Novel insights into the activating transcription factor 4 in Alzheimer's disease and associated aging-related diseases: Mechanisms and therapeutic implications

Ageing is inherent to all human beings, most mechanistic explanations of ageing results from the combined effects of various physiological and pathological processes. Additionally, aging pivotally contributes to several chronic diseases. Activating transcription factor 4 (ATF4), a member of the ATF/cAMP response element-binding protein family, has recently emerged as a pivotal player owing to its indispensable role in the pathophysiological processes of Alzheimer's disease and aging-related diseases. Moreover, ATF4 is integral to numerous biological processes. Therefore, this article aims to comprehensively review relevant research on the role of ATF4 in the onset and progression of aging-related diseases, elucidating its potential mechanisms and therapeutic approaches. Our objective is to furnish scientific evidence for the early identification of risk factors in aging-related diseases and pave the way for new research directions for their treatment. By elucidating the signaling pathway network of ATF4 in aging-related diseases, we aspire to gain a profound understanding of the molecular and cellular mechanisms, offering novel strategies for addressing aging and developing related therapeutics.

Phenotypic high-throughput screening identifies aryl hydrocarbon receptor agonism as common inhibitor of toxin-induced retinal pigment epithelium cell death

The retinal pigment epithelium (RPE) is essential to maintain retinal function, and RPE cell death represents a key pathogenic stage in the progression of several blinding ocular diseases, including age-related macular degeneration (AMD). To identify pathways and compounds able to prevent RPE cell death, we developed a phenotypic screening pipeline utilizing a compound library and high-throughput screening compatible assays on the human RPE cell line, ARPE-19, in response to different disease relevant cytotoxic stimuli. We show that the metabolic by-product of the visual cycle all-trans-retinal (atRAL) induces RPE apoptosis, while the lipid peroxidation by-product 4-hydroxynonenal (4-HNE) promotes necrotic cell death. Using these distinct stimuli for screening, we identified agonists of the aryl hydrocarbon receptor (AhR) as a consensus target able to prevent both atRAL mediated apoptosis and 4-HNE-induced necrotic cell death. This works serves as a framework for future studies dedicated to screening for inhibitors of cell death, as well as support for the discussion of AhR agonism in RPE pathology.

Azoramide ameliorates cadmium-induced cytotoxicity by inhibiting endoplasmic reticulum stress and suppressing oxidative stress

Background

Cadmium (Cd) is hazardous to human health because of its cytotoxicity and long biological half-life. Azoramide is a small molecular agent that targets the endoplasmic reticulum (ER) and moderates the unfolded protein response. However, its role in Cd-induced cytotoxicity remains unclear. This study was performed to investigate the protective effect of azoramide against Cd-induced cytotoxicity and elucidate its underlying mechanisms.

Methods

Inductively coupled plasma‒mass spectrometry was used to measure Cd concentrations in each tissue of ICR male mice. The human proximal tubule epithelial cell line HK-2 and the human retinal pigment epithelial cell line ARPE-19 were used in the in vitro study. Cell apoptosis was determined by DAPI staining, JC-1 staining, and annexin V/propidium iodide double staining. Intracellular oxidative stress was detected by MitoSOX red staining, western blot, and quantitative real-time PCR. Moreover, ER stress signaling, MAPK cascades, and autophagy signaling were analyzed by western blot.

Results

The present data showed that Cd accumulated in various organs of ICR mice, and the concentrations of Cd in the studied organs, from high to low, were as follows: liver > kidney > testis > lung > spleen > eye. Our study demonstrated that azoramide inhibited ER stress by promoting BiP expression and suppressing the PERK-eIF2α-CHOP pathway. Additionally, we also found that azoramide significantly decreased ER stress-associated radical oxidative species production, attenuated p38 MAPK and JNK signaling, and inhibited autophagy, thus suppressing apoptosis in HK-2 and ARPE-19 cells.

Conclusion

Our study investigated the effect of azoramide on Cd-induced cytotoxicity and revealed that azoramide may be a therapeutic drug for Cd poisoning.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

eIF2α incites photoreceptor cell and retina damage by all-trans-retinal

Dry age-related macular degeneration (AMD) and recessive Stargardt's disease (STGD1) lead to irreversible blindness in humans. The accumulation of all-trans-retinal (atRAL) induced by chaos in visual cycle is closely associated with retinal atrophy in dry AMD and STGD1, but its critical downstream signaling molecules remain ambiguous. Here, we reported that activation of eukaryotic translation initiation factor 2α (eIF2α) by atRAL promoted retinal degeneration and photoreceptor loss through activating c-Jun N-terminal kinase (JNK) signaling-dependent apoptosis and gasdermin E (GSDME)-mediated pyroptosis. We determined that eIF2α activation by atRAL in photoreceptor cells resulted from endoplasmic reticulum (ER) homeostasis disruption caused at least in part by reactive oxygen species (ROS) production, and it activated JNK signaling independent of and dependent on activating transcription factor 4 (ATF4) and the ATF4/transcription factor C/EBP homologous protein (CHOP) axis. CHOP overexpression induced apoptosis of atRAL-loaded photoreceptor cells through activating JNK signaling rather than inhibiting the expression of anti-apoptotic gene Bcl2. JNK activation by eIF2α facilitated photoreceptor cell apoptosis caused by atRAL via caspase-3 activation and DNA damage. Additionally, we demonstrated that eIF2α was activated in neural retina of light-exposed Abca4^-/-Rdh8^-/- mice, a model that shows severe defects in atRAL clearance and displays primary features of human dry AMD and STGD1. Of note, inhibition of eIF2α activation by salubrinal effectively ameliorated retinal degeneration and photoreceptor apoptosis in Abca4^-/-Rdh8^-/- mice upon light exposure. The results of this study suggest that eIF2α is an important target to develop drug therapies for the treatment of dry AMD and STGD1.

