Expression of Orai genes and ICRAC activation in the human retinal pigment epithelium

Retinal electrophysiology in central nervous system disorders. A review of human and mouse studies

The retina and brain share similar neurochemistry and neurodevelopmental origins, with the retina, often viewed as a "window to the brain." With retinal measures of structure and function becoming easier to obtain in clinical populations there is a growing interest in using retinal findings as potential biomarkers for disorders affecting the central nervous system. Functional retinal biomarkers, such as the electroretinogram, show promise in neurological disorders, despite having limitations imposed by the existence of overlapping genetic markers, clinical traits or the effects of medications that may reduce their specificity in some conditions. This narrative review summarizes the principal functional retinal findings in central nervous system disorders and related mouse models and provides a background to the main excitatory and inhibitory retinal neurotransmitters that have been implicated to explain the visual electrophysiological findings. These changes in retinal neurochemistry may contribute to our understanding of these conditions based on the findings of retinal electrophysiological tests such as the flash, pattern, multifocal electroretinograms, and electro-oculogram. It is likely that future applications of signal analysis and machine learning algorithms will offer new insights into the pathophysiology, classification, and progression of these clinical disorders including autism, attention deficit/hyperactivity disorder, bipolar disorder, schizophrenia, depression, Parkinson's, and Alzheimer's disease. New clinical applications of visual electrophysiology to this field may lead to earlier, more accurate diagnoses and better targeted therapeutic interventions benefiting individual patients and clinicians managing these individuals and their families.

Klotho: A new therapeutic target in diabetic retinopathy?

Klotho (Kl) is considered an antiaging gene, mainly for the inhibition of the insulin-like growth factor-1 signaling. Kl exists as full-length transmembrane, which acts as co-receptor for fibroblast growth factor receptor, and in soluble forms (sKl). The sKl may exert pleiotropic effects on organs and tissues by regulating several pathways involved in the pathogenesis of diseases associated with oxidative and inflammatory state. In diabetic Patients, serum levels of Kl are significantly decreased compared to healthy subjects, and are related to duration of diabetes. In diabetic retinopathy (DR), one of the most common microvascular complications of type 2 diabetes, serum Kl levels are negatively correlated with progression of the disease. A lot of evidences showed that Kl regulates several mechanisms involved in maintaining homeostasis and functions of retinal cells, including phagocytosis, calcium signaling, secretion of vascular endothelial growth factor A (VEGF-A), maintenance of redox status, and melanin biosynthesis. Experimental data have been shown that Kl exerts positive effects on several mechanisms involved in onset and progression of DR. In particular, treatment with Kl: (1) Prevents apoptosis induced by oxidative stress in human retinal endothelial cells and in retinal pigment epithelium (RPE) cells; (2) reduces secretion of VEGF-A by RPE cells; and (3) decreases subretinal fibrosis and preserves autophagic activity. Therefore, Kl may become a novel biomarker and a good candidate for the treatment of DR.

Is white the right light for the clinical electrooculogram?

PURPOSE

To investigate if a lower luminance monochromatic LED stimulus could be used as an alternative to a high luminance white light for the clinical electrooculogram.

METHODS

Clinical electrooculograms were recorded in color normal participants (N = 23) aged 22.6 ± 1.2 years, 7 male and 16 female using the standard 100 cd.m^-2 white illuminant and four monochromatic LEDs with peak wavelengths of 448, 534, 596 and 634 nm at 30 cd.m^-2. Pupils were dilated and there was a 30 cd.m^-2pre-adaptation to white light for 2 min followed by 15 min dark adaptation and 20 min recording in the light stimulus using a Ganzfeld stimulator.

RESULTS

The normalized LP:DTratio for the short wavelength LED (448 nm) was equivalent in amplitude and timing to the ISCEV standard EOG (p = .99). The LP:DTratio for the white (100 cd.m^-2) and 448 nm (30 cd.m^-2) were (median ± SEM): 2.49 ± .11 and 2.47 ± .11. The time to light-rise peak was also equivalent being 9.0 ± .2 and 8.0 ± .4 min (p = .54).

CONCLUSIONS

Consideration may be given to using a short wavelength monochromatic stimulus that is more comfortable for the subject than the current 100 cd.m^-2 illuminant.

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

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.

