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

Protein kinase A regulation of pigment granule motility in retinal pigment epithelial cells from fish, Lepomis spp

Retinomotor movements include elongation and contraction of rod and cone photoreceptors, and mass migration of melanin-containing pigment granules (melanosomes) of the retinal pigment epithelium (RPE) within the eyes of fish, frogs, and other lower vertebrates. Eyes of these animals do not contain dilatable pupils; therefore the repositioning of the rods and cones and a moveable curtain of pigment granules serve to modulate light intensity within the eye. RPE from sunfish (Lepomis spp.) can be isolated from the eye and dissociated into single cells, allowing in vitro studies of the cytoskeletal and regulatory mechanisms of organelle movement. Pigment granule aggregation from distal tips of apical projections into the cell body can be triggered by the application of underivatized cAMP, and dispersion is effected by cAMP washout in the presence of dopamine. While the phenomenon of cAMP-dependent pigment granule aggregation in isolated RPE was described many years ago, whether cAMP acts through the canonical cAMP-PKA pathway to stimulate motility has never been demonstrated. Here, we show that pharmacological inhibition of PKA blocks pigment granule aggregation, and microinjection of protein kinase A catalytic subunit triggers pigment granule aggregation. Treatment with a cAMP agonist that activates the Rap GEF, Epac (Effector protein activated by cAMP), had no effect on pigment granule position. Taken together, these results confirm that cAMP activates RPE pigment granule motility by the canonical cAMP-PKA pathway. Isolated RPE cells labeled with antibodies against PKA RIIα and against PKA-phosphorylated serine/threonine amino acids show diffuse, punctate labeling throughout the RPE cell body and apical projections. Immunoblotting of RPE lysates using the anti-PKA substrate antibody demonstrated seven prominent bands; two bands in particular at 27 and 64 kD showed increased levels of phosphorylation in the presence of cAMP, indicating their phosphorylation could contribute to the pigment granule aggregation mechanism.

Effect of metabolic and respiratory acidosis on intracellular calcium in osteoblasts

In vivo, metabolic acidosis {decreased pH from decreased bicarbonate concentration ([HCO(3)(-)])} increases urine calcium (Ca) without increased intestinal Ca absorption, resulting in a loss of bone Ca. Conversely, respiratory acidosis [decreased pH from increased partial pressure of carbon dioxide (Pco(2))] does not appreciably alter Ca homeostasis. In cultured bone, chronic metabolic acidosis (Met) significantly increases cell-mediated net Ca efflux while isohydric respiratory acidosis (Resp) does not. The proton receptor, OGR1, appears critical for cell-mediated, metabolic acid-induced bone resorption. Perfusion of primary bone cells or OGR1-transfected Chinese hamster ovary (CHO) cells with Met induces transient peaks of intracellular Ca (Ca(i)). To determine whether Resp increases Ca(i), as does Met, we imaged Ca(i) in primary cultures of bone cells. pH for Met = 7.07 ([HCO(3)(-)] = 11.8 mM) and for Resp = 7.13 (Pco(2) = 88.4 mmHg) were similar and lower than neutral (7.41). Both Met and Resp induced a marked, transient increase in Ca(i) in individual bone cells; however, Met stimulated Ca(i) to a greater extent than Resp. We used OGR1-transfected CHO cells to determine whether OGR1 was responsible for the greater increase in Ca(i) in Met than Resp. Both Met and Resp induced a marked, transient increase in Ca(i) in OGR1-transfected CHO cells; however, in these cells Met was not different than Resp. Thus, the greater induction of Ca(i) by Met in primary bone cells is not a function of OGR1 alone, but must involve H(+) receptors other than OGR1, or pathways sensitive to Pco(2), HCO(3)(-), or total CO(2) that modify the effect of H(+) in primary bone cells.

Acetylcholine induces Ca2+ signaling in chicken retinal pigmented epithelial cells during dedifferentiation

Retinal pigmented epithelial cells exchange their cellular phenotypes into lens cells and neurons, via depigmented and non-epithelial-shaped dedifferentiated intermediates. Because these dedifferentiated cells can either revert to pigmented epithelial cells or transdifferentiate into lens cells and/or neurons, they are recognized as candidates for lens and retinal cell regeneration. The purpose of the present study was to elucidate the signal transduction pathways between chicken retinal pigmented epithelial cells and their dedifferentiated intermediates. We monitored intracellular Ca(2+) concentrations using Fluo-4-based Ca(2+) optical imaging and focused on cellular responses to the neurotransmitter acetylcholine. Muscarinic Ca(2+) mobilization was observed both in retinal pigmented epithelial cells and in dedifferentiated cells, and was inhibited by atropine. The muscarine-dependent acetylcholine response depended on Ca(2+) release from intracellular Ca(2+) stores, which was completely blocked by thapsigargin. In contrast, the nicotine-dependent acetylcholine response that led to Ca(2+) influx through L-type Ca(2+) channels was inhibited by alpha-bungarotoxin and attenuated by nifedipine, and it was detected only in the dedifferentiated intermediates. Application of (S)-(-)-BayK8644 elevated intracellular Ca(2+) both in retinal pigmented epithelial cells and in dedifferentiated intermediates; however, the nicotinic response was not observed in pigmented epithelial cells. Another L-type Ca(2+) channel blocker, diltiazem, also blocked the nicotine-dependent acetylcholine response in dedifferentiated cells and maintained the epithelial-like morphology of retinal pigmented epithelial cells. Our results indicate that an alternative acetylcholine signaling pathway is used during the dedifferentiation process of retinal pigmented epithelial cells.