Induction of ferroptosis by HO-1 contributes to retinal degeneration in mice with defective clearance of all-trans-retinal

The accumulation of all-trans-retinal (atRAL) in photoreceptors and the retinal pigment epithelium (RPE), which is induced by chaos in visual (retinoid) cycle, is closely associated with the pathogenesis of dry age-related macular degeneration (AMD) and autosomal recessive Stargardt's disease (STGD1). Although we have reported that the induction of ferroptosis by atRAL is an important cause of photoreceptor loss, but its mechanisms still remain unclear. In this study, we identified heme oxygenase-1 (HO-1) as an inducer of photoreceptor ferroptosis elicited by atRAL. In atRAL-loaded photoreceptor cells, inhibition of Kelch-like ECH-associated protein 1 (KEAP1) at least in part by reactive oxygen species (ROS) production evoked the release of nuclear factor-erythroid 2-related factor-2 (NRF2) from KEAP1, followed by the translocation of active NRF2 into the nucleus where it promoted the transcription of the Ho-1 gene, thereby leading to HO-1 overexpression in the cytosol. A significant elevation of Fe²⁺ levels in photoreceptor cells resulted from activation of HO-1 by atRAL, and it facilitated ROS overproduction and then triggered ferroptotic cell death through ROS-mediated lipid peroxidation. Both treatment with HO-1 repressor Zinc protoporphyrin IX (ZnPP) and knockout of Ho-1 gene clearly rescued photoreceptor cells against ferroptosis arising from atRAL overload. Light-exposed Abca4^-/-Rdh8^-/- mice rapidly display severe defects in atRAL clearance, and serve as an acute model of dry AMD and STGD1. HO-1 activation was distinctly observed in neural retina of Abca4^-/-Rdh8^-/- mice after exposure to light, and it was visibly relieved by intraperitoneally injected ferroptosis inhibitor ferrostatin-1. More notably, intraperitoneal administration of ZnPP effectively alleviated both photoreceptor degeneration and RPE atrophy in Abca4^-/-Rdh8^-/- mice in response to light exposure by repressing HO-1-mediated ferroptosis. Our study suggests that HO-1 is a key factor that regulates atRAL-induced ferroptosis in photoreceptors and the RPE, and its inhibition may hold promises for the therapy of dry AMD and STGD1.

SHP-1 knockdown suppresses mitochondrial biogenesis and aggravates mitochondria-dependent apoptosis induced by all trans retinal through the STING/AMPK pathways

BACKGROUND

Oxidative stress-caused damage to the retinal pigment epithelium (RPE) underlies the onset and progression of age-related macular degeneration (AMD). Impaired mitochondrial biogenesis sensitizes RPE cells to mitochondrial dysfunction, energy insufficiency and death. Src-homology 2 domain-containing phosphatase (SHP)-1 is important in regulating immune responses and cell survival. However, its roles in cell survival are not always consistent. Until now, the effects of SHP-1 on RPE dysfunction, especially mitochondrial homeostasis, remain to be elucidated. We sought to clarify the effects of SHP-1 in RPE cells in response to atRAL-induced oxidative stress and determine the regulatory mechanisms involved.

METHODS

In the all trans retinal (atRAL)-induced oxidative stress model, we used the vector of lentivirus to knockdown the expression of SHP-1 in ARPE-19 cells. CCK-8 assay, Annexin V/PI staining and JC-1 staining were utilized to determine the cell viability, cell apoptosis and mitochondrial membrane potential. We also used immunoprecipitation to examine the ubiquitination modification of stimulator of interferon genes (STING) and its interaction with SHP-1. The expression levels of mitochondrial marker, proteins related to mitochondrial biogenesis, and signaling molecules involved were examined by western blotting analysis.

RESULTS

We found that SHP-1 knockdown predisposed RPE cells to apoptosis, aggravated mitochondrial damage, and repressed mitochondrial biogenesis after treatment with atRAL. Immunofluoresent staining and immunoprecipitation analysis confirmed that SHP-1 interacted with the endoplasmic reticulum-resident STING and suppressed K63-linked ubiquitination and activation of STING. Inhibition of STING with the specific antagonist H151 attenuated the effects of SHP-1 knockdown on mitochondrial biogenesis and oxidative damage. The adenosine monophosphate-activated protein kinase (AMPK) pathway acted as the crucial downstream target of STING and was involved in the regulatory processes.

CONCLUSIONS

These findings suggest that SHP-1 knockdown potentiates STING overactivation and represses mitochondrial biogenesis and cell survival, at least in part by blocking the AMPK pathway in RPE cells. Therefore, restoring mitochondrial health by regulating SHP-1 in RPE cells may be a potential therapeutic strategy for degenerative retinal diseases including AMD.

Role of the AMPK signalling pathway in the aetiopathogenesis of ocular diseases

BACKGROUND

AMP-activated protein kinase (AMPK) plays a precise role as a master regulator of cellular energy homeostasis. AMPK is activated in response to the signalling cues that exhaust cellular ATP levels such as hypoxia, ischaemia, glucose depletion and heat shock. As a central regulator of both lipid and glucose metabolism, AMPK is considered to be a potential therapeutic target for the treatment of various diseases, including eye disorders.

OBJECTIVE

To review all the shreds of evidence concerning the role of the AMPK signalling pathway in the pathogenesis of ocular diseases.

METHOD

Scientific data search and review of available information evaluating the influence of AMPK signalling on ocular diseases.

RESULTS

Review highlights the significance of AMPK signalling in the aetiopathogenesis of ocular diseases, including cataract, glaucoma, diabetic retinopathy, retinoblastoma, age-related macular degeneration, corneal diseases, etc. The review also provides the information on the AMPK-associated pathways with reference to ocular disease, which includes mitochondrial biogenesis, autophagy and regulation of inflammatory response.