Thrombin-Induced Calpain Activation Promotes Protease-Activated Receptor 1 Internalization

The serine protease thrombin activates Protease-Activated Receptors (PARs), a family of G-protein-coupled receptors (GPCRs) activated by the proteolytic cleavage of their extracellular N-terminal domain. Four members of this family have been identified: PAR1–4. The activation of Protease-Activated Receptor 1(PAR1), the prototype of this receptor family, leads to an increase in intracellular Ca⁺² concentration ([Ca⁺²]i) mediated by G_q11α coupling and phospholipase C (PLC) activation. We have previously shown that the stimulation of PAR1 by thrombin promotes intracellular signaling leading to RPE cell transformation, proliferation, and migration which characterize fibroproliferative eye diseases leading to blindness. Within this context, the elucidation of the mechanisms involved in PAR1 inactivation is of utmost importance. Due to the irreversible nature of PAR1 activation, its inactivation must be efficiently regulated in order to terminate signaling. Using ARPE-19 human RPE cell line, we characterized thrombin-induced [Ca⁺²]i increase and demonstrated the calcium-dependent activation of μ-calpain mediated by PAR1. Calpains are a family of calcium-activated cysteine proteases involved in multiple cellular processes including the internalization of membrane proteins through clathrin-coated vesicles. We demonstrated that PAR1-induced calpain activation results in the degradation of α-spectrin by calpain, essential for receptor endocytosis, and the consequent decrease in PAR1 membrane expression. Collectively, the present results identify a novel μ-calpain-dependent mechanism for PAR1 inactivation following exposure to thrombin.

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 neglected CRAC proteins: Orai2, Orai3, and STIM2

Plasma-membrane-localized Orai1 ion channel subunits interacting with ER-localized STIM1 molecules comprise the major subunit composition responsible for calcium release-activated calcium channels. STIM1 "translates" the Ca(2+) store content into Orai1 activity, making it a store-operated channel. Surprisingly, in addition to being the physical activator, STIM1 also modulates Orai1 properties, including its inactivation and permeation (see Chapter 1). STIM1 is thus more than a pure Orai1 activator. Within the past 7 years following the discovery of STIM and Orai proteins, the molecular mechanisms of STIM1/Orai1 activity and their functional importance have been studied in great detail. Much less is currently known about the other isoforms STIM2, Orai2, and Orai3. In this chapter, we summarize the current knowledge about STIM2, Orai2, and Orai3 properties and function. Are these homologues mainly modulators of predominantly STIM1/Orai1-mediated complexes or do store-dependent or -independent functions such as regulation of basal Ca(2+) concentration and activation of Orai3-containing complexes by arachidonic acid or by estrogen receptors point toward their "true" physiological function? Is Orai2 the Orai1 of neurons? A major focus of the review is on the functional relevance of STIM2, Orai2, and Orai3, some of which still remains speculative.

Involvement of STIM1 and Orai1 in EGF-mediated cell growth in retinal pigment epithelial cells

BACKGROUND

In non-excitable cells, one major route for calcium entry is through store-operated calcium (SOC) channels in the plasma membrane. These channels are activated by the emptying of intracellular Ca²⁺ store. STIM1 and Orai1 are major regulators of SOC channels. In this study, we explored the functions of STIM1 and Orai1 in epidermal growth factor (EGF)-induced cell proliferation and migration in retinal pigment epithelial cells (ARPE-19 cell line).

RESULTS

EGF triggers cell proliferation and migration in ARPE-19 cells. Cell proliferation and migration involve STIM1 and Orai1, as well as phosphorylation of extracellular signal-regulated protein kinase (ERK) 1/2, and Akt. Pharmacological inhibitors of SOC channels and siRNA of Orai1 and STIM1 suppress cell proliferation and migration. Pre-treatment of mitogen-activated protein kinase kinase (MEK) inhibitors and a phosphatidylinositol 3 kinases (PI3K) inhibitor attenuated cell proliferation and migration. However, inhibition of the SOC channels failed to prevent EGF-mediated ERK 1/2 and Akt phosphorylation.

CONCLUSIONS

Our results showed that STIM1, Orai1, ERK 1/2, and Akt are key determinants of EGF-mediated cell growth in ARPE-19 cells. EGF is a potent growth molecule that has been linked to the development of PVR, and therefore, STIM1, Orai1, as well as the MEK/ERK 1/2 and PI3K/Akt pathways, might be potential therapeutic targets for drugs aimed at treating such disorders.

