Apoptotic Photoreceptor Cell Death Research Articles

Apoptotic Photoreceptor Death in the Rhodopsin Knockout Mouse in the Presence and Absence of c-fos

A combined total of approximately 100 mutations have been encountered within the rhodopsin gene in retinitis pigmentosa (RP) and congenital night blindness. Mice carrying a targeted disruption of the rhodopsin gene phenotypically mimic RP, losing their photoreceptors over a period of 3 months and having no recordable rod electroretinogram. These animals will serve as a model for both recessive and dominant disease (in the latter case, the presence of normal and mutant human rod opsin transgenes on the murine Rho−/−background). Precise knowledge of apoptotic photoreceptor cell death, together with factors which may influence apoptosis will be required for optimum utility of Rho−/−mice as a model for therapeutic genetic intervention. A peak phase of apoptosis of the photoreceptors of Rho−/−mice was shown to occur at 24 days post-birth. The extent of apoptosis appeared to be similar, irrespective of whether or not the rod opsin knockout was present on a c-fos+/+orc-fos−/− genetic background, the latter known to favor survival of photoreceptors following exposure of mouse retinas to excessive light. These data clearly support the existence in animals of distinct apoptotic pathways in light-induced, as opposed to mutation-induced apoptosis, and together with similar observations recently reported in studies of the naturally occurring rd mouse, may assist in focusing future research on precisely defining the distinct molecular pathways giving rise to such dichotomy.

Genetic Evidence for Selective Transport of Opsin and Arrestin by Kinesin-II in Mammalian Photoreceptors

To test whether kinesin-II is important for transport in the mammalian photoreceptor cilium, and to identify its potential cargoes, we used Cre-loxP mutagenesis to remove the kinesin-II subunit, KIF3A, specifically from photoreceptors. Complete loss of KIF3A caused large accumulations of opsin, arrestin, and membranes within the photoreceptor inner segment, while the localization of α-transducin was unaffected. Other membrane, organelle, and transport markers, as well as opsin processing appeared normal. Loss of KIF3A ultimately caused apoptotic photoreceptor cell death similar to a known opsin transport mutant. The data suggest that kinesin-II is required to transport opsin and arrestin from the inner to the outer segment and that blocks in this transport pathway lead to photoreceptor cell death as found in retinitis pigmentosa.

Tapetoretinal degenerations: experiences, experiments and expectations.

Tapetoretinal degenerations are a common cause for vision problems, but have until recently not been amenable to rational treatment. With rapidly increasing insights into basic neurobiology and pathobiology this has now begun to change. From having been a relatively small group of largely unknown yet fairly prevalent disorders, they are rapidly forming a large set of well defined diseases, and it is easy to predict that our knowledge about them will continue to increase for many years to come. Vitamin A (15,000 IU daily) is currently the only rational treatment available. However, in experimental animals, therapy strategies are now actively being developed along several different lines. Apoptotic photoreceptor cell death can be delayed with different drugs, and at least one of them, diltiazem, is approved for human use in cardiovascular diseases. It remains to be seen if it has any clinically significant effect in human tapetoretinal degenerations. Other strategies aim at counteracting the production of harmful protein variants, acting either on DNA or mRNA levels. Transgenes can also be used to induce the production of important but missing metabolic components. Finally, cells or retina sheets can be transplanted, either to replace failing cells or as a source for missing trophic factors. Neither of these strategies has yet been transferred to humans, but trials are under way. With the high increase in the flow of new information on tapetoretinal disorders, much more precise diagnoses and much improved treatments are soon to be expected, augmenting considerably the possibilities for ophthalmologists to help patients with such diseases. It is not likely that there will be a single treatment for all the many varieties. Instead, we are most likely going to see pharmacological treatments for some of them, DNA transfers for some, and transplantations for others.

Gene therapy for ocular disease.

Gene therapy for ocular disease.

Therapeutic strategies in RP (retinitis pigmentosa): light at the end of the tunnel?

Retinitis pigmentosa (RP) is a hereditary retinal dystrophy which leads to severe visual impairment or blindness and affects about 3.5/1000 of individuals in the industrial world. During the past decades, numerous animal models carrying mutations analogous to mutations in human RP have been studied to elucidate the molecular mechanisms leading to apoptotic photoreceptor cell death in this disease. Up to date, there is no effective treatment to influence the fatal outcome of RP. Recent progress in basic research promotes the development of new therapeutic strategies. In order to restore visual function in blind individuals, the development of electronic photoreceptor prosthesis is being investigated by several researchgroups. Other promising approaches are somatic gene therapy, the application of growth factors and/or pharmacological agents and the inhibition of photoreceptor cell death by interfering with the apoptotic pathway. However, a better understanding of the molecular events leading to cell loss due to photoreceptor apoptosis will be essential for the development of effective treatment.