CONCLUSION

The study concludes the role of AMPK in ocular diseases. There is growing interest in the therapeutic utilization of the AMPK pathway for ocular disease treatment. Furthermore, inhibition of AMPK signalling might represent more pertinent strategy than AMPK activation for ocular disease treatment. Such information will guide the development of more effective AMPK modulators for ocular diseases.[Formula: see text].

Repressing c-Jun N-terminal kinase signaling mitigates retinal pigment epithelium degeneration in mice with failure to clear all-trans-retinal

Retinal pigment epithelium (RPE) cell apoptosis arising from all-trans-retinal (atRAL) is in close contact with the etiology of dry age-related macular degeneration (AMD) and autosomal recessive Stargardt's disease (STGD1), but its underlying mechanisms remain elusive. In this study, we reported that c-Jun N-terminal kinase (JNK) activation facilitated atRAL-induced apoptosis of RPE cells. Reactive oxygen species production and endoplasmic reticulum stress were identified as two of major upstream events responsible for activating JNK signaling in atRAL-loaded RPE cells. Inhibiting JNK signaling rescued RPE cells from apoptosis induced by atRAL through attenuating caspase-3 activation leading to poly-ADP-ribose polymerase (PARP) cleavage, and DNA damage response. Abca4^-/-Rdh8^-/- mice upon light exposure exhibit rapidly increased accumulation of atRAL in the retina, and display severe RPE degeneration, a primary attribute of dry AMD and STGD1. Reducing JNK signaling by intraperitoneally injected JNK-IN-8 was highly effective in preventing RPE atrophy and apoptosis in light-exposed Abca4^-/-Rdh8^-/- mice. These findings afford a further understanding for contribution of JNK activation by atRAL to retinal damage.

Involvement of Oxidative and Endoplasmic Reticulum Stress in RDH12-Related Retinopathies

Retinol dehydrogenase 12 (RDH12) is expressed in photoreceptor inner segments and catalyses the reduction of all-trans retinal (atRAL) to all-trans retinol (atROL), as part of the visual cycle. Mutations in RDH12 are primarily associated with autosomal recessive Leber congenital amaurosis. To further our understanding of the disease mechanisms, HEK-293 cell lines expressing wildtype (WT) and mutant RDH12 were created. The WT cells afforded protection from atRAL-induced toxicity and oxidative stress. Mutant RDH12 cells displayed reduced protein expression and activity, with an inability to protect cells from atRAL toxicity, inducing oxidative and endoplasmic reticulum (ER) stress, with upregulation of sXBP1, CHOP, and ATF4. Pregabalin, a retinal scavenger, attenuated atRAL-induced ER stress in the mutant RDH12 cell lines. A zebrafish rdh12 mutant model (rdh12^u533 c.17_23del; p.(Val6AlafsTer5)) was generated through CRISPR-Cas9 gene editing. Mutant fish showed disrupted phagocytosis through transmission electron microscopy, with increased phagosome size at 12 months post-fertilisation. Rhodopsin mislocalisation and reduced expression of atg12 and sod2 indicated early signs of a rod-predominant degeneration. A lack of functional RDH12 results in ER and oxidative stress representing key pathways to be targeted for potential therapeutics.

Protective effects of NAC and salubrinal on apoptosis of retinal pigment epithelial cells induced by all-trans retinoic acid

PURPOSE

Accumulation of endogenous all-trans retinoic acid (ATRA) plays a role in the degeneration of photoreceptor cells and retinal pigment epithelium (RPE) cells, contributing to age-related macular degeneration (AMD). This study attempted to investigate the influence of antioxidant N-acetylcysteine (NAC) and selective endoplasmic reticulum stress (ERS) inhibitor salubrinal on apoptosis of ARPE-19 cells induced by ATRA.

METHODS

The RPE cell line (ARPE-19) was treated with ATRA, ATRA+NAC, ATRA+salubrinal or ATRA+NAC+salubrinal and the control was untreated. After 24 h of cell culture, the levels of apoptosis, multicaspase and reactive oxygen species (ROS) were detected by flow cytometry. Western blot analysis was employed to detect the expression of vascular endothelial growth factor-A (VEGF-A), C/EBP homologous protein (CHOP) and cleaved caspase-3 in the groups.

RESULTS

The results of flow cytometry showed that NAC and salubrinal decreased the levels of apoptosis, ROS and multicaspase. ATRA increased VEGF-A levels associated with neovascularisation. NAC and salubrinal inhibited an increase in VEGF-A, CHOP and caspase-3 caused by ATRA in ARPE-19 cells.

CONCLUSIONS

In ARPE-19 cells, the levels of ROS and ERS can be increased by ATRA, contributing to apoptosis, which can be effectively inhibited by NAC and salubrinal. Thus, ATRA may play an important role in the prevention, diagnosis and treatment of age-related macular degeneration.

Recessive multiple epiphyseal dysplasia and Stargardt disease in two sisters

BACKGROUND

The rapid spread of genome-wide next-generation sequencing in the molecular diagnosis of rare genetic disorders has produced increasing evidence of multilocus genomic variations in cases with a previously well-characterized molecular diagnosis. Here, we describe two patients with a rare combination of skeletal abnormalities and retinal dystrophy caused by variants in the SLC26A2 and ABCA4 genes, respectively, in a family with parental consanguinity.

METHODS

Next-generation sequencing and Sanger sequencing were performed to obtain a molecular diagnosis for the retinal and skeletal phenotypes, respectively.

RESULTS

Genetic testing revealed that the sisters were homozygous for the p.(Cys653Ser) variant in SLC26A2 and heterozygous for the missense p.(Pro68Leu) and splice donor c.6386+2C>G variants in ABCA4. Segregation analysis confirmed the carrier status of the parents.

CONCLUSION

Despite low frequency of occurrence, the detection of multilocus genomic variations in a single disease gene-oriented approach can provide accurate diagnosis even in cases with high phenotypic complexity. A targeted sequencing approach can detect relationships between observed phenotypes and underlying genotypes, useful for clinical management.