Carbachol-mediated pigment granule dispersion in retinal pigment epithelium requires Ca2+ and calcineurin

Background

Inside bluegill (Lepomis macrochirus) retinal pigment epithelial cells, pigment granules move in response to extracellular signals. During the process of aggregation, pigment motility is directed toward the cell nucleus; in dispersion, pigment is directed away from the nucleus and into long apical processes. A number of different chemicals have been found to initiate dispersion, and carbachol (an acetylcholine analog) is one example. Previous research indicates that the carbachol-receptor interaction activates a G_q-mediated pathway which is commonly linked to Ca²⁺ mobilization. The purpose of the present study was to test for involvement of calcium and to probe calcium-dependent mediators to reveal their role in carbachol-mediated dispersion.

Results

Carbachol-induced pigment granule dispersion was blocked by the calcium chelator BAPTA. In contrast, the calcium channel antagonist verapamil, and incubation in Ca²⁺-free medium failed to block carbachol-induced dispersion. The calcineurin inhibitor cypermethrin blocked carbachol-induced dispersion; whereas, two protein kinase C inhibitors (staurosporine and bisindolylmaleimide II) failed to block carbachol-induced dispersion, and the protein kinase C activator phorbol 12-myristate 13-acetate failed to elicit dispersion.

Conclusion

A rise in intracellular calcium is necessary for carbachol-induced dispersion; however, the Ca²⁺ requirement is not dependent on extracellular sources, implying that intracellular stores are sufficient to enable pigment granule dispersion to occur. Calcineurin is a likely Ca²⁺-dependent mediator involved in the signal cascade. Although the pathway leads to the generation of diacylglycerol and calcium (both required for the activation of certain PKC isoforms), our evidence does not support a significant role for PKC.

Conservation of retinal circadian rhythms during cavefish eye degeneration

Regressive evolution of morphological features is a common evolutionary event. However, the relationship between structural degeneration and loss of physiological function is often unclear because the ancestral and derived states of a character are usually not available for comparison. Here, we report studies on retinomotor rhythms during development of the blind cavefish Astyanax mexicanus, a single teleost species consisting of a sighted surface-dwelling form (surface fish) and several blind cave-dwelling (cavefish) forms. The eyed and blind forms of Astyanax diverged from a common sighted ancestor within the past million years. Despite the absence of functional eyes in cavefish adults, optic primordia are formed in embryos, but then gradually arrest in development, degenerate, and sink into the orbits. Although a layered retina is formed in cavefish embryos, it is deficient in photoreceptor cells, and in some cases the retinal pigment epithelium has lost its pigmentation. We show that the capacity to exhibit light-entrained retinomotor rhythms has been conserved in the degenerating embryonic eyes of two different Astyanax cavefish populations. The results indicate that loss of circadian retinal function does not precede and is therefore not required for eye degeneration in the blind cavefish.

Molecular and pharmacological characterization of muscarinic receptors in retinal pigment epithelium: role in light-adaptive pigment movements

Muscarinic receptors are the predominant cholinergic receptors in the central and peripheral nervous systems. Recently, activation of muscarinic receptors was found to elicit pigment granule dispersion in retinal pigment epithelium isolated from bluegill fish. Pigment granule movement in retinal pigment epithelium is a light-adaptive mechanism in fish. In the present study, we used pharmacological and molecular approaches to identify the muscarinic receptor subtype and the intracellular signaling pathway involved in the pigment granule dispersion in retinal pigment epithelium. Of the muscarinic receptor subtype-specific antagonists used, only antagonists specific for M1 and M3 muscarinic receptors were found to block carbamyl choline (carbachol)-induced pigment granule dispersion. A phospholipase C inhibitor also blocked carbachol-induced pigment granule dispersion, and a similar result was obtained when retinal pigment epithelium was incubated with an inositol trisphosphate receptor inhibitor. We isolated M2 and M5 receptor genes from bluegill and studied their expression. Only M5 was found to be expressed in retinal pigment epithelium. Taken together, pharmacological and molecular evidence suggest that activation of an odd subtype of muscarinic receptor, possibly M5, on fish retinal pigment epithelium induces pigment granule dispersion.