Anti-recoverin antibodies cause the apoptotic death of mammalian photoreceptor cells in vitro

We developed an in vitro model to study the effect of anti-recoverin antibodies on retinal cells and the mechanism(s) by which they kill photoreceptors in cancer-associated retinopathy (CAR). Rat retinal cells were grown in a defined medium, and cell types were identified by using antibodies against rhodopsin, recoverin, syntaxin, and thy-1. Purified immunoglobulin (IgG) against recoverin was added to the cultures at different concentrations for 24, 48, or 72 hr, and the survival of the cells was determined by fluorescence microscopy. Preimmune IgG and normal medium were used as controls. The cell death detection enzyme-linked immunosorbent assay and the terminal deoxyuridine triphosphate nick-end labeling assay were used to demonstrate cells undergoing apoptosis. Double labeling was used to visualize cell types and apoptotic death. Rods, amacrine cells, and ganglion cells were identified in the cultures. Rod cells, but not ganglion cells and amacrine cells, markedly decreased in the presence of 200 microg/ml of anti-recoverin IgG for 24, 48, and 72 hr. Anti-recoverin antibodies caused apoptosis in rod cells but not in amacrine cells. Almost all cells were shown to take up IgG from the medium. In conclusion, our retinal cell cultures provide a system for investigating antibody-mediated photoreceptor cell death and demonstrate that anti-recoverin antibodies cause the apoptotic death of rod cells, with no effect on amacrine cells. The results suggest that anti-recoverin antibodies play a key role in the apoptotic death of photoreceptors in CAR.

Retinal Cell Death by Light Damage

Purpose: To determine the relationship between apoptotic photoreceptor cell death and the duration of light exposure. Methods: Ten-week-old male albino rats (Wistar strain) were dark-adapted for 2 days and then exposed to intense light for 12 hours, and 1, 2, 3, 7, 14, 21, and 28 days. The presence of apoptosis was confirmed by electron microscopy and the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) method. Differences in the apoptotic processes of the photoreceptor cells in the superior, posterior pole, and inferior portions of the retina were determined. Results: Photoreceptor cells showed TUNEL-positive staining, whereas the cells in the inner nuclear layer, ganglion cell layer and retinal pigment epithelia exhibited weak positive or negative TUNEL staining. By electron microscopy, photoreceptor cells showed typical apoptotic nuclear changes and formation of apoptotic bodies. Conclusions: The sensitivity to light damage and style of death differed among retinal cells by location and cell type.

Apoptotic photoreceptor cell death induced by quinolone phototoxicity in mice

We examined retinal degeneration induced by phototoxicity of quinolone antibacterial agents. Albino Balb/c and pigmented DBA/2 mice fasted overnight were given a single oral administration of ciprofloxacin (CPFX), levofloxacin (LVFX), enoxacin (ENX), lomefloxacin (LFLX) or sparfloxacin (SPFX), followed by ultraviolet-A (UVA) irradiation at 1.5 mW/cm 2 for 4 h (21.6 J/cm 2). At 24 h after quinolone administration, the mice were sacrificed, and the eyes were then histologically examined. ENX or LFLX at 200 or 400 mg/kg or SPFX at 50 or 100 mg/kg plus UVA induced retinal degeneration in Balb/c mice, whereas no histological change was observed in the eyes of DBA/2 mice. CPFX and LVFX at 800 mg/kg plus UVA had no effect on the eyes in either Balb/c or DBA/2 mice. Agarose gel electrophoresis showed that chromosomal DNA extracted from the eyes of Balb/c mice was fragmented in the SPFX 100 mg/kg group, but not in the LVFX 800 mg/kg group. In the electron microscopic examination, swelling of mitochondria and disruption of the cytoplasm were observed in the photoreceptor inner segment (PIS) at 2 h, and disarrangement of lamellar disks in the outer segment (POS) and condensed chromatin in photoreceptor cell nuclei in the outer nuclear layer (ONL) were observed at 4 h after 100 mg/kg SPFX administration to Balb/c mice. These results suggest that quinolone plus UVA irradiation induces retinal degeneration in albino Balb/c mice, but not in DBA/2 mice, and this degeneration is associated with apoptotic photoreceptor cell death.