IL-1/IL-1R signaling induced by all-trans-retinal contributes to complement alternative pathway activation in retinal pigment epithelium

The underlying mechanisms of complement activation in Stargardt disease type 1 (STGD1) and age-related macular degeneration (AMD) are not fully understood. Overaccumulation of all-trans-retinal (atRAL) has been proposed as the pathogenic factor in both diseases. By incubating retinal pigment epithelium (RPE) cells with atRAL, we showed that C5b-9 membrane attack complexes (MACs) were generated mainly through complement alternative pathway. An increase in complement factor B (CFB) expression as well as downregulation of complement regulatory proteins CD46, CD55, CD59, and CFH were observed in RPE cells after atRAL treatment. Furthermore, interleukin-1β production was provoked in both atRAL-treated RPE cells and microglia/macrophages. Coincubation of RPE cells with interleukin-1 receptor antagonist (IL1Ra) and atRAL ameliorated complement activation and downregulated CFB expression by attenuating both p38 and c-Jun N-terminal kinase (JNK) signaling pathways. Our findings demonstrate that atRAL induces an autocrine/paracrine IL-1/IL-1R signaling to promote complement alternative pathway activation in RPE cells and provide a novel perspective on the pathomechanism of macular degeneration.

All-trans retinoic acid increases ARPE-19 cell apoptosis via activation of reactive oxygen species and endoplasmic reticulum stress pathways

AIM

To explore the apoptosis of ARPE-19 cells after the treatment with different doses of all-trans-retinoic acid (ATRA).

METHODS

ARPE-19 cells were used in the in-vitro experiment. Flow cytometry assay was employed to evaluate the level of reactive oxygen species (ROS) and apoptosis. The effects of ATRA (concentrations from 2.5 to 20 µmol/L) on the expression of endoplasmic reticulum stress (ERS) markers in vitro were evaluated by Western blot and real-time quantitative polymerase chain reaction (qRT-PCR) assays. The contribution of ROS and ERS-induced apoptosis in vitro was determined by using N-acetyl-L-cysteine (NAC) and Salubrinal, an antagonist of NAC and ERS, respectively.

RESULTS

Flow cytometry showed that ATRA significantly increased ARPE-19 cell apoptosis and ROS levels in each group (F=86.39, P<0.001; F=116.839, P<0.001). Western blot and qRT-PCR revealed that levels of CHOP and BIP were elevated in a concentration-dependent pattern after the cells were incubated with ATRA (2.5-20 µmol/L). The upregulation of VEGF-A and CHOP induced by ATRA could be inhibited by NAC (antioxidant) and Salubrinal (ERS inhibitor) in vitro.

CONCLUSION

ATRA induces the apoptosis of ARPE-19 cells via activated ROS and ERS signaling pathways.

All-trans-Retinaldehyde Contributes to Retinal Vascular Permeability in Ischemia Reperfusion

Purpose

Extracellular accumulation of all-trans-retinaldehyde (atRAL), a highly reactive visual cycle intermediate, is toxic to cells of the outer retina and contributes to retinal and macular degenerations. However, the contribution of atRAL to retinal capillary function has not been studied. We hypothesized that atRAL released from the outer retina can contribute to retinal vascular permeability. We, therefore, tested the contribution of atRAL to retinal ischemia-reperfusion (IR)-induced vascular permeability.

Methods

IR was induced in mice by transient increase in intraocular pressure followed by natural reperfusion. The visual cycle was ablated in the Lrat-/- mice, reduced by dark adaptation or the use of the RPE65 inhibitor and atRAL scavenger emixustat. Accumulation of FITC-BSA was used to assess vascular permeability and DNA fragmentation quantified cell death after IR. Primary bovine retinal endothelial cell (BREC) culture was used to measure the direct effects of atRAL on endothelial permeability and cell death.

Results

Inhibition of the visual cycle by Lrat-/-, dark adaptation, or with emixustat, all reduced approximately half of IR induced vascular permeability at 48 hours. An increase in BREC permeability with atRAL coincided with lactate dehydrogenase (LDH) release, a measure of cell death. Both permeability and toxicity were blocked by emixustat.

Conclusions

Outer retinal pathology may contribute to vascular permeability by release of atRAL, which can act directly on vascular endothelial cells to alter barrier properties and induce cell death. These studies may have implications for a variety of blinding eye diseases that include outer retinal damage and retinal vascular permeability.

Activation of JNK signaling promotes all-trans-retinal-induced photoreceptor apoptosis in mice

Disrupted clearance of all-trans-retinal (atRAL), a component of the visual (retinoid) cycle in the retina, may cause photoreceptor atrophy in autosomal recessive Stargardt disease (STGD1) and dry age-related macular degeneration (AMD). However, the mechanisms underlying atRAL-induced photoreceptor loss remain elusive. Here, we report that atRAL activates c-Jun N-terminal kinase (JNK) signaling at least partially through reactive oxygen species production, which promoted mitochondria-mediated caspase- and DNA damage-dependent apoptosis in photoreceptor cells. Damage to mitochondria in atRAL-exposed photoreceptor cells resulted from JNK activation, leading to decreased expression of Bcl2 apoptosis regulator (Bcl2), increased Bcl2 antagonist/killer (Bak) levels, and cytochrome c (Cyt c) release into the cytosol. Cytosolic Cyt c specifically provoked caspase-9 and caspase-3 activation and thereby initiated apoptosis. Phosphorylation of JNK in atRAL-loaded photoreceptor cells induced the appearance of γH2AX, a sensitive marker for DNA damage, and was also associated with apoptosis onset. Suppression of JNK signaling protected photoreceptor cells against atRAL-induced apoptosis. Moreover, photoreceptor cells lacking Jnk1 and Jnk2 genes were more resistant to atRAL-associated cytotoxicity. The Abca4 ^-/- Rdh8 ^-/- mouse model displays defects in atRAL clearance that are characteristic of STGD1 and dry AMD. We found that JNK signaling was activated in the neural retina of light-exposed Abca4 ^-/- Rdh8 ^-/- mice. Of note, intraperitoneal administration of JNK-IN-8, which inhibits JNK signaling, effectively ameliorated photoreceptor degeneration and apoptosis in light-exposed Abca4 ^-/- Rdh8 ^-/- mice. We propose that pharmacological inhibition of JNK signaling may represent a therapeutic strategy for preventing photoreceptor loss in retinopathies arising from atRAL overload.

