PKG activity causes photoreceptor cell death in two retinitis pigmentosa models

Photoreceptor degeneration in retinitis pigmentosa is one of the leading causes of hereditary blindness in the developed world. Although causative genetic mutations have been elucidated in many cases, the underlying neuronal degeneration mechanisms are still unknown. Here, we show that activation of cGMP-dependent protein kinase (PKG) hallmarks photoreceptor degeneration in rd1 and rd2 human homologous mouse models. When induced in wild-type retinae, PKG activity was both necessary and sufficient to trigger cGMP-mediated photoreceptor cell death. Target-specific, pharmacological inhibition of PKG activity in both rd1 and rd2 retinae strongly reduced photoreceptor cell death in organotypic retinal explants. Likewise, inhibition of PKG in vivo, using three different application paradigms, resulted in robust photoreceptor protection in the rd1 retina. These findings suggest a pivotal role for PKG activity in cGMP-mediated photoreceptor degeneration mechanisms and highlight the importance of PKG as a novel target for the pharmacological intervention in RP.     

The role of mislocalized phototransduction in photoreceptor cell death of retinitis pigmentosa

Most of inherited retinal diseases such as retinitis pigmentosa (RP) cause photoreceptor cell death resulting in blindness. RP is a large family of diseases in which the photoreceptor cell death can be caused by a number of pathways. Among them, light exposure has been reported to induce photoreceptor cell death. However, the detailed mechanism by which photoreceptor cell death is caused by light exposure is unclear. In this study, we have shown that even a mild light exposure can induce ectopic phototransduction and result in the acceleration of rod photoreceptor cell death in some vertebrate models. In ovl, a zebrafish model of outer segment deficiency, photoreceptor cell death is associated with light exposure. The ovl larvae show ectopic accumulation of rhodopsin and knockdown of ectopic rhodopsin and transducin rescue rod photoreceptor cell death. However, knockdown of phosphodiesterase, the enzyme that mediates the next step of phototransduction, does not. So, ectopic phototransduction activated by light exposure, which leads to rod photoreceptor cell death, is through the action of transducin. Furthermore, we have demonstrated that forced activation of adenylyl cyclase in the inner segment leads to rod photoreceptor cell death. For further confirmation, we have also generated a transgenic fish which possesses a human rhodopsin mutation, Q344X. This fish and rd10 model mice show photoreceptor cell death caused by adenylyl cyclase. In short, our study indicates that in some RP, adenylyl cyclase is involved in photoreceptor cell death pathway; its inhibition is potentially a logical approach for a novel RP therapy.     

Blockade of neuronal nitric oxide synthase reduces cone cell death in a model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of diseases in which many different mutations cause rod photoreceptor cells to die and then gradually cone photoreceptors die due to progressive oxidative damage. In this study, we have shown that peroxynitrite-induced nitrosative damage also occurs. In the rd1 mouse model of RP, there was increased staining for S-nitrosocysteine and nitrotyrosine protein adducts that are generated by peroxynitrite. Peroxynitrite is generated from nitric oxide (NO) and superoxide radicals. After degeneration of rods, injection of hydroethidine resulted in strong fluorescence in the retina of rd1 mice, indicating high levels of superoxide radicals, and this was reduced, as was nitrotyrosine staining, by apocynin, suggesting that overaction of NADP(H) oxidase is at least partially responsible. Treatment of rd1 mice with a mixture of nitric oxide synthase (NOS) inhibitors markedly reduced S-nitrosocysteine and nitrotyrosine staining and significantly increased cone survival, indicating that NO-derived peroxynitrite contributes to cone cell death. Treatment with 7-nitroindazole, a relatively specific inhibitor of neuronal NOS, also significantly reduced cone cell death, but aminoguanidine, a relatively specific inhibitor of inducible NOS, did not. These data suggest that NO generated by neuronal NOS exacerbates oxidative damage to cones in RP and that combined therapy to reduce NO and oxidative stress should be considered.     

Altered retinal microRNA expression profile in a mouse model of retinitis pigmentosa

Background  

The role played by microRNAs (miRs) as common regulators in physiologic processes such as development and various disease states was recently highlighted. Retinitis pigmentosa (RP) linked to RHO (which encodes rhodopsin) is the most frequent form of inherited retinal degeneration that leads to blindness, for which there are no current therapies. Little is known about the cellular mechanisms that connect mutations within RHO to eventual photoreceptor cell death by apoptosis.     