Expression of alphaB-crystallin in a mouse model of inherited retinal degeneration.

AlphaB-crystallin, which is abundantly expressed in the lens but also in a diversity of other tissues, functions as a stress-inducible molecular chaperone and is increased in brain neurodegenerative diseases. We compared retinal alphaB-crystallin expression in a model of inherited retinal degeneration, the rd mouse, and controls. Northern and in situ hybridization analysis showed alphaB-crystallin mRNA to have an altered spatio-temporal pattern with increased levels localized to glial cells in the degenerative state. Immunocytochemistry confirmed increased expression at Müller cells and astrocytes, together with transiently increased localization to the degenerating photoreceptors. These findings suggest that increased alphaB-crystallin expression is associated with glial cell reaction to neuronal damage in the retina, and may comprise part of the retina's overall defensive response to the stress of apoptotic photoreceptor cell death.

Na,K-ATPase Subunit β1knock-inPrevents Lethality of β2 Deficiency in Mice

The beta2 subunit of the Na,K-ATPase displays functional properties of both an integral constituent of an ion pump and an adhesion and neurite outgrowth-promoting molecule in vitro. To investigate whether the beta1 subunit of the Na,K-ATPase can functionally substitute for the beta2 isoform in vivo, we have generated beta2/beta1 knock-in mice by homologous recombination in embryonic stem cells. In beta2/beta1 knock-in mice, expression of beta2 was abolished, whereas beta1 mRNA expression from the mutated gene amounted to approximately 15% of the normal expression of beta2 in the adult mouse brain and prevented the juvenile lethality observed for beta2 null mutant mice. In contrast to beta2 null mutant mice, the overall morphological structure of all analyzed brain regions was normal. By immunohistochemical analysis, beta1 expression was detected in photoreceptor cells in the retina of knock-in mice at an age when expression of beta1 and beta2, respectively, is downregulated and persisting in the wild-type mice. Morphological analysis by light and electron microscopy revealed a progressive degeneration of photoreceptor cells. Apoptotic death of photoreceptor cells determined quantitatively by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling analysis increased in beta2/beta1 knock-in mice with age. These observations suggest that the beta1 subunit of the Na,K-ATPase can substitute sufficiently, at least in certain cell types, for the role of the beta2 subunit as a component of a functional Na,K-ATPase, but they do not allow us to determine the possible role of the beta2 subunit as an adhesion molecule in vivo.

Adenovirus-mediated delivery of rhodopsin-promoted bcl-2 results in a delay in photoreceptor cell death in the rd/rd mouse.

Gene transfer to retinal cells may provide a means to retard photoreceptor cell death and thus prevent blindness in diseases such as retinitis pigmentosa. We tested the possibility of interfering with apoptotic photoreceptor cell death in the rd mouse through subretinal delivery of a recombinant replication-defective adenovirus containing the human cDNA for bcl-2, Ad.2.5HRPbcl-2. Photoreceptor-specific transgene expression was accomplished through incorporation of the 2.5 kb human rhodopsin upstream fragment (HRP). Ad.2.5HRPbcl-2 was injected alone or in combination with Ad.CMVPDE beta. Ad.CMVPDE beta contains a cDNA encoding the beta subunit of cGMP phosphodiesterase (PDE beta). Recombinant viruses containing lacZ (driven either by the cytomegalovirus (CMV) promoter/enhancer or HRP) and of Ad.CMVPDE beta and vehicle alone were injected in contralateral eyes as control. Injection of Ad.2.5HRPbcl-2 in the rd mouse resulted in histologically detectable rescue lasting 6 weeks after birth. Extent of rescue was not as large as after delivery of wildtype PDE beta, the gene defective in the rd mouse. However, delivery of genes which prevent apoptotic cell death may have broad application to gene therapy of retinal degenerative diseases.