Activation of muscarinic acetylcholine receptors elicits pigment granule dispersion in retinal pigment epithelium isolated from bluegill

BACKGROUND

In fish, melanin pigment granules in the retinal pigment epithelium disperse into apical projections as part of the suite of responses the eye makes to bright light conditions. This pigment granule dispersion serves to reduce photobleaching and occurs in response to neurochemicals secreted by the retina. Previous work has shown that acetylcholine may be involved in inducing light-adaptive pigment dispersion. Acetylcholine receptors are of two main types, nicotinic and muscarinic. Muscarinic receptors are in the G-protein coupled receptor superfamily, and five different muscarinic receptors have been molecularly cloned in human. These receptors are coupled to adenylyl cyclase, calcium mobilization and ion channel activation. To determine the receptor pathway involved in eliciting pigment granule migration, we isolated retinal pigment epithelium from bluegill and subjected it to a battery of cholinergic agents.

RESULTS

The general cholinergic agonist carbachol induces pigment granule dispersion in isolated retinal pigment epithelium. Carbachol-induced pigment granule dispersion is blocked by the muscarinic antagonist atropine, by the M1 antagonist pirenzepine, and by the M3 antagonist 4-DAMP. Pigment granule dispersion was also induced by the M1 agonist 4-[N-(4-chlorophenyl) carbamoyloxy]-4-pent-2-ammonium iodide. In contrast the M2 antagonist AF-DX 116 and the M4 antagonist tropicamide failed to block carbachol-induced dispersion, and the M2 agonist arecaidine but-2-ynyl ester tosylate failed to elicit dispersion.

CONCLUSIONS

Our results suggest that carbachol-mediated pigment granule dispersion occurs through the activation of Modd muscarinic receptors, which in other systems couple to phosphoinositide hydrolysis and elevation of intracellular calcium. This conclusion must be corroborated by molecular studies, but suggests Ca2+-dependent pathways may be involved in light-adaptive pigment dispersion.

Elevated free calcium levels in the subretinal space elevate the absolute dark-adapted threshold in hypopigmented mice

Abundant evidence spanning 25 years demonstrates that hypopigmentation is associated with sensory abnormalities manifested most clearly as elevated absolute dark-adapted thresholds in hypopigmented mice. Here we show that when ocular melanin is increased in the himalayan mouse via alpha-melanocyte stimulating hormone (alpha-MSH) injections, dark-adapted thresholds drop in proportion to the change in ocular melanin. We further measured free calcium concentration with calcium-sensitive microelectrodes in both albino and black mouse retinal eyecups in living subjects. The recordings were done in anesthetized animals as the defect is not present in isolated retinas or in the superfused eye preparation. A double-barreled electrode--pCa and Vref--was used to simultaneously record the calcium concentration and the electroretinogram (ERG) at each of many depths as the electrode was driven through the retina. The position of the electrode was confirmed with ERG and 1,1'-dioctadecyl-3, 3,3',3'-tetramethylindocarbocyanine perchlorate electrode tract reconstruction. Dark-adapted albinos (n = 6) had 1.4 +/- 0.015 mM calcium in the subretinal space compared with 0.80 +/- 0.025 mM in black mice (n = 6). The results of these experiments are consistent with the hypothesis that ocular hypopigmentation causes elevated calcium levels in the subretinal space that in turn mimic light adaptation in hypopigmented mice.

Bidirectional pigment granule migration in isolated retinal pigment epithelial cells requires actin but not microtubules

In the teleost retinal pigment epithelium (RPE), melanin pigment granules disperse into long apical projections in the light and reaggregate into the cell body in the dark. To investigate the cytoskeletal mechanisms responsible for these movements, we have examined the effects of cytoskeletal inhibitors on pigment granule transport in cultured, dissociated RPE cells using time-lapse video microscopy. The kinetics of pigment granule transport during normal aggregation and dispersion are quite distinct: during aggregation, all pigment granules undergo simultaneous, nonsaltatory centripetal movement (mean velocity 3.6 microm/min); during dispersion, individual granules undergo independent, bidirectional saltatations (mean velocities 3.7 microm/min centrifugal; 1.1 microm/min centripetal). Nocodazole disruption of microtubules within the RPE apical projections had little effect on the kinetics of pigment granule movement, and essentially no effect on extent of pigment granule aggregation or dispersion, or on maintenance of the fully aggregated or fully dispersed states. In contrast, cytochalasin D (CD) treatment blocked net aggregation and dispersion of pigment granules, and compromised maintenance of the fully aggregated and dispersed states. These observations suggest that the actin cytoskeleton plays an important role in both centripetal and centrifugal transport of pigment granules in teleost RPE cells.

Molecular mechanisms of pigment transport in melanophores

We present an overview of the research on intracellular transport in pigment cells, with emphasis on the most recent discoveries. Pigment cells of lower vertebrates have been traditionally used as a model for studies of intracellular transport mechanisms, because these cells transport pigment organelles to the center or to the periphery of the cell in a highly co-ordinated fashion. It is now well established that both aggregation and dispersion of pigment in melanophores require two elements of the cytoskeleton: microtubules and actin filaments. Melanosomes are moved along these cytoskeletal tracks by motor proteins. Recent studies have identified the motors responsible for pigment dispersion and aggregation in melanophores. We propose a model for the possible roles of the two cytoskeletal transport systems and how they might interact. We also discuss the putative mechanisms of regulation of pigment transport, especially phosphorylation. Last, we suggest areas of research that will receive attention in the future in order to elucidate the mechanisms of organelle transport.