NADPH oxidase plays a central role in cone cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is a collection of diseases in which rod photoreceptors die from a variety of mutations. After rods die, the level of tissue oxygen in the outer retina becomes elevated and there is progressive oxidative damage to cones that ultimately triggers apoptosis. In this study, we investigated the hypothesis that NADPH oxidase (Nox) and/or xanthine oxidase serve as critical intermediaries between increased tissue oxygen and the generation of excessive reactive oxygen species that cause oxidative damage to cones. Apocynin, a blocker of Nox, but not allopurinol, a blocker of xanthine oxidase, markedly reduced the superoxide radicals visualized by hydroethidine in the outer retina in the retinal degeneration-1 ( rd1 ^+/+ ) model of RP. Compared to rd1 ^+/+ mice treated with vehicle, those treated with apocynin, but not those treated with allopurinol, had significantly less oxidative damage in the retina measured by ELISA for carbonyl adducts. Apocynin-treated, but not allopurinol-treated, rd1 ^+/+ mice had preservation of cone cell density, increased mRNA levels for m- and s-cone opsin, and increased mean photopic b-wave amplitude. In Q344ter mice, a model of dominant RP in which mutant rhodopsin is expressed, apocynin treatment preserved photopic electroretinogram b-wave amplitude compared to vehicle-treated controls. These data indicate that Nox, but not xanthine oxidase, plays a critical role in generation of the oxidative stress that leads to cone cell death in RP and inhibition of Nox provides a new treatment strategy.     

Mechanisms of cell death in rhodopsin retinitis pigmentosa: implications for therapy

Retinitis pigmentosa (RP) is a group of retinal degenerative diseases that are characterised primarily by the loss of rod photoreceptor cells. Mutations in rhodopsin are the most common cause of autosomal-dominant RP (ADRP). Here, we propose a new classification for rhodopsin mutations based on their biochemical and cellular properties. Several different potential gain-of-function mechanisms for rhodopsin ADRP are described and discussed. Possible dominant-negative mechanisms, which affect the processing, translocation or degradation of wild-type rhodopsin, are also considered. Understanding the molecular and cellular consequences of rod-opsin mutations and the underlying disease mechanisms in ADRP are essential to develop future therapies for this class of retinal dystrophies.     

Probing mechanisms of photoreceptor degeneration in a new mouse model of the common form of autosomal dominant retinitis pigmentosa due to P23H opsin mutations

Rhodopsin, the visual pigment mediating vision under dim light, is composed of the apoprotein opsin and the chromophore ligand 11-cis-retinal. A P23H mutation in the opsin gene is one of the most prevalent causes of the human blinding disease, autosomal dominant retinitis pigmentosa. Although P23H cultured cell and transgenic animal models have been developed, there remains controversy over whether they fully mimic the human phenotype; and the exact mechanism by which this mutation leads to photoreceptor cell degeneration remains unknown. By generating P23H opsin knock-in mice, we found that the P23H protein was inadequately glycosylated with levels 1-10% that of wild type opsin. Moreover, the P23H protein failed to accumulate in rod photoreceptor cell endoplasmic reticulum but instead disrupted rod photoreceptor disks. Genetically engineered P23H mice lacking the chromophore showed accelerated photoreceptor cell degeneration. These results indicate that most synthesized P23H protein is degraded, and its retinal cytotoxicity is enhanced by lack of the 11-cis-retinal chromophore during rod outer segment development.     

Lysosomal membrane permeabilization contributes to elemene emulsion-induced apoptosis in A549 cells

Elemene is a broad-spectrum antitumor agent. In the present study, lysosomal membrane permeabilization (LMP) was detected after short elemene emulsion--exposure (12 h) that preceded a decrease of the mitochondrial membrane potential and DNA damage (24 h) in A549 cells. At later time points (36 h) elemene emulsion caused the appearance of A549 cells with apoptotic features, including apoptotic morphology, phosphatidylserine exposure, and caspase-3 activation. A significant increase in protein expression for cathepsin D was also observed utilizing Western blot analysis after exposure to elemene emulsion for 12 h. The present study showed that elemene emulsion induced the increased levels of reactive oxygen species (ROS) and depletion of glutathione (GSH) in A549 cells. Cells treated with pepstatin A, an inhibitor for cathepsin D, showed a significant inhibition in DNA damage, mitochondrial membrane permeabilization, caspase-3 activation, and phosphatidylserine exposure. These results demonstrated that apoptosis induced by elemene emulsion in A549 cells is mediated in part through LMP and lysosomal protease cathepsin D.     