Absence of p53 delays apoptotic photoreceptor cell death in the rds mouse

PURPOSE. This study was aimed at determining whether or not apoptotic photoreceptor cell death in a mouse model of inherited retinal degeneration is p53 dependent. METHODS. A colony of p53-deficient rds mice were obtained by crossing homozygous rds mice with animals homozygous for a targeted disruption of the p53 gene and genotyping the off-spring of the F1 cross. Both parental strains were on a BALB/c background. Age matched p53-deficient rds mice and controls (p53-deficient, rds and BALB/c mice), were sacrificed from day 1 to day 58 after birth. Eyes were paraffin-embedded and a modified terminal dUTP nick-end labeling (TUNEL) technique was used to detect the number of cells displaying DNA fragmentation within the sectioned retina. Eyes were also resin-embedded for semi-thin and ultra-thin sectioning. RESULTS. The peak in photoreceptor apoptosis, which occurs at 16 days in the rds mouse, was delayed by 3 days in p53-deficient rds mice. In addition, there was also a delay in the loss of photoreceptor cells between 16 and 26 days. However, absence of p53 did not prevent retinal degeneration in the rds mouse. The number of photoreceptor cells in p53-deficient rds mice at 35 days was very similar to that in the controls. CONCLUSIONS. We have demonstrated that absence of p53 delays but does not prevent photoreceptor cell loss in the rds mouse. Our results provide evidence for plasticity in the mechanism by which apoptosis proceeds in retinal degeneration.

Photoreceptor decay over time and apoptosis in experimental retinal detachment.

Data are scarce on the actual rate and mode of outer nuclear layer decay in retinal detachment (RD). We used an experimental rabbit model to assess the presence of apoptosis and rate of photoreceptor death following RD. This model included the creation of localized and stable retinal blebs, while controlling for any decline of retinal elevation over time. RD was produced in New Zealand white rabbits by injecting 0.05 ml of 15% sodium hyaluronate (Healon GV) under the neural retina using a microsurgical technique. Animals were killed at 1, 2, 4, 7, 14 and 29 days. Retinal tissue was processed for light and electron microscopy and for in situ end labeling of fragmented DNA using a modification of the TUNEL technique. Photoreceptor cell nuclei were counted in the RD areas of maximum retinal elevation of 28 eyes, and an additional 4 eyes were used for nick end labeling. Positive DNA nick end labeling, ultrastructural features and absence of necrotic cells indicated apoptotic photoreceptor cell death. Also, there was a rapid, almost linear elimination of photoreceptor nuclei over time. At 14 days only half of the number of nuclei were discernible, while approximately one tenth remained after 29 days. There was a statistically significant, but minimal decline in RD height over the 4 weeks of study. Following experimental RD in rabbits, apoptotic cell death is associated with an almost linear elimination of photoreceptor cells over time. The use of highly viscous sodium hyaluronate in separating the neural retina from the retinal pigment epithelium allows the RD to maintain a nearly constant height over a period of 4 weeks.

Bcl-2 overexpression reduces apoptotic photoreceptor cell death in three different retinal degenerations.

Apoptosis of photoreceptors occurs infrequently in adult retina but can be triggered in inherited and environmentally induced retinal degenerations. The protooncogene bcl-2 is known to be a potent regulator of cell survival in neurons. We created lines of transgenic mice overexpressing bcl-2 to test for its ability to increase photoreceptor survival. Bcl-2 increased photoreceptor survival in mice with retinal degeneration caused by a defective opsin or cGMP phosphodiesterase. Overexpression of Bcl-2 in normal photoreceptors also decreased the damaging effects of constant light exposure. Apoptosis was induced in normal photoreceptors by very high levels of bcl-2. We conclude that bcl-2 is an important regulator of photoreceptor cell death in retinal degenerations.

Apoptotic cell death of photoreceptor cells in mice deficient for the adhesion molecule on glia (AMOG, the beta 2- subunit of the Na, K-ATPase).

Disruption of the gene for the adhesion molecule on glia (AMOG, the beta 2-subunit of the Na, K-ATPase) in mice results in swelling and subsequent degeneration of astrocyte endfeet in the brainstem and in cell death of photoreceptor cells in the retina. In the present study, we demonstrate that photoreceptor cells in the mutant develop normally during the first postnatal week. Compared to wild-type mice, a slightly increased density of degenerating photoreceptor cells became apparent in 9-day-old mutants and numerous degenerating photoreceptor cells were present in the retina of 16-day-old AMOG/beta 2-deficient mice. In situ labelling of degenerating cells by terminal dUTP nick end labelling and electron microscopic analysis revealed apoptotic cell death of photoreceptor cells. Massive degeneration of photoreceptor cells in the mutant at postnatal day 16 correlated with elevated levels of glial fibrillary acidic protein in retinal astrocytes and with expression of this protein by Muller cells. No evidence was found for degeneration of other retinal cell types or for glial cell death in the optic nerve. Our observations demonstrate that the pathological death of cells induced by disruption of the AMOG/beta 2 gene results from activation of an intrinsic death program, similar to what has been shown to occur during normal development.