Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: differential sensitivity to extracellular calcium

The effect of the nonspecific nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester on the choroidal compensatory response to myopic defocus in chickens

PURPOSE

Chick eyes show rapid compensation to retinal defocus. One component of this mechanism involves changes in the thickness of the choroid: when the retina is exposed to myopic defocus, the choroid thickens, pushing the retina forward; conversely, when the eye is exposed to hyperopic defocus, the choroid thins. The underlying mechanism(s) for these changes are unknown. We tested the hypothesis that nitric oxide might play a role.

METHODS

We examined the effect of the nonspecific nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the compensatory choroidal thickening in response to myopic defocus using two visual paradigms: first, in previously form-deprived "recovering" eyes and, second, in eyes wearing +15 D spectacle lenses. L-NAME was injected intravitreally after removal of the diffuser or immediately before putting on the lenses. In addition, we looked at the effect of L-NAME on experimentally thickened choroids (induced by 1 week of recovery from deprivation myopia or 1 week of +15 D lens wear) and on choroids of normal eyes. Eyes were measured using A-scan ultrasonography before the injections and at subsequent intervals for several days. As a control for the injection procedure, eyes with the same visual conditions were injected with saline. Fellow eyes were untreated and uninjected.

RESULTS

L-NAME inhibited choroidal thickening in both previously form-deprived eyes (2 vs. 117 microm; p < 0.001) and eyes wearing +15 D lenses (3 vs. 137 microm; p < 0.02). The effect was rapid, transient, and dose dependent (ED50, 0.26 micromoles). L-NAME produced thinning in experimentally thickened choroids (recovering: -116 microm; lenses: -219 microm) and in normal choroids (-47 microm) within 7 hours.

CONCLUSIONS

Nitric oxide may play a role in modulating choroidal thickness. The mechanism is as yet unknown.

Cellular Deflagellation

Deciliation, also known as deflagellation, flagellar autotomy, flagellar excision, or flagellar shedding, refers to the process whereby eukaryotic cells shed their cilia or flagella, often in response to stress. Used for many decades as a tool for scientists interested in the structure, function, and genesis of cilia, deciliation itself is a process worthy of scientific investigation. Deciliation has numerous direct medical implications, but more profoundly, intriguing relationships between deciliation, ciliogenesis, and the cell cycle indicate that understanding the mechanism of deciliation will contribute to a deeper understanding of broad aspects of cell biology. This review provides a critical examination of diverse data bearing on this problem. It also highlights current deficiencies in our understanding of the mechanism of deciliation.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

Excitatory amino acid receptors in membranes from cultured human retinal pigment epithelium

The presence of specific, saturable receptor sites for excitatory amino acids (EAA) in membranes from cultured human retinal pigment epithelium (RPE) was established through the binding of [3H]L-glutamate (L-Glu). The age of the donors ranged from 6 days to 33 years. The affinity of the binding (KB) sites was between 1.2 and 1.5 microM, and did not change with the age of the donor, whereas the Bmax was slightly increased (8.6 to 13.0 pmol/mg) in membranes from the 33 year-old compared to the 29 day-old donor. The efficacy profile of agonists and antagonists acting at EAA receptors for displacing [3H]L-Glu was L-Glu = L-Aspartate > 2-amino-4-phosphonovalerate (AP5) > N-methyl-D-Aspartate (NMDA) > 1-aminocyclopentane-1,3 dicarboxylate (trans-ACPD) > 2-amino-3-phosphonopropionate (AP3). These data suggest the presence of either an NMDA-receptor sensitive to the metabotropic agonist trans-ACPD or alternatively, the presence of two different populations of receptors with similar affinity for the agonist: NMDA and metabotropic. Glycine highly stimulated Glu-binding; this effect was inversely related to the age of the donor. Taurine and to a lesser extent GABA, mimicked this effect. Stimulation by glycine was dose-dependent, insensitive to strychnine and 80% inhibited by 7-chlorokynurenate. This effect was also present in human RPE-derived fibroblasts, human scleral fibroblasts and the human lymphoblastoid cell line NB76, all continuously dividing cells. The results further support the possibility of the participation of EAA receptors in the regulation of phagocytosis in RPE.