Lysosomal membrane permeabilization contributes to elemene emulsion-induced apoptosis in A549 cells

Abstract

Elemene is a broad-spectrum antitumor agent. In the present study, lysosomal membrane permeabilization (LMP) was detected after short elemene emulsion - exposure (12 h) that preceded a decrease of the mitochondrial membrane potential and DNA damage (24 h) in A549 cells. At later time points (36 h) elemene emulsion caused the appearance of A549 cells with apoptotic features, including apoptotic morphology, phosphatidylserine exposure, and caspase-3 activation. A significant increase in protein expression for cathepsin D was also observed utilizing Western blot analysis after exposure to elemene emulsion for 12 h. The present study showed that elemene emulsion induced the increased levels of reactive oxygen species (ROS) and depletion of glutathione (GSH) in A549 cells. Cells treated with pepstatin A, an inhibitor for cathepsin D, showed a significant inhibition in DNA damage, mitochondrial membrane permeabilization, caspase-3 activation, and phosphatidylserine exposure. These results demonstrated that apoptosis induced by elemene emulsion in A549 cells is mediated in part through LMP and lysosomal protease cathepsin D.     

Lysosomal membrane permeabilization causes oxidative stress and ferritin induction in macrophages

Moderate lysosomal membrane permeabilization (LMP) is an important inducer of apoptosis. Macrophages are professional scavengers and are rich in hydrolytic enzymes and iron. In the present study, we found that LMP by lysosomotropic detergent MSDH resulted in early up-regulation of lysosomal cathepsins, oxidative stress and ferritin up-regulation, and cell death. Lysosomotropic base NH₄Cl reduced the ferritin induction and oxidative stress in apoptotic cells induced by MSDH. Cysteine cathepsin inhibitors significantly protected cell death and oxidative stress, but had less effect on ferritin induction. We conclude that oxidative stress induced by lysosomal rupture causes ferritin induction with concomitant mitochondrial damage, which are the potential target for prevention of cellular oxidative stress and cell death induced by typical lysosomotropic substances in different disorders.     

Zinc-desferrioxamine attenuates retinal degeneration in the rd10 mouse model of retinitis pigmentosa

Iron-associated oxidative injury plays a role in retinal degeneration such as age-related macular degeneration and retinitis pigmentosa. The metallo-complex zinc-desferrioxamine (Zn/DFO) may ameliorate such injury by chelation of labile iron in combination with release of zinc. We explored whether Zn/DFO can affect the course of retinal degeneration in the rd10 mouse model of retinitis pigmentosa. Zn/DFO-treated animals showed significantly higher electroretinographic responses at 3 and 4.5 weeks of age compared with saline-injected controls. Corresponding retinal (photoreceptor) structural rescue was observed by quantitative histological and immunohistochemical techniques. When administered alone, the components of the complex, Zn and DFO, showed a lesser, partial effect. TBARS, a marker of lipid peroxidation, and levels of oxidative DNA damage as quantified by 8-OHdG immunostaining were significantly lower in Zn/DFO-treated retinas compared with saline-injected controls. Reduced levels of retinal ferritin as well as reduced iron content within ferritin molecules were measured in Zn/DFO-treated retinas. The data, taken together, suggest that the protective effects of the Zn/DFO complex are mediated through modulation of iron bioavailability, leading to attenuation of oxidative injury. Reducing iron-associated oxidative stress using complexes such as Zn/DFO may serve as a “common pathway” therapeutic approach to attenuate injury in retinal degeneration.     

p73 regulates DRAM-independent autophagy that does not contribute to programmed cell death

