCO3/PCl was calculated from the change in reversal potential (Erev) produced by replacing extracellular Cl(-) with HCO3(-). hBest1 was highly permeable to HCO3(-) (P HCO3)/PCl = approximately 0.44). hBest2, hBest4, and mBest2 had an even higher relative HCO3(-) permeability (P HCO3/PCl = 0.6-0.7). All four bestrophins had HCO3(-) conductances that were nearly the same as Cl(-) (G HCO3/GCl = 0.9-1.1). Extracellular Na+ did not affect the permeation of hBest1 to HCO3(-). At physiological HCO3(-) concentration, HCO3(-) was also highly conductive. The hBest1 disease-causing mutations Y85H, R92C, and W93C abolished both Cl(-) and HCO3(-) currents equally. The V78C mutation changed P HCO3/PCl and G HCO3/GCl of mBest2 channels. These results raise the possibility that disease-causing mutations in hBest1 produce disease by altering HCO3(-) homeostasis as well as Cl(-) transport in the retina.

Drosophila bestrophin-1 chloride current is dually regulated by calcium and cell volume

Mutations in the human bestrophin-1 (hBest1) gene are responsible for Best vitelliform macular dystrophy, however the mechanisms leading to retinal degeneration have not yet been determined because the function of the bestrophin protein is not fully understood. Bestrophins have been proposed to comprise a new family of Cl(-) channels that are activated by Ca(2+). While the regulation of bestrophin currents has focused on intracellular Ca(2+), little is known about other pathways/mechanisms that may also regulate bestrophin currents. Here we show that Cl(-) currents in Drosophila S2 cells, that we have previously shown are mediated by bestrophins, are dually regulated by Ca(2+) and cell volume. The bestrophin Cl(-) currents were activated in a dose-dependent manner by osmotic pressure differences between the internal and external solutions. The increase in the current was accompanied by cell swelling. The volume-regulated Cl(-) current was abolished by treating cells with each of four different RNAi constructs that reduced dBest1 expression. The volume-regulated current was rescued by transfecting with dBest1. Furthermore, cells not expressing dBest1 were severely depressed in their ability to regulate their cell volume. Volume regulation and Ca(2+) regulation can occur independently of one another: the volume-regulated current was activated in the complete absence of Ca(2+) and the Ca(2+)-activated current was activated independently of alterations in cell volume. These two pathways of bestrophin channel activation can interact; intracellular Ca(2+) potentiates the magnitude of the current activated by changes in cell volume. We conclude that in addition to being regulated by intracellular Ca(2+), Drosophila bestrophins are also novel members of the volume-regulated anion channel (VRAC) family that are necessary for cell volume homeostasis.

Regulation of basolateral Cl(-) channels in airway epithelial cells: the role of nitric oxide

The presence of basolateral Cl(-) channels in airway epithelium has been reported in several studies, but little is known about their role in the regulation of anion secretion. The purpose of this study was to characterize regulation of these channels by nitric oxide (NO) in Calu-3 cells. Transepithelial measurements revealed that NO donors activated a basolateral Cl(-) conductance sensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and anthracene-9-carboxylic acid. Apical membrane permeabilization studies confirmed the basolateral localization of NO-activated Cl(-) channels. Experiments using 8-bromo cyclic guanosine monophosphate (8Br-cGMP) and selective inhibitors of soluble guanylyl cyclase and inducible NO synthase (1H-[1, 2, 4] oxadiazolol-[4, 3-a] quinoxalin-1-one [ODQ] and 1400W [N-(3-Aminomethyl)benzyl)acetamidine], respectively) demonstrated that NO activated Cl(-) channels via a cGMP-dependent pathway. Anion replacement and (36)Cl(-) flux studies showed that NO affected both Cl(-) and HCO (3) (-) secretion. Two different types of Cl(-) channels are known to be present in the basolateral membrane of epithelial cells: Zn(2+)-sensitive ClC-2 and DIDS-sensitive bestrophin channels. S-Nitrosoglutathione (GSNO) activated Cl(-) conductance in the presence of Zn(2+) ions, indicating that ClC-2 channel function was not affected by GSNO. In contrast, DIDS completely inhibited GSNO-activated Cl(-) conductance. Bestrophin immunoprecipitation studies showed that under control conditions bestrophin channels were not phosphorylated but became phosphorylated after GSNO treatment. The presence of bestrophin in airway epithelia was confirmed using immunohistochemistry. We conclude that basolateral Cl(-) channels play a major role in the NO-dependent regulation of anion secretion in Calu-3 cells.

Development and role of tight junctions in the retinal pigment epithelium

The outer blood-retinal barrier is formed by the retinal pigment epithelium. In any epithelial monolayer, the tight junctions enable the epithelium to form a barrier by joining neighboring cells together and regulating transepithelial diffusion through the paracellular spaces. Tight junctions are complex, dynamic structures that regulate cell proliferation, polarity, and paracellular diffusion. The specific properties of tight junctions vary among epithelia, according to the physiological role of the epithelium. Unlike other epithelia, the apical surface of the retinal pigment epithelium interacts with a solid tissue, the neural retina. Secretions of the developing neural retina regulate the assembly, maturation, and tissue-specific properties of these tight junctions. The slow time course of development allows investigators to dissect the mechanisms of junction assembly and function. These studies are aided by culture systems that model different stages of development.