Isopropyl-phloroglucinol-DHA protects outer retinal cells against lethal dose of all-trans-retinal

All‐trans‐retinal (atRAL) is a highly reactive carbonyl specie, known for its reactivity on cellular phosphatidylethanolamine in photoreceptor. It is generated by photoisomerization of 11‐cis‐retinal chromophore linked to opsin by the Schiff's base reaction. In ABCA4‐associated autosomal recessive Stargardt macular dystrophy, atRAL results in carbonyl and oxidative stress, which leads to bisretinoid A2E, accumulation in the retinal pigment epithelium (RPE). This A2E‐accumulation presents as lipofuscin fluorescent pigment, and its photooxidation causes subsequent damage. Here we describe protection against a lethal dose of atRAL in both photoreceptors and RPE in primary cultures by a lipidic polyphenol derivative, an isopropyl‐phloroglucinol linked to DHA, referred to as IP‐DHA. Next, we addressed the cellular and molecular defence mechanisms in commonly used human ARPE‐19 cells. We determined that both polyunsaturated fatty acid and isopropyl substituents bond to phloroglucinol are essential to confer the highest protection. IP‐DHA responds rapidly against the toxicity of atRAL and its protective effect persists. This healthy effect of IP‐DHA applies to the mitochondrial respiration. IP‐DHA also rescues RPE cells subjected to the toxic effects of A2E after blue light exposure. Together, our findings suggest that the beneficial role of IP‐DHA in retinal cells involves both anti‐carbonyl and anti‐oxidative capacities.

All-trans-retinal induces autophagic cell death via oxidative stress and the endoplasmic reticulum stress pathway in human retinal pigment epithelial cells

Failure of all-trans-retinal (atRAL) clearance contributes to retina degeneration. However, whether autophagy can be activated by excess atRAL accumulation in retinal pigment epithelial (RPE) cells is not known. This study showed that atRAL provoked mitochondria-associated reactive oxygen species (ROS) production, activated the nuclear factor (erythroid-derived 2)-like 2 and apoptosis in a human RPE cell line, ARPE-19 cells. Moreover, we found that autophagic flux was functionally activated after atRAL treatment. The antioxidant N-acetylcysteine attenuated the expression of autophagy markers, suggesting that ROS triggered atRAL-activated autophagy. In addition, autophagic cell death was observed in atRAL-treated RPE cells, while inhibition of autophagy with 3-methyladenine or LC3, Beclin1, p62 silencing ameliorated atRAL-induced cytotoxicity. Suppression of autophagy quenched mitochondrial ROS and inhibited HO-1 and γ-GCSh expression, indicating that atRAL-activated autophagy enhances intracellular oxidative stress, thereby promoting RPE cell apoptosis. Furthermore, we found that inhibiting endoplasmic reticulum (ER) stress suppressed atRAL-induced mitochondrial ROS generation, subsequently attenuated autophagy and apoptosis in RPE cells. Taken together, these results suggest that atRAL-induced oxidative stress and ER stress modulate autophagy, which may contribute to RPE degeneration. There may be positive feedback regulatory mechanisms between atRAL-induced oxidative stress and autophagy or ER stress.

Retinal Pigment Epithelium Cell Death Is Associated With NLRP3 Inflammasome Activation by All-trans Retinal

Purpose

Visual (retinoid) cycle anomalies induce aberrant build-up of all-trans retinal (atRAL) in the retinal pigment epithelium (RPE), which is a cause of RPE atrophy in Stargardt disease type 1 and age-related macular degeneration. NLR family pyrin domain containing 3 (NLRP3) inflammasome activation is implicated in the etiology of age-related macular degeneration. Here, we elucidated the relationship between NLRP3 inflammasome activation and atRAL-induced death of RPE cells.

Methods

Cellular toxicities were assessed by MTS or MTT assays. Expression levels of mRNAs and proteins were determined by quantitative reverse transcription-polymerase chain reaction, Western blotting, or enzyme-linked immunosorbent assay. Fluorescence microscopy was used to examine intracellular signals. Ultrastructural features of organelles were examined by transmission electron microscope.

Results

Abnormal accumulation of atRAL was associated with a significant increase in the proportion of human ARPE-19 cells exhibiting features of apoptosis and Caspase-3/gasdermin E (GSDME)-mediated pyroptosis. These cells also exhibited elevated expression of NLRP3, ASC, cleaved Caspase-1/poly ADP-ribose polymerase (PARP)/Caspase-3/GSDME, interleukin-1β (IL-1β), and IL-18, as well as NLRP3 inflammasome-related genes (IL1B and IL18). After exposure of human ARPE-19 cells to excess atRAL, reactive oxygen species (ROS) (including mitochondrial ROS) and cathepsins released from lysosomes transmitted signals leading to NLRP3 inflammasome activation. Suppressing the production of ROS, NLRP3 inflammasome, Caspase-1, cathepsin B, or cathepsin D protected ARPE-19 cells against atRAL-associated cytotoxicity. Damage to mitochondria, lysosomes, and endoplasmic reticulum in atRAL-exposed ARPE-19 cells was partially alleviated by treatment with MCC950, a selective NLRP3 inflammasome inhibitor.

Conclusions

Aberrant build-up of atRAL promotes the death of RPE cells via NLRP3 inflammasome activation.

All-trans-retinoic acid generation is an antidotal clearance pathway for all-trans-retinal in the retina

The present study was designed to analyze the metabolites of all-trans-retinal (atRal) and compare the cytotoxicity of atRal versus its derivative all-trans-retinoic acid (atRA) in human retinal pigment epithelial (RPE) cells. We confirmed that atRA was produced in normal pig neural retina and RPE. The amount of all-trans-retinol (atROL) converted from atRal was about 2.7 times that of atRal-derived atRA after incubating RPE cells with 10 μmol/L atRal for 24 h, whereas atRA in medium supernatant is more plentiful (91 vs. 29 pmol/mL), suggesting that atRA conversion facilitates elimination of excess atRal in the retina. Moreover, we found that mRNA expression of retinoic acid-specific hydroxylase CYP26b1 was dose-dependently up-regulated by atRal exposure in RPE cells, indicating that atRA inactivation may be also initiated in atRal-accumulated RPE cells. Our data show that atRA-caused viability inhibition was evidently reduced compared with the equal concentration of its precursor atRal. Excess accumulation of atRal provoked intracellular reactive oxygen species (ROS) overproduction, heme oxygenase-1 (HO-1) expression, and increased cleaved poly(ADP-ribose) polymerase 1 (PARP1) expression in RPE cells. In contrast, comparable dosage of atRA-induced oxidative stress was much weaker, and it could not activate apoptosis in RPE cells. These results suggest that atRA generation is an antidotal metabolism pathway for atRal in the retina. Moreover, we found that in the eyes of ABCA4^-/-RDH8^-/- mice, a mouse model with atRal accumulation in the retina, the atRA content was almost the same as that in the wild type. It is possible that atRal accumulation simultaneously and equally promotes atRA synthesis and clearance in eyes of ABCA4^-/-RDH8^-/- mice, thus inhibiting the further increase of atRA in the retina. Our present study provides further insights into atRal clearance in the retina.

Adeno-associated virus (AAV)-mediated neuroprotective effects on the degenerative retina: the therapeutic potential of erythropoietin

Retinal degeneration (RD) results in photoreceptor loss and irreversible visual impairments. This study sought to alleviate the photoreceptor degeneration via the adeno-associated virus (AAV)-mediated erythropoietin (EPO) therapy. AAV-2/2-mCMV-EPO vectors were constructed and delivered into the subretinal space of a RD model. The retinal morphology, optokinetic behaviour and electrophysiological function of the treated animals were analysed. The subretinal delivery of AAV-2/2 vectors induced robust EPO gene expressions in the retinas. AAV2/2-mediated EPO therapy ameliorated the photoreceptor degeneration and visual impairments of the RD animal model. Furthermore, the multi-electrodes array (MEA) was used to detect the firing activities of retinal ganglion cells. MEA recording showed that the EPO therapy could restrain the spontaneous firing response, enhance the light-induced firing response and preserve the basic configurations of visual signal pathway in RD model. Our MEA assay provided an example to evaluate the potency of pharmacological compounds on retinal plasticity. In conclusion, AAV2/2-mediated EPO therapy can ameliorate the photoreceptor degeneration and rectify the abnormities in visual signal transmission. These beneficial results suggest the AAV vector is a viable therapeutic option for retinopathies with rapidly degenerating kinetics and lay the groundwork for future development of EPO gene therapy.

Simvastatin protects photoreceptors from oxidative stress induced by all-trans-retinal, through the up-regulation of interphotoreceptor retinoid binding protein

BACKGROUND AND PURPOSE

Simvastatin is a 3-hydroxy-3-methylglutaryl CoA reductase inhibitor with multiple targets and effects. It protects neurons in the brain, but its protective effects on photoreceptors are unclear. In this study, we evaluated the neuroprotective effect of simvastatin on photoreceptors exposed to stress induced by all-trans-retinal (atRAL).

EXPERIMENTAL APPROACH

AlamarBlue and LDH assays were used to evaluate the viability and metabolic activity of Y79 cells (a retinoblastoma cell line) exposed to atRAL-induced stress with or without simvastatin pretreatment. Changes in cellular ROS were evaluated using flow cytometry and mitochondrial stress markers JC-1 and HSP60. Changes in levels of two photoreceptor-specific markers, cone-rod homeobox protein (CRX) and interphotoreceptor retinoid binding protein (IRBP), were evaluated with western blot. The results were validated in ex vivo human retinal explants and a mouse model of photoreceptor degeneration.

KEY RESULTS

Simvastatin improved mitochondrial function, alleviated oxidative stress and up-regulated the photoreceptor-specific markers IRBP and its upstream regulator CRX in Y79 cells and ex vivo human retinal explants under atRAL-induced stress. Simvastatin attenuated photoreceptor degeneration in association with up-regulation of IRBP and CRX expression after knockdown of IRBP in a murine model.

CONCLUSION AND IMPLICATIONS

Our findings suggest that simvastatin has a novel role in protecting photoreceptors from atRAL-induced stress. Simvastatin treatment resulted in up-regulation of IRBP and its upstream transcription factor CRX in Y79 cells, ex vivo human retinal explants, and murine retinas in vivo. Further studies of simvastatin to treat photoreceptor degeneration are warranted.

Photo-Oxidative Blue-Light Stimulation in Retinal Pigment Epithelium Cells Promotes Exosome Secretion and Increases the Activity of the NLRP3 Inflammasome

PURPOSE

Age-related macular degeneration (AMD) is a major cause of blindness in the elderly, and the activation of the NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasome is involved in AMD pathogenesis. We investigated whether photooxidative blue-light stimulation in retinal pigment epithelium (RPE) cells promotes exosome secretion and modulates the activity of the NLRP3 inflammasome in vitro.

METHODS

Exosomes were isolated from ARPE-19 cultures stimulated or not with blue-light photostimulation (488 nm). Isolated exosomes were characterized by transmission electron microscope and Western blot analyses. The contents of the NLRP3 inflammasome (IL-1β, IL-18, and caspase-1 as markers of the inflammasome) in exosomes were analyzed by Western blotting. After culture, IL-1β, IL-18, and caspase-1 in RPE cells were analyzed by both immunofluorescence and Western blotting. RT-PCR and Western blotting were conducted to assess the contents of NLRP3 in RPE cells.

RESULTS

Exosomes exhibited a typical characteristic morphology (cup-shaped) and size (diameter between 50 and 150 nm) in both groups. The exosome markers CD9, CD63, and CD81 were strongly present. After blue-light photostimulation, ARPE-19 cells were noted to release exosomes with higher levels of IL-1β, IL-18, and caspase-1 than those in the control group. The levels of IL-1β, IL-18, and caspase-1 in ARPE-19 cells were significantly enhanced when treated with stressed RPE exosomes. Additionally, the NLRP3 mRNA and protein levels were found to be markedly higher in the treated group than in the control group.

CONCLUSIONS

Under photooxidative blue-light stimulation, RPE-derived exosomes may aggravate a potentially harmful oxidative response through the upregulation of the NLRP3 inflammasome.

Paeoniflorin attenuates atRAL-induced oxidative stress, mitochondrial dysfunction and endoplasmic reticulum stress in retinal pigment epithelial cells via triggering Ca2+/CaMKII-dependent activation of AMPK

Abnormal accumulation of the free-form all-trans-retinal (atRAL), a major intermediate of human visual cycle, is considered to be a key cause of retinal pigment epithelial (RPE) dysfunction in the pathogenesis of retinal degenerative diseases such as age-related macular degeneration (AMD). Paeoniflorin (PF), a monoterpene glucoside isolated from Paeonia lactiflora Pall., has been used in clinical treatment of retinal degenerative diseases in China for several years; however, the underlying mechanism remains unclear. The aim of this study is to investigate the protective effect of PF against atRAL toxicity in human ARPE-19 cells and its molecular mechanism. The results of our study showed that the pre-treatment of PF dose-dependently attenuated atRAL-induced cell injury by the reduction of Nox1/ROS-associated oxidative stress, mitochondrial dysfunction and GRP78-PERK-eIF2α-ATF4-CHOP-regulated endoplasmic reticulum (ER) stress in ARPE-19 cells. Additionally, our data showed that PF mainly exerted its activity via triggering calcium-calmodulin dependent protein kinase II (CaMKII)-mediated activation of AMP-activated protein kinase (AMPK). AMPK inhibition significantly reversed the protective effect of PF against atRAL toxicity in ARPE-19 cells. Overall, our findings provided the novel mechanism of PF protecting human RPE cells, which may prevent the progression of retinal degenerative diseases.

Chrysin Ameliorates Malfunction of Retinoid Visual Cycle through Blocking Activation of AGE-RAGE-ER Stress in Glucose-Stimulated Retinal Pigment Epithelial Cells and Diabetic Eyes

Diabetes-associated visual cycle impairment has been implicated in diabetic retinopathy, and chronic hyperglycemia causes detrimental effects on visual function. Chrysin, a naturally occurring flavonoid found in various herbs, has anti-inflammatory, antioxidant, and neuroprotective properties. The goal of the current study was to identify the retinoprotective role of chrysin in maintaining robust retinoid visual cycle-related components. The in vitro study employed human retinal pigment epithelial (RPE) cells exposed to 33 mM of glucose or advanced glycation end products (AGEs) in the presence of 1–20 μM chrysin for three days. In the in vivo study, 10 mg/kg of chrysin was orally administrated to db/db mice. Treating chrysin reversed the glucose-induced production of vascular endothelial growth factor, insulin-like growth factor-1, and pigment epithelium-derived factor (PEDF) in RPE cells. The outer nuclear layer thickness of chrysin-exposed retina was enhanced. The oral gavage of chrysin augmented the levels of the visual cycle enzymes of RPE65, lecithin retinol acyltransferase (LRAT), retinol dehydrogenase 5 (RDH5), and rhodopsin diminished in db/db mouse retina. The diabetic tissue levels of the retinoid binding proteins and the receptor of the cellular retinol-binding protein, cellular retinaldehyde-binding protein-1, interphotoreceptor retinoid-binding protein and stimulated by retinoic acid 6 were restored to those of normal mouse retina. The presence of chrysin demoted AGE secretion and AGE receptor (RAGE) induction in glucose-exposed RPE cells and diabetic eyes. Chrysin inhibited the reduction of PEDF, RPE 65, LRAT, and RDH5 in 100 μg/mL of AGE-bovine serum albumin-exposed RPE cells. The treatment of RPE cells with chrysin reduced the activation of endoplasmic reticulum (ER) stress. Chrysin inhibited the impairment of the retinoid visual cycle through blocking ER stress via the AGE-RAGE activation in glucose-stimulated RPE cells and diabetic eyes. This is the first study demonstrating the protective effects of chrysin on the diabetes-associated malfunctioned visual cycle.

PGC-1α Protects RPE Cells of the Aging Retina against Oxidative Stress-Induced Degeneration through the Regulation of Senescence and Mitochondrial Quality Control. The Significance for AMD Pathogenesis

PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) is a transcriptional coactivator of many genes involved in energy management and mitochondrial biogenesis. PGC-1α expression is associated with cellular senescence, organismal aging, and many age-related diseases, including AMD (age-related macular degeneration), an important global issue concerning vision loss. We and others have developed a model of AMD pathogenesis, in which stress-induced senescence of retinal pigment epithelium (RPE) cells leads to AMD-related pathological changes. PGC-1α can decrease oxidative stress, a key factor of AMD pathogenesis related to senescence, through upregulation of antioxidant enzymes and DNA damage response. PGC-1α is an important regulator of VEGF (vascular endothelial growth factor), which is targeted in the therapy of wet AMD, the most devastating form of AMD. Dysfunction of mitochondria induces cellular senescence associated with AMD pathogenesis. PGC-1α can improve mitochondrial biogenesis and negatively regulate senescence, although this function of PGC-1α in AMD needs further studies. Post-translational modifications of PGC-1α by AMPK (AMP kinase) and SIRT1 (sirtuin 1) are crucial for its activation and important in AMD pathogenesis.