Evading programmed cell death is a common event in tumour development. The p53 family member, p73, is a potent inducer of death and a determinant of chemotherapeutic response, but different to p53, is rarely mutated in cancer. Understanding cell death pathways downstream of p53 and p73 is therefore pivotal to understand both the development and treatment of malignant disease. Recently, p53 has been shown to modulate autophagy--a membrane trafficking process, which degrades long-lived proteins and organelles. This requires a p53 target gene, DRAM, and both DRAM and autophagy are critical for p53-mediated death. We report here that TA-p73 also regulates DRAM and autophagy, with different TA-p73 isoforms regulating DRAM and autophagy to varying extents. RNAi knockdown of DRAM, however, revealed that p73's modulation of autophagy is DRAM-independent. Also, p73's ability to induce death, again different to p53, is neither dependent on DRAM nor autophagy. In contrast to TA-p73, deltaN-p73 is a negative regulator of p53-induced and p73-induced autophagy, but does not affect autophagy induced by amino-acid starvation. These studies, therefore, represent not only the first report that p73 modulates autophagy but also highlight important differences in the mechanism by which starvation, p53 and p73 regulate autophagy and how this contributes to programmed cell death.     

Induction of unspecific permeabilization of mitochondrial membrane and its role in cell death

Mitochondria participate in various vital cellular processes. Violation of their functions can lead to the development of cardiovascular and neurodegenerative diseases and malignancies. One of the key events responsible for mitochondrial damage—induction of Ca²⁺-dependent mitochondrial permeability transition, due to the opening of a nonspecific pore in the inner mitochondrial membrane. Despite active studies of pore components, its detailed structure has not yet been established. This review analyzes possible constituents and regulators of the pore, the role of the pore in various pathologies, and hypotheses that explain the organization of the pores. Elucidation of these questions can help developing strategies for the treatment of a wide range of pathologies—from Alzheimer and Parkinson’s disease to cancer.     

The role of subunit assembly in peripherin-2 targeting to rod photoreceptor disk membranes and retinitis pigmentosa

Peripherin-2 is a member of the tetraspanin family of membrane proteins that plays a critical role in photoreceptor outer segment disk morphogenesis. Mutations in peripherin-2 are responsible for various retinal degenerative diseases including autosomal dominant retinitis pigmentosa (ADRP). To identify determinants required for peripherin-2 targeting to disk membranes and elucidate mechanisms underlying ADRP, we have generated transgenic Xenopus tadpoles expressing wild-type and ADRP-linked peripherin-2 mutants as green fluorescent fusion proteins in rod photoreceptors. Wild-type peripherin-2 and P216L and C150S mutants, which assemble as tetramers, targeted to disk membranes as visualized by confocal and electron microscopy. In contrast the C214S and L185P mutants, which form homodimers, but not tetramers, were retained in the rod inner segment. Only the P216L disease mutant induced photoreceptor degeneration. These results indicate that tetramerization is required for peripherin-2 targeting and incorporation into disk membranes. Tetramerization-defective mutants cause ADRP through a deficiency in wild-type peripherin-2, whereas tetramerization-competent P216L peripherin-2 causes ADRP through a dominant negative effect, possibly arising from the introduction of a new oligosaccharide chain that destabilizes disks. Our results further indicate that a checkpoint between the photoreceptor inner and outer segments allows only correctly assembled peripherin-2 tetramers to be incorporated into nascent disk membranes.     

A diffusible factor from normal retinal cells promotes rod photoreceptor survival in an in vitro model of retinitis pigmentosa

