Ca(2+)-conductances in cultured rat retinal pigment epithelial cells

Functional Voltage-Gated Calcium Channels Are Present in Human Embryonic Stem Cell-Derived Retinal Pigment Epithelium

Retinal pigment epithelium (RPE) performs important functions for the maintenance of photoreceptors and vision. Malfunctions within the RPE are implicated in several retinal diseases for which transplantations of stem cell‐derived RPE are promising treatment options. Their success, however, is largely dependent on the functionality of the transplanted cells. This requires correct cellular physiology, which is highly influenced by the various ion channels of RPE, including voltage‐gated Ca²⁺ (Ca_V) channels. This study investigated the localization and functionality of Ca_V channels in human embryonic stem cell (hESC)‐derived RPE. Whole‐cell patch‐clamp recordings from these cells revealed slowly inactivating L‐type currents comparable to freshly isolated mouse RPE. Some hESC‐RPE cells also carried fast transient T‐type resembling currents. These findings were confirmed by immunostainings from both hESC‐ and mouse RPE that showed the presence of the L‐type Ca²⁺ channels Ca_V1.2 and Ca_V1.3 as well as the T‐type Ca²⁺ channels Ca_V3.1 and Ca_V3.2. The localization of the major subtype, Ca_V1.3, changed during hESC‐RPE maturation co‐localizing with pericentrin to the base of the primary cilium before reaching more homogeneous membrane localization comparable to mouse RPE. Based on functional assessment, the L‐type Ca²⁺ channels participated in the regulation of vascular endothelial growth factor secretion as well as in the phagocytosis of photoreceptor outer segments in hESC‐RPE. Overall, this study demonstrates that a functional machinery of voltage‐gated Ca²⁺ channels is present in mature hESC‐RPE, which is promising for the success of transplantation therapies. stem cells translational medicine2019;8:179&15

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.

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.

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.

Ca2+ channel characteristics in neuroendocrine tumor cell cultures analyzed by color contour plots

Neuroendocrine tumor (NET) cells express several voltage-operated Ca2+ channels (VOCCs). To address the question, if clinically distinct entities of NETs could be associated by specific patterns of VOCC expression, electrophysiological properties of NET primary cultures derived from consecutive surgical resections and permanent NET cell cultures were determined and analyzed by a color-coding contour diagram method. Using whole-cell patch-clamp technique, electrophysiological data were obtained from human pancreatic (foregut) BON and mouse intestinal (midgut) STC-1 cells as well as from human primary cultivated NET cells from fore- and midgut tumors or liver metastasis. To describe definite Ca2+ channel characteristics, we suggested a color-coding method to depict the contours of time- and voltage-dependence. In this study, we could demonstrate specific Ca2+ channel properties in NET cells from midgut tumors which were not found in NET cells from foregut location. This may be important functionally in respect of different cell biological functions of NET cells such as release of bioamines and neuropeptides. In addition, definite differences between the effect of specific and unspecific Ca2+ channel modulators on NET cells were detected. Although most of this information can be obtained by superimposing current-voltage curves at different times after the onset of the voltage step, the color-coding contour diagrams clearly provides a better visual summary, and hence might be generally quite useful.

Calcium influx induced by activation of receptor tyrosine kinases in SV40-transfected human corneal endothelial cells

This study was undertaken to investigate electrophysiological properties of immortalized SV40-transfected human corneal endothelial cells (HCEC-SV40) combined with the analysis of intracellular Ca(2+) responses mediated by ligands for receptor tyrosine kinases (RTK). In addition, the effects of several tyrosine kinase inhibitors were tested on Ca(2+) inflow mediated by induction of capacitative calcium entry (CCE). Patch-clamp techniques and measurements of the intracellular free Ca(2+) ([Ca(2+)](i)) by fura-2 were performed using HCEC-SV40. Stimulation of fibroblast growth factor receptors (FGFR) (e.g. by basic-FGF) (10 ng ml(-1)) elicited activation of Ca(2+) permeable channels and a subsequent increase of cytosolic free Ca(2+) in HCEC-SV40. This effect could be disrupted by the L-type Ca(2+) channel blocker nifedipine (5 microM). In addition, nifedipine significantly reduced the magnitude of CCE. Inhibition of protein tyrosine kinases (PTKs) by genistein, lavendustin A, or tyrphostin 51 (all 5 microM) also led to a reduction of CCE in HCEC-SV40. This study demonstrates for the first time that L-type Ca(2+) channel activity in HCEC-SV40 is linked to the activity of FGF receptor tyrosine kinases. These data regarding Ca(2+) inflow through Ca(2+) channels could be useful for investigation of culture and vitality conditions of HCEC.