Single Cl- channels activated by Ca2+ in Drosophila S2 cells are mediated by bestrophins

Mutations in human bestrophin-1 (VMD2) are genetically linked to several forms of retinal degeneration but the underlying mechanisms are unknown. Bestrophin-1 (hBest1) has been proposed to be a Cl⁻ channel involved in ion and fluid transport by the retinal pigment epithelium (RPE). To date, however, bestrophin currents have only been described in overexpression systems and not in any native cells. To test whether bestrophins function as Ca²⁺-activated Cl⁻ (CaC) channels physiologically, we used interfering RNA (RNAi) in the Drosophila S2 cell line. S2 cells express four bestrophins (dbest1–4) and have an endogenous CaC current. The CaC current is abolished by several RNAi constructs to dbest1 and dbest2, but not dbest3 or dbest4. The endogenous CaC current was mimicked by expression of dbest1 in HEK cells, and the rectification and relative permeability of the current were altered by replacing F81 with cysteine. Single channel analysis of the S2 bestrophin currents revealed an ∼2-pS single channel with fast gating kinetics and linear current–voltage relationship. A similar channel was observed in CHO cells transfected with dbest1, but no such channel was seen in S2 cells treated with RNAi to dbest1. This provides definitive evidence that bestrophins are components of native CaC channels at the plasma membrane.

Photochemical damage of the retina

Visual perception occurs when radiation with a wavelength between 400 and 760 nm reaches the retina. The retina has evolved to capture photons efficiently and initiate visual transduction. The retina, however, is vulnerable to damage by light, a vulnerability that has long been recognized. Photochemical damage has been widely studied, because it can cause retinal damage within the intensity range of natural light. Photochemical lesions are primarily located in the outer layers at the central region of the retina. Two classes of photochemical damage have been recognized: Class I damage, which is characterized by the rhodopsin action spectrum, is believed to be mediated by visual pigments, with the primary lesions located in the photoreceptors; whereas Class II damage is generally confined to the retinal pigment epithelium. The action spectrum peaks in the short wavelength region, providing the basis for the concept of blue light hazard. Several factors can modify the susceptibility of the retina to photochemical damage. Photochemical mechanisms, in particular mechanisms that arise from illumination with blue light, are responsible for solar retinitis and for iatrogenic retinal insult from ophthalmological instruments. Further, blue light may play a role in the pathogenesis of age-related macular degeneration. Laboratory studies have suggested that photochemical damage includes oxidative events. Retinal cells die by apoptosis in response to photic injury, and the process of cell death is operated by diverse damaging mechanisms. Modern molecular biology techniques help to study in-depth the basic mechanism of photochemical damage of the retina and to develop strategies of neuroprotection.

Ionic control of ocular growth and refractive change

The physiological mechanisms underlying the abnormal vitreal and ocular growth and myopic refractive errors induced under conditions of visual form deprivation in many animal species, including humans, are unknown. This study demonstrates, using energy dispersive x-ray microanalysis, a systematic pattern of changes in the elemental distribution of K, Na, and Cl across the entire retina in experimental form deprivation myopia and in the 5 days required for refractive normalization after occluder removal. In our report we link the ionic environment associated with physiological activity of the retina under a translucent occluder to refractive change and describe large but reversible environmentally driven increases in potassium, sodium, and chloride abundances in the neural retina. Our results are consistent with the notion of ionically driven fluid movements as the vector underlying the myopic increase in ocular size. New treatments for myopia, which currently affects nearly half of the human population, may result.

Aquaporins and CFTR in ocular epithelial fluid transport

Aquaporins (AQPs) and the cystic fibrosis transmembrane conductance regulator (CFTR) provide the molecular routes for transport of water and chloride, respectively, through many epithelial tissues. In ocular epithelia, fluid transport generally involves secondary active chloride transport, which creates the osmotic gradient to drive transepithelial water transport. This review is focused on the role of AQPs and CFTR in water and ion transport across corneal/conjunctival epithelia, corneal endothelium, ciliary epithelium, and retinal pigment epithelium. The potential relevance of water and chloride transport to common disorders of ocular fluid balance is also considered. Recent data suggest AQPs and CFTR as attractive targets for drug development for therapy of keratoconjunctivitis sicca, recurrent corneal erosions, corneal edema, glaucoma, retinal detachment, and retinal ischemia.

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.

Expression of bestrophin‐1, the product of the VMD2 gene, modulates voltage‐dependent Ca 2+ channels in retinal pigment epithelial cells

Mutations in the VMD2 gene cause Best's disease, an inherited form of macular degeneration. The reduction in the light-peak amplitude in the patient's electro-oculogram suggests that bestrophin-1 influences the membrane conductance of the retinal pigment epithelium (RPE). Systemic application of the L-type Ca2+ channel blocker nimodipine reduced the light-peak amplitude in the rat electroretinogram but not a- and b-waves. Expression of bestrophin-1 in a RPE cell line (RPE-J) led to changes in L-type channel properties. Wild-type bestrophin-1 induced an acceleration of activation kinetics of Ba2+ currents through L-type Ca2+ channels and a shift of the voltage-dependent activation to more negative values, closer to the resting potential of RPE cells. Expression of bestrophin-1 with Best disease-causing mutations led to comparable shifts in voltage-dependent activation but different effects on activation and inactivation kinetics. Bestrophin W93C exhibited slowed activation and inactivation, and bestrophin R218C accelerated the activation and inactivation. Thus, transfection of RPE cells with bestrophin-1 distinctively changed L-type Ca2+ channel kinetics and voltage-dependence. On the basis of these data, we propose that presence of bestrophin-1 influences kinetics and voltage-dependence of voltage-dependent Ca2+ channels and that these effects might open new ways to understand the mechanisms leading to retinal degeneration in Best's disease.