Transgenic mice expressing a dominant mutation in the gene for the phototransduction molecule rhodopsin undergo retinal degeneration similar to that experienced by patients with the retinal degenerative disease, retinitis pigmentosa (RP). Although the mutation is thought to cause photoreceptor degeneration in a cell‐autonomous manner, the fact that rod photoreceptor degeneration is slowed in chimeric wild‐type/mutant mice suggests that cellular interactions are also important for maintaining photoreceptor survival. To more fully characterize the nature of the cellular interactions important for rod degeneration in the RP mutant mice, we have used an in vitro approach. We found that when the retinas of the transgenic mice were isolated from the pigmented epithelium and cultured as explants, the rod photoreceptors underwent selective degeneration with a similar time course to that observed in vivo. This selective rod degeneration also occurred when the cells were dissociated and cultured as monolayers. These data indicate that the mutant rod photoreceptors degenerate when removed from their normal cellular relationships and without contact with the pigmented epithelium, thus confirming the relative cell autonomy of the mutant phenotype. We next tested whether normal retinal cells could rescue the mutant photoreceptors in a coculture paradigm. Coculture of transgenic mouse with wild‐type mouse or rat retinal cells significantly enhanced transgenic rod photoreceptor survival; this survival‐promoting activity was diffusible through a filter, was heat labile, and not present in transgenic retinal cells. Several peptide growth factors known to be present in the retina were tested as the potential survival‐promoting molecule responsible for the effects of the conditioned medium; however, none of them promoted survival of the photoreceptors expressing the Pro23His mutant rhodopsin. Nevertheless, we were able to demonstrate that the mutant photoreceptors could be rescued by an antagonist to a retinoic acid receptor, suggesting that the endogeneous survival‐promoting activity may function through this pathway. These data thus confirm and extend the findings of previous work that local trophic interactions are important in regulating rod photoreceptor degeneration in retinitis pigmentosa. A diffusible factor found in normal but not transgenic retinal cells has a protective effect on the survival of rod photoreceptors from Pro23His mutant rhodopsin mice. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 475–490, 1999     

Development and degeneration of cone bipolar cells are independent of cone photoreceptors in a mouse model of retinitis pigmentosa

Retinal photoreceptors die during retinal synaptogenesis in a portion of retinal degeneration. Whether cone bipolar cells establish regular retinal mosaics and mature morphologies, and resist degeneration are not completely understood. To explore these issues, we backcrossed a transgenic mouse expressing enhanced green fluorescent protein (EGFP) in one subset of cone bipolar cells (type 7) into rd1 mice, a classic mouse model of retinal degeneration, to examine the development and survival of cone bipolar cells in a background of retinal degeneration. Our data revealed that both the development and degeneration of cone bipolar cells are independent of the normal activity of cone photoreceptors. We found that type 7 cone bipolar cells achieved a uniform tiling of the retinal surface and developed normal dendritic and axonal arbors without the influence of cone photoreceptor innervation. On the other hand, degeneration of type 7 cone bipolar cells, contrary to our belief of central-to-peripheral progression, was spatially uniform across the retina independent of the spatiotemporal pattern of cone degeneration. The results have important implications for the design of more effective therapies to restore vision in retinal degeneration.     

ER stress and autophagy contribute to CsA-induced death of malignant glioma cells

Cyclosporine A (CsA), which revolutionized transplantology due to its ability to block the activation of lymphocytes and other immune system cells, triggers autophagy in malignant glioma cell lines via stimulation of endoplasmic reticulum (ER) stress. We also found that autophagy serves as a protective mechanism against CsA toxicity.     

Lysosomal involvement in cell death and cancer

Lysosomes, with their arsenal of degradative enzymes are increasingly becoming an area of interest in the field of oncology. The changes induced in this compartment upon transformation are numerous and whereas most are viewed as pro-oncogenic the same processes also render cancer cells susceptible to lysosomal death pathways. This review will provide an overview of the pro- and anti-oncogenic potential of this compartment and how these might be exploited for cancer therapy, with special focus on lysosomal death pathways.     

The role of caspases in photoreceptor cell death of the retinoschisin-deficient mouse

Early schisis cavities in the retinal bipolar cell layer accompanied by progressive loss of cone and rod photoreceptor cells are the hallmark of the retinoschisin-deficient (Rs1h(-/Y)) murine retina. With this study we aimed at elucidating the molecular events underlying the photoreceptor cell death in this established murine model of X-linked juvenile retinoschisis. We show that photoreceptor degeneration in the Rs1h(-/Y) mouse is due to apoptotic events peaking around postnatal day 18. Cell death is accompanied by increased expression of initiator and inflammatory caspases but not by downstream effector caspases. The strong induction of caspase-1 (Casp1) prompted us to explore its involvement in the apoptotic process. We therefore generated double knock-out mice deficient for both retinoschisin and Casp1. No direct influence of the Casp1 genotype on apoptosis could be identified although striking differences in the overall number of resident microglia were observed independent of the Rs1h genotype.