Ca2+-imaging techniques to analyze Ca2+ signaling in cells and to monitor neuronal activity in the retina

Ca(2+) is an important regulator of many cell functions including proliferation, apoptosis, movements, secretion, contraction, excitation, and differentiation. The regulation of these different cell functions is encoded by the specific temporal and spatial distribution of Ca(2+) signals. In degenerative diseases mutations can lead to changes in cell functions in the worst case to apoptosis. Thus analysis of signals arising as changes in intracellular free Ca(2+) represent an important step towards the understanding of mutation-dependent or environmental impact into cell function. The classic approach to study changes in intracellular free Ca(2+) is the measurement of intracellular Ca(2+) by using Ca(2+)-sensitive fluorescence dyes in conjunction with fluorescence microscopy as a method called Ca(2+) imaging. In this chapter the basic method and a short theoretical background will be provided to perform Ca(2+)-imaging experiments. As a model cultured retinal pigment epithelial cells will be used. The basic steps of the method are the loading of the cells with the fluorescence dye by incubation with a membrane permeable ester of the dye. The next step would be the application of an agonist which can be further analyzed by blockers of enzymes or by manipulating the different Ca(2+)-storing compartments which contribute to changes in intracellular free Ca(2+). At the end of an experiment an on-cell type of calibration will be performed to calculate the underlying concentration of intracellular free Ca(2+). Furthermore, the successful calibration of an experiment can be used as a measure of a reliable experiment. In addition to that, three examples for basic experiments will be given which can lead to a first insight into the mechanism underlying changes in cytosolic free Ca(2+)as a second messenger.

Role of bestrophin-1 in store-operated calcium entry in retinal pigment epithelium

The retinal pigment epithelium (RPE) expresses bestrophin-1 where mutant bestrophin cause retinal degenerations. Overexpression of bestrophin-1 demonstrated Ca(2+)-dependent Cl(-) channel function, whereas the RPE in bestrophin-1 knockout or mutant bestrophin-1 knock-in mice showed no change in Cl(-) conductance. To account for these apparently mutually exclusive findings, we investigated the function of endogenously expressed bestrophin-1 in a short-time RPE cell culture system by means of immunocytochemistry, Ca(2+) imaging, and siRNA knockdown. Immunocytochemical quantification of bestrophin-1 localization demonstrated 2.5 times higher co-localization with the endoplasmic reticulum (ER) Ca(2+)-sensor protein, Stim-1, than with the membrane protein β-catenin, implicating it in store-operated Ca(2+) entry (SOCE). Ca(2+) release from ER stores under extracellular Ca(2+)-free conditions using thapsigargin (1 μM) to inhibit endoplasmic Ca(2+) ATPase (SERCA) followed by re-adjustment of extracellular Ca(2+) to physiological levels activated SOCE, which was insensitive to the blocker of numerous transient receptor potential channels and voltage-dependent Ca(2+) channels SKF96563 (1 μM). SOCE was augmented at 5 μM and inhibited at 75 μM by 2-aminoethoxydiphenyl borate which indicates the involvement Orai-1 channels. In confirmation, SOCE was decreased by siRNA knockdown of Orai-1 expression. SOCE amplitude was strongly reduced by siRNA knockdown of bestrophin-1 expression, which was due to neither changes in Stim-1/Orai-1 expression nor Stim-1/bestrophin-1 interaction. The amount of Ca(2+) released by SERCA inhibition was reduced after siRNA knockdown of bestrophin-1, but not of Orai-1. In conclusion we found that a proportion of bestrophin-1 is functionally localized to ER Ca(2+) stores where it influences the amount of Ca(2+) and therefore Ca(2+) signals which result from activation of Orai-1 via Stim-1.

Unraveling STIM2 function

The discovery of molecular players in capacitative calcium (Ca(2+)) entry, also referred to as store-operated Ca(2+) entry (SOCE), supposed a great advance in the knowledge of cellular mechanisms of Ca(2+) entry, which are essential for a broad range of cellular functions. The identification of STIM1 and STIM2 proteins as the sensors of Ca(2+) stored in the endoplasmic reticulum unraveled the mechanism by which depletion of intracellular Ca(2+) stores is communicated to store-operated Ca(2+) channels located in the plasma membrane, triggering the activation of SOCE and intracellular Ca(2+)-dependent signaling cascades. Initial studies suggested a dominant function of STIM1 in SOCE and SOCE-dependent cellular functions compared to STIM2, especially those that participate in immune responses. Consequently, most of the subsequent studies focused on STIM1. However, during the last years, STIM2 has been demonstrated to play a more relevant and complex function than initially reported, being even important to sustain normal life in mice. These studies have led to reconsider the role of STIM2 in SOCE and its relevance in cellular physiology. This review is intended to summarize and provide an overview of the current data available about this exciting isoform, STIM2, and its actual position together with STIM1 in the mechanism of SOCE.