Specific interaction of glutamate with membranes from cultured retinal pigment epithelium

Excitatory amino acids (EAA) have been shown to induce phagocytosis in retinal pigment epithelial (RPE) cells. In order to explore if this action is receptor-mediated, we have identified and characterized receptors for L-glutamate through the binding of [3H]L-glutamate to membranes from chick RPE cells in primary culture. Specific binding was found saturable, with KB = 333nM and Bmax = 3.2 pmol/mg protein in frozen/thawed membranes. Na(+)-independent binding was present in cultures of 16 and 25 days in vitro, and was not affected by temperature. Pharmacological profile of analogues of EAA at different receptor types suggests the presence of a metabotropic type receptor (L-glutamate > S-2-amino-3-phosphonopropionate > 2-amino-4-phosphonobutyrate = trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylate > quisqualate). Excitatory amino acid analogues acting at the NMDA-receptor also displaced bound L-glutamate, and a noticeable stimulation of specific binding of this ligand by glycine was shown; this effect was mimicked by D-serine and 1-hydroxy-3-aminopyrrolidone-2 (HA-966) but not by 7-chlorokynurenate, and was not inhibited by strychnine. Since taurine and GABA also increased specific binding, it is likely that modulation of EAA receptors in RPE differs from that in neurons.

Glutamate and taurine uptake by retinal pigment epithelium during rat development

1. The rat retinal pigment epithelium accumulated glutamate and taurine by saturable, temperature and Na(+)-dependent mechanisms. 2. Glutamate and taurine showed high and low affinity transport systems, with a Km of 30 microM and 80 microM, respectively. 3. The transport rates of both amino acids decreased during maturation of retinal pigment epithelial cells while their kinetic characteristics were not modified. 4. The results suggest an involvement of the retinal pigment epithelium in the regulation of glutamate and taurine levels in the neural retina and support its role as part of the blood retinal barrier.

Photomembrane turnover in frog retina: light intensity and spectral correlates

Light regulates membrane turnover in vertebrate rod photoreceptor cells. Rods shed membrane-filled tips immediately after light onset, with light inhibiting the dark priming phase but initiating the light induction phase. This study examines the intensities and wavelengths of light that control these two shedding requirements, and demonstrates unexpected situations where red or dim lights are simultaneously dark to the dark priming mechanism and light to the light induction process. Since shedding takes place immediately following darkness we asked if dim or red light could substitute for true darkness and dark prime the retinas: our results confirm this. White light, less than 0.7 microE m m-2 sec-1 (0.15 W m2 or 40 lx), allows dark priming, and even 15 microE m-2 sec-1 of red fluorescent light dark primes as effectively as true darkness. Conversely, bright white light and wavelengths from 480 to 560 nm inhibit dark priming, implying that dark priming inhibition is a photopic mechanism transduced by photopigment in the 502-cone. We also asked if dim or red light could induce shedding, substituting for the bright light usually employed: again, the results confirm thus. White light as dim as 0.15 microE m-2 sec-1 induces shedding and red light is an effective light trigger. This light induction is initiated at all wavelengths tested (420-640 nm), with a maximum effect between 540 and 600 nm. Finally, we find that retinas shed continuously in red or dim white light. These lights substitute both for the darkness necessary for dark priming and for the light of light induction, extending shedding from the 20 min dark-light transition period to hours or days. We also find that the dim, red light of natural dawn is as effective a shedding stimulus as the sudden onset of bright laboratory light.