Tunicamycin-induced Endoplasmic Reticulum Stress Upregulates the Expression of Pentraxin 3 in Human Retinal Pigment Epithelial Cells

PURPOSE

To investigate the production of long pentraxin 3 (PTX3) in response to tunicamycin-induced endoplasmic reticulum (ER) stress and its role in ER stress-associated cell death, PTX3 expression was evaluated in the human retinal pigment epithelial cell line, ARPE-19.

METHODS

PTX3 production in ARPE-19 cells was analyzed in the absence or presence of tunicamycin treatment by enzyme-linked immunosorbent assay. PTX3 protein and mRNA levels were estimated using western blot analysis and real-time reverse transcription-polymerase chain reaction, respectively. Protein and mRNA levels of CCAAT-enhancer-binding protein homologous protein (CHOP) and ARPE-19 cell viability were measured in the presence of tunicamycin-induced ER stress in control or PTX3 small hairpin RNA (shRNA)-transfected ARPE-19 cells.

RESULTS

The protein and mRNA levels of PTX3 were found to be significantly increased by tunicamycin treatment. PTX3 production was significantly decreased in inositol-requiring enzyme 1α shRNA-transfected ARPE-19 cells compared to control shRNA-transfected cells. Furthermore, pretreatment with the NF-κB inhibitor abolished tunicamycin-induced PTX3 production. Decreased cell viability and prolonged protein and mRNA expression of CHOP were observed under tunicamycin-induced ER stress in PTX3 shRNA transfected ARPE-19 cells.

CONCLUSIONS

These results suggest that PTX3 production increased in the presence of tunicamycin-induced ER stress. Therefore, PTX3 could be an important protector of ER stress-induced cell death in human retinal pigment epithelial cells. Inositol-requiring enzyme 1α and the NF-κB signaling pathway may serve as potential targets for regulation of PTX3 expression in the retina. Therefore, their role in PTX3 expression needs to be further investigated.

All-trans-retinal dimer formation alleviates the cytotoxicity of all-trans-retinal in human retinal pigment epithelial cells

Effective clearance of all-trans-retinal (atRAL) from retinal pigment epithelial (RPE) cells is important for avoiding its cytotoxicity. However, the metabolism of atRAL in RPE cells is poorly clarified. The present study was designed to analyze metabolic products of atRAL and to compare the cytotoxicity of atRAL versus its derivative all-trans-retinal dimer (atRAL-dimer) in human RPE cells. We found that all-trans-retinol (atROL) and a mixture of atRAL condensation metabolites including atRAL-dimer and A2E were generated after incubating RPE cells with atRAL for 6h, and the amount of atRAL-dimer was significantly higher than that of A2E. In the eyes of Rdh8^-/- Abca4^-/- mice, a mouse model with defects in retinoid cycle that displays some symbolic characteristics of age-related macular degeneration (AMD), the level of atRAL-dimer was increased compared to wild-type mice, and was even much greater than that of A2E & isomers. The cytotoxicity of atRAL-dimer was reduced compared with its precursor atRAL. The latter could provoke intracellular reactive oxygen species (ROS) overproduction, increase the mRNA expression of several oxidative stress related genes (Nrf2, HO-1, and γ-GCSh), and induce ΔΨm loss in RPE cells. By contrast, the abilities of atRAL-dimer to induce intracellular ROS and oxidative stress were much weaker versus that of concentration-matched atRAL, and atRAL-dimer exhibited no toxic effect on mitochondrial function at higher concentrations. In conclusion, the formation of atRAL-dimer during atRAL metabolic process ameliorates the cytotoxicity of atRAL by reducing oxidative stress.

Dengue-induced autophagy, virus replication and protection from cell death require ER stress (PERK) pathway activation

A virus that reproduces in a host without killing cells can easily establish a successful infection. Previously, we showed that dengue-2, a virus that threatens 40% of the world, induces autophagy, enabling dengue to reproduce in cells without triggering cell death. Autophagy further protects the virus-laden cells from further insults. In this study, we evaluate how it does so; we show that dengue upregulates host pathways that increase autophagy, namely endoplasmic reticulum (ER) stress and ataxia telangiectasia mutated (ATM) signaling followed by production of reactive oxygen species (ROS). Inhibition of ER stress or ATM signaling abrogates the dengue-conferred protection against other cell stressors. Direct inhibition of ER stress response in infected cells decreases autophagosome turnover, reduces ROS production and limits reproduction of dengue virus. Blocking ATM activation, which is an early response to infection, decreases transcription of ER stress response proteins, but ATM has limited impact on production of ROS and virus titers. Production of ROS determines only late-onset autophagy in infected cells and is not necessary for dengue-induced protection from stressors. Collectively, these results demonstrate that among the multiple autophagy-inducing pathways during infection, ER stress signaling is more important to viral replication and protection of cells than either ATM or ROS-mediated signaling. To limit virus production and survival of dengue-infected cells, one must address the earliest phase of autophagy, induced by ER stress.

Systemic Retinaldehyde Treatment Corrects Retinal Oxidative Stress, Rod Dysfunction, and Impaired Visual Performance in Diabetic Mice

PURPOSE

Diabetes appears to induce a visual cycle defect because rod dysfunction is correctable with systemic treatment of the visual cycle chromophore 11-cis-retinaldehyde. However, later studies have found no evidence for visual cycle impairment. Here, we further examined whether photoreceptor dysfunction is corrected with 11-cis-retinaldehyde. Because antioxidants correