Depolarization and activation of dihydropyridine-sensitive Ca2+ channels stimulate inositol phosphate accumulation in photoreceptor-enriched chick retinal cell cultures

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

Influence of spontaneous calcium events on cell-cycle progression in embryonal carcinoma and adult stem cells

Spontaneous Ca(2+) events have been observed in diverse stem cell lines, including carcinoma and mesenchymal stem cells. Interestingly, during cell cycle progression, cells exhibit Ca(2+) transients during the G(1) to S transition, suggesting that these oscillations may play a role in cell cycle progression. We aimed to study the influence of promoting and blocking calcium oscillations in cell proliferation and cell cycle progression, both in neural progenitor and undifferentiated cells. We also identified which calcium stores are required for maintaining these oscillations. Both in neural progenitor and undifferentiated cells calcium oscillations were restricted to the G1/S transition, suggesting a role for these events in progression of the cell cycle. Maintenance of the oscillations required calcium influx only through inositol 1,4,5-triphosphate receptors (IP(3)Rs) and L-type channels in undifferentiated cells, while neural progenitor cells also utilized ryanodine-sensitive stores. Interestingly, promoting calcium oscillations through IP(3)R agonists increased both proliferation and levels of cell cycle regulators such as cyclins A and E. Conversely, blocking calcium events with IP(3)R antagonists had the opposite effect in both undifferentiated and neural progenitor cells. This suggests that calcium events created by IP(3)Rs may be involved in cell cycle progression and proliferation, possibly due to regulation of cyclin levels, both in undifferentiated cells and in neural progenitor cells.

Expression of calcium transporters in the retina of the tiger salamander (Ambystoma tigrinum)

Changes in intracellular calcium concentration, [Ca2+]i, modulate the flow of visual signals across all stages of processing in the retina, yet the identities of Ca2+ transporters responsible for these changes are still largely unknown. In the current study, the distribution of plasma membrane and intracellular Ca2+ transporters in the retina of tiger salamander, a model system for physiological studies of retinal function, was determined. Plasma membrane calcium ATPases (PMCAs), responsible for high-affinity Ca2+ extrusion, were highly expressed in the salamander retina. PMCA isoforms 1, 2, and 4 were localized to photoreceptors, whereas the inner retina expressed all four isoforms. PMCA3 was expressed in a sparse population of amacrine and ganglion neurons, whereas PMCA2 was expressed in most amacrine and ganglion cells. Na+/Ca2+ exchangers, a high-capacity Ca2+ extrusion system, were expressed in the outer plexiform layer and in a subset of inner nuclear and ganglion layer cells. Intracellular Ca2+ store transporters were also represented prominently. SERCA2a, a splice variant of the sarcoplasmic-endoplasmic Ca2+ ATPase, was found mostly in photoreceptors, whereas SERCA2b was found in the majority of retinal neurons and in glial cells. The predominant endoplasmic reticulum (ER) Ca2+ channels in the salamander retina are represented by the isoform 2 of the IP3 receptor family and the isoform 2 of the ryanodine receptor family. These results indicate that Ca2+ transporters in the salamander retina are expressed in a cell type-specific manner.

Circadian rhythm and photic control of cAMP level in chick retinal cell cultures: a mechanism for coupling the circadian oscillator to the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, in photoreceptor cells

Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Intracellular Ca(2+) changes induced by in vitro ischemia in rat retinal slices

The effect of ischemia on intracellular Ca(2+)concentration [[Ca(2+)](i)] in retinal slices was investigated. [Ca(2+)](i)in each layer of the retina was determined from fluorescence images in rat retinal slices loaded with fura2-AM. Ischemic like conditions were imposed on the retinal slice in vitro by perfusion with an oxygen/glucose deprived solution. All measurements were taken at 25 degrees C except when temperature dependence was examined. In response to 100 m M K(+)or 0.2 m M glutamate under normoxic conditions, the [Ca(2+)](i)increase was higher in the inner retinal layers. Fifteen min ischemia evoked an increase in Ca(2+)levels in all layers of the retina, and the rates of increase were especially high in the outer/inner segment layer and the outer nuclear layer. Ischemia in the absence of extracellular Ca(2+)also induced a Ca(2+)rise, but at lower rates than with standard ischemia. Intermittent ischemia, composed of three 15 min bursts of ischemia at 10 min intervals, promoted the Ca(2+)rise. There was a more marked rise in [Ca(2+)](i)when the temperature was increased to 29 or 33 degrees C. Thus, in the rat retinal slice, in vitro ischemia evoked a more marked Ca(2+)rise in the outer retina, which was in contrast to the Ca(2+)responses to glutamate or high K(+). The rates of increase in [Ca(2+)](i)with ischemia were larger at higher temperatures, and intermittent ischemia also promoted the Ca(2+)rise. These increases appear to be derived from predominant influx of extracellular Ca(2+)rather than release of intracellular Ca(2+)stores.

Caffeine-sensitive calcium stores regulate synaptic transmission from retinal rod photoreceptors

We investigated the role of caffeine-sensitive intracellular stores in regulating intracellular calcium ([Ca(2+)](i)) and glutamatergic synaptic transmission from rod photoreceptors. Caffeine transiently elevated and then markedly depressed [Ca(2+)](i) to below prestimulus levels in rod inner segments and synaptic terminals. Concomitant with the depression was a reduction of glutamate release and a hyperpolarization of horizontal cells, neurons postsynaptic to rods. Caffeine did not affect the rods' membrane potentials indicating that caffeine likely acted via some mechanism(s) other than a voltage-dependent deactivation of the calcium channels. Most of caffeine's depressive action on [Ca(2+)](i), on glutamate release, and on I(Ca) in rods can be attributed to calcium release from stores: (1) caffeine's actions on [Ca(2+)](i) and I(Ca) were reduced by intracellular BAPTA and barium substitution for calcium, (2) other nonxanthine store-releasing compounds, such as thymol and chlorocresol, also depressed [Ca(2+)](i), and (3) the magnitude of [Ca(2+)](i) depression depended on basal [Ca(2+)](i) before caffeine. We propose that caffeine-released calcium reduces I(Ca) in rods by an as yet unidentified intracellular signaling mechanism. To account for the depression of [Ca(2+)](i) below rest levels and the increased fall rate of [Ca(2+)](i) with higher basal calcium, we also propose that caffeine-evoked calcium release from stores activates a calcium transporter that, via sequestration into stores or extrusion, lowers [Ca(2+)](i) and suppresses glutamate release. The effects of store-released calcium reported here operate at physiological calcium concentrations, supporting a role in regulating synaptic signaling in vivo.

Inhibition of carbachol-induced inositol phosphate accumulation in the embryonic retina promoted by kainate and veratridine

In the present study, we report that low concentrations of the glutamate ionotropic agonist kainate decreased the turnover of [3H]-phosphoinositides ([3H]-InsPs) induced by muscarinic receptors in the chick embryonic retina. When 100 microM carbachol was used, the estimated IC50 value for kainate was 0.2 microM and the maximal inhibition of approximately 50% was obtained with 1 microM or higher concentrations of the glutamatergic agonist. Our data also show that veratridine, a neurotoxin that increases the permeability of voltage-sensitive sodium channels, had no effect on [3H]-InsPs levels of the embryonic retina. However, 50 microM veratridine, but not 50 mM KCl, inhibited approximately 65% of the retinal response to carbachol. While carbachol increased [3H]-InsPs levels from 241.2 +/- 38.0 to 2044.5 +/- 299.9 cpm/mg protein, retinal response decreased to 861.6 +/- 113.9 cpm/mg protein when tissues were incubated with carbachol plus veratridine. These results suggest that the accumulation of phosphoinositides induced by activation of muscarinic receptors can be inhibited by the influx of Na+ ions triggered by activation of kainate receptors or opening of voltage-sensitive sodium channels in the chick embryonic retina.