Reconstitution of the photoreceptor-pigment epithelium interface: l-glutamate stimulation of adhesive interactions and rod disc shedding after recombination of dissociated Xenopus laevis eyecups

In order to investigate adhesive interactions between photoreceptor and pigment epithelial cells, we have mechanically separated neural retinas from Xenopus laevis eyecups and then recombined the tissues in vitro. When tissue pairs are incubated in a defined medium, cell-cell contact is achieved within 3 hr. However, the average proportion of reassembled eyecups in which photoreceptor outer segments interdigitate with epithelial microvilli is limited. Furthermore, rod disc shedding does not take place in these cultures, even following a dark to light transition. When recombined tissues are placed in medium supplemented with 12 mM L-glutamate, retinal reattachment is enhanced and there is a four-fold increase in epithelial phagosome content. The positive effect of excitatory amino acid exposure on shedding, however, is restricted to regions where visual and epithelial cells interdigitate. These results indicate that re-establishment of cell contact may be necessary for shedding of apical disc membranes prior to their engulfment by the epithelium. While reattachment is not affected by pre-incubation of separated tissues in normal medium, rod photoreceptors fail to undergo membrane turnover in response to L-glutamate if a delay of 1 hr or more is interposed between isolation of the retina and its recombination with the pigment epithelium. This is probably due to a decline in retinal function in culture, since a similar preincubation of the pigment epithelium prior to reassembly with a freshly isolated retina does not inhibit the shedding response.(ABSTRACT TRUNCATED AT 250 WORDS)

Zymosan induced 45Ca uptake by retinal pigment epithelial cells

45Ca uptake was studied in isolated frog retinal pigment epithelial cells in response to the phagocytic stimuli, zymosan. 45Ca uptake was strongly stimulated immediately in the presence of zymosan particles. Calcium uptake was proportional to the zymosan concentrations. After 60 min in the presence of zymosan acid phosphatase and beta-glucuronidase activities showed a 25% and 50% increase, respectively. Rod outer segments induced a similar increase of these enzyme activities. The zymosan-induced lysosomal enzyme activities was inhibited by cytochalasin B and ruthenium red. The ionophore A23187 produced a remarkable increase in 45Ca uptake but did not affect the lysosomal enzyme activities. These results suggest that in vitro RPE cells are able to respond to zymosan as phagocytosable stimuli and that calcium mediate that response.

Increased levels of leukotriene C4 in retinal pigment epithelium are correlated with early events in photoreceptor shedding in Xenopus laevis

The levels of 5-lipoxygenase products of arachidonic acid, leukotriene (LT) B4 and LTC4 in retinal pigment epithelia (RPE) from Xenopus laevis were measured by radioimmunoassay (RIA). RPE were isolated during various stages of photoreceptor renewal to determine possible alterations in 5-lipoxygenase activity concurrent with photoreceptor detachment and phagocytosis. Both LTC4 and LTB4 were released to RPE incubation media, although levels of LTB4 in unstimulated RPE were close to the limits of detection by RIA. Incubation of RPE with the calcium ionophore A23187 increased the levels of both LTB4 and LTC4. When animals were maintained on a cycle of 12 hr light/dark, normal photoreceptor shedding, as measured by histological quantitation of the appearance of phagosomes in the RPE, occurred 1 hr after light onset. Levels of LTC4 in RPE were lower 1 hr after light onset, as compared to 1 hr prior to light onset. Due to the low levels of LTB4, no significant differences could be detected. However, when LTB4 levels were elevated with A23187, LTB4 also declined 1 hr after light onset. When animals were maintained in constant light for 5 days, then exposed to 2 hr dark and 2 hr light, a massive shedding response occurred. Levels of LTC4 were stimulated 5 min after light onset (prior to detectable shedding) and declined below dark levels as shedding progressed. These data suggest a correlation between 5-lipoxygenase activity and the events of photoreceptor shedding and phagocytosis.