Stimulation of L-type Ca(2+) channels by increase of intracellular InsP3 in rat retinal pigment epithelial cells

The purpose of this study was to investigate the role of voltage-dependent L-type Ca(2+)channels in intracellular Ca(2+)signaling of the retinal pigment epithelium (RPE). Patch-clamp techniques in conjunction with measurements of the intracellular free Ca(2+)using the Ca(2+)-sensitive fluorescence dye fura-2 were performed using cultured rat RPE cells. Intracellular application of inositol-1,4,5-trisphosphate (InsP3; 10 microM) via the patch-pipette during the whole-cell configuration led to an increase in the intracellular free Ca(2+)([Ca(2+)](i)). This effect could be reduced by the L-type Ca(2+)channel blocker nifedipine (2 microM). At the moment of the maximal rise in [Ca(2+)](i)L-type currents displayed an increase in the current density and shifts in the activation curve and of the steady-state inactivation. Comparable changes of L-type channel activity could be observed by induction of capacitative Ca(2+)entry, a maneuver to release Ca(2+)from intracellular Ca(2+)stores independently from InsP3. The increase in L-type Ca(2+)channel activity and [Ca(2+)](i)by intracellular application of InsP3 or induction of capacitative Ca(2+)entry could be inhibited by blocking tyrosine kinase activity using genistein (5 microM) or tyrphostin 51 (10 microM). It is concluded that L-type Ca(2+)channels are involved in the Ca(2+)/InsP3 second messenger system by generating an influx of extracellular Ca(2+)into the cell. This is enabled by depletion of cytosolic Ca(2+)stores and tyrosine kinase-dependent activation of L-type channels.

Fibroblast growth factor receptor 2 (FGFR2) in brain neurons and retinal pigment epithelial cells act via stimulation of neuroendocrine L-type channels (Ca(v)1.3)

In contrast to the fibroblast growth factor receptor 1 (FGFR1), little is known about intracellular signaling of FGFR2. The signaling cascade of FGFR2 was studied using the perforated patch configuration of the patch-clamp technique in cultured rat retinal pigment epithelial (RPE) cells that express both FGFR1 and FGFR2. Interaction of signaling proteins was studied using immunoprecipitation techniques with membrane proteins from RPE cells and freshly isolated rat brain. When Ba(2+) currents through L-type channels were studied, extracellular application of bFGF (10 ng/ml) led to a shift of the steady-state activation to more negative values. In 50% of cells, an additional increase in maximal current amplitude was observed. This effect was blocked by the tyrosine kinase inhibitor lavendustin A (10(-5) M) but was not influenced by the FGFR1 blocker SU5402 (2 x 10(-5) M) or by the blocker for src-kinase herbimycin A (10(-5) M). Immunoprecipitation of FGFR2 led to coprecipitation of alpha 1D Ca(2+) channel subunits and precipitation of alpha 1D subunits led to coprecipitation of FGFR2. Immunoprecipitation of FGFR1 did not result in the coprecipitation with alpha 1D Ca(2+) channel subunits. The coprecipitation results were comparable when using brain tissue and RPE cells. The alpha 1D subunit-specific band were stained with antiphosphotyrosine antibodies. We conclude that FGFR2 acts via a different signaling cascade than FGFR1. This cascade involves an src-kinase-independent, close functional interaction of FGFR2 and the alpha subunit of neuroendocrine L-type channels.

Activation of neuroendocrine L-type channels (alpha1D subunits) in retinal pigment epithelial cells and brain neurons by pp60(c-src)

The aim of this study is to characterize the subtype of tyrosine kinase-regulated L-type Ca(2+) channels in retinal pigment epithelial (RPE) cells. Ca(2+) channel alpha1D-subunits were enriched by immunoprecipitation from membrane proteins isolated from rat RPE cells. Western blot analysis of the precipitates revealed coprecipitation of pp60(c-src). In addition, in precipitates obtained with antibodies against pp60(c-src), alpha1D-subunits were identified. The same was observed in immunoprecipitations from rat brain neurons. Tyrosine phosphorylation of alpha1D-subunits was confirmed using anti-phosphotyrosine antibodies. Ba(2+) currents through L-type channels in cultured rat RPE cells were increased by intracellular application of active pp60(c-src) (30 U/ml) (heat-inactivated pp60(c-src) had no effect). Thus, L-type channels of the neuroendocrine subtype can be expressed in epithelial cells and are activated by tyrosine kinase of the src subtype. This kind of regulation is also suggested for brain-derived neurons.

Altered regulation of L-type channels by protein kinase C and protein tyrosine kinases as a pathophysiologic effect in retinal degeneration