Cytochalasin D inhibits L-glutamate-induced disc shedding without altering L-glutamate-induced increase in adhesiveness

Excitatory amino acid-stimulated disc shedding is correlated with the appearance of microfilament-rich ensheathing processes of the retinal pigment epithelium (RPE) and increased apparent adhesiveness between photoreceptors and RPE in explanted eyecups of Xenopus laevis. We have compared the time course of disc shedding and increased retinal adherence during L-glutamate treatment. Increased adherence was measured on the basis of the tendency of the apical RPE domains to partition with isolated neural retinas. In medium supplemented with L-glutamate (12 mM) or kainate (100 microM), a glutamate analog, the time course of increased partitioning of melanin pigment-rich cell fragments which contain ensheathing processes differs, even though the kinetics of induced disc shedding is the same in either case. Co-treatment with cytochalasin D (5 microM) completely blocks L-glutamate-induced disc shedding, as well as formation of microfilament-rich ensheathing processes, even though it has little effect upon apparent adhesiveness. The virtually complete dissociation of the effects of L-glutamate on disc shedding from that on increased adhesiveness of photoreceptors to RPE suggests that increased retinal adherence and pseudopod formation may be unrelated causally.

Excitatory amino acids and rod photoreceptor disc shedding: analysis using specific agonists

L-Glutamate and L-aspartate stimulate photoreceptor disc shedding. In order to evaluate the possible involvement of a receptor, we examined the effects of specific excitatory amino acid agonists on rod photoreceptor disc shedding and neural retinal toxicity. Using eyecups from both Xenopus laevis and Rana pipiens, we found that kainate, quisqualate, and N-methyl-D-aspartate (NMDA) were all neurotoxic, but that kainate caused a more extensive inner retinal lesion. Kainate also caused disc shedding at concentrations as low as 10 microM; dihydrokainate, a structural analogue, was at least 100-fold less potent. In contrast, quisqualate induced disc shedding only at concentrations above 5.0 mM, and NMDA had no effect on disc shedding at any concentration examined. Our results suggest that excitatory amino acids act via a receptor of the kainate type to effect disc shedding. The mechanism in the retina or photoreceptor-pigment epithelial complex by which an excitatory amino acid receptor system influences disc shedding remains to be identified.

Effects of kynurenate and other excitatory amino acid antagonists as blockers of light- and kainate-induced retinal rod photoreceptor disc shedding

Photoreceptor disc shedding in the retina involves detachment of discs from distal outer segments and phagocytosis of those discs by adjacent pigment epithelial cells. The disc-shedding process balances the continuous renewal of photosensitive membrane. In amphibians, rod disc shedding normally is light-stimulated. However, excitatory amino acids such as kainate stimulate disc shedding independent of a dark-light transition. Excitatory amino acid-induced disc shedding is accompanied by toxic changes within the retina. To evaluate the possible role of an endogenous excitatory amino acid in the regulation of light-evoked disc shedding, we examined the effects of excitatory amino acid antagonists on kainate-induced and light-evoked disc shedding and on retinal toxicity. Using eyecups from Rana pipiens, we found that kynurenate, D-O-phosphoserine, and cis-2,3-piperidine dicarboxylic acid (cis-PDA) all block both the neurotoxic and disc-shedding effects of kainate. Kynurenate and D-O-phosphoserine, but not cis-PDA, also block light-evoked disc shedding. Our analysis suggests that kynurenate blocks the mechanism by which light "triggers" disc shedding rather than directly inhibiting disc detachment and phagocytosis. The observation that cis-PDA antagonizes the effects of kainate, but not light, suggests that the receptor mediating the kainate effect on disc shedding may not be involved in the normal initiation of the response by light. In contrast, our data on kynurenate suggest that it antagonizes an endogenous mechanism involved in the normal control of disc shedding. Thus, analysis of the differences between cis-PDA and kynurenate as antagonists in the retina may yield important insight into the mechanism by which light initiates disc shedding.