The effect of protein tyrosine kinases (PTK) on L-type calcium channels in cultured retinal pigmented epithelium (RPE) from rats with retinal dystrophy was investigated. Barium currents through Bay K 8644 (10(-6) M) sensitive L-type channels were measured using the patch-clamp technique. The current density of L-type currents is twice as high and the inactivation time constants are much slower than in cells from nondystrophic control rats. Application of the PTK blockers genistein, lavendustin A, and herbimycin A (all 5 x 10(-6) M) led to an increase of L-type currents. Intracellular application of pp60c-src (30 U/ml) via the patch pipette led to a transient decrease of L-type currents. The protein kinase A (PKA) and PKG blocker H9 (10(-6) M) showed no effect on L-type currents. However, the protein kinase C blocker chelerythrine (10(-5) M) reduced these currents. Up-regulation of PKC by 10(-6) M 4beta-phorbol-12 myristate-13 acetate (PMA) led to a decrease of L-type currents. Additional application of genistein led to a further decrease of these currents. However, intracellular application of pp60(c-src) in PMA-treated cells led to a transient increase of L-type currents. Investigating the calcium response to bFGF application showed that RPE cells from RCS rats used different pathways than control RPE cells to increase cytosolic free calcium. This different pathway does not involve the activation of L-type channels. The present study with RPE cells from rats with retinal dystrophy shows a changed integration of PTK and PKC in channel regulation. Considering the altered response to bFGF in RCS-RPE cells, this disturbed regulation of L-type channels by tyrosine kinases is involved in the etiology of retinal degeneration in RCS rats.

Calcium-independent regulation of pigment granule aggregation and dispersion in teleost retinal pigment epithelial cells

In the eyes of teleosts and amphibians, melanin pigment granules of the retinal pigment epithelium (RPE) migrate in response to changes in light conditions. In the light, pigment granules disperse into the cells' long apical projections, thereby shielding the rod photoreceptor outer segments and reducing their extent of bleach. In darkness, pigment granules aggregate towards the base of the RPE cells. In vitro, RPE pigment granule aggregation can be induced by application of nonderivatized cAMP, and pigment granule dispersion can be induced by cAMP washout. In previous studies based on RPE-retina co-cultures, extracellular calcium was found to influence pigment granule migration. To examine the role of calcium in regulation of RPE pigment granule migration in the absence of retinal influences, we have used isolated RPE sheets and dissociated, cultured RPE cells. Under these conditions depletion of extracellular or intracellular calcium ([Ca2+]o, [Ca2+]i) had no effect on RPE pigment granule aggregation or dispersion. Using the intracellular calcium dye fura-2 and a new dye, fura-pe3, to monitor calcium dynamics in isolated RPE cells, we found that [Ca2+]i did not change from basal levels when pigment granule aggregation was triggered by cAMP, or dispersion was triggered by cAMP washout. Also, no change in [Ca2+]i was detected when dispersion was triggered by cAMP washout in the presence of 10 microM dopamine, a treatment previously shown to enhance dispersion. In addition, elevation of [Ca2+]i by addition of ionomycin neither triggered pigment movements, nor interfered with pigment granule motility elicited by cAMP addition or washout. Since other studies have indicated that actin plays a role in both pigment granule dispersion and aggregation in RPE, our findings suggest that RPE pigment granule migration depends on an actin-based motility system that is not directly regulated by calcium.

Evidence for voltage-sensitive, calcium-conducting channels in airway epithelial cells

In airways epithelial cultures, mechanical stimulation induces intracellular Ca2+ concentration ([Ca2+]i) changes by causing Ca2+ entry and intracellular Ca2+ release. Mechanically induced Ca2+ fluxes across the plasma membrane are blocked by Ni2+ (Boitano, S., M. J. Sanderson, and E. R. Dirksen. J. Cell. Sci. 107: 3037-3044, 1994). In this report we use fluorescence imaging microscopy with fura 2 and intracellular recording of the transmembrane potential to further characterize Ca2+ flux in the plasma membrane of these cells. Mechanically induced Ca2+ influx is blocked by nifedipine. Addition of the dihydropyridine agonist BAY K8644 (2 microM) leads to a delayed increase of [Ca2+]i that is dependent on extracellular Ca2+. Switching to high extracellular K+ concentration ([K+]o) causes depolarization of the plasma membrane and a transient increase in [Ca2+]i. The number of cells that respond to high [K+]o is significantly decreased by Ni2+ (1 mM) or nifedipine (10 microM). Mechanical stimulation causes a rapid depolarization of the stimulated cell that can be suppressed by the K+ ionophore valinomycin. Valinomycin treatment also blocks mechanically induced Ca2+ dux. These results suggest that voltage-sensitive Ca(2+)-conducting channels exist in airway epithelial cells, and these channels contribute to the [Ca2+]i changes observed after mechanical stimulation or depolarization of the plasma membrane.

Extracellular matrix proteins as substrate modulate the pattern of calcium channel expression in cultured rat retinal pigment epithelial cells

We investigated the effect of different culture substrates on the expression of membrane conductances for calcium in cultured rat retinal pigment epithelial (RPE) cells using the perforated patch technique and barium as charge carrier. In younger cultures (up to 12 days old) the RPE cells expressed L-type calcium channels, in older cultures (more than 12 days old) LVA-type channels. The LVA-type channels have been characterized as a tetrodotoxin sensitive Ca2+ channels. Coating the culture substrate with laminin, shifted the culture age for expression of LVA-type channels to 7 days. When collagen type 4 was used as substrate LVA-type channels and L-type channels were expressed simultaneously in 7 days old cultures. We concluded that proteins of the extracellular matrix which are known to enhance cell differentiation in culture, enhance the expression of LVA-type channels in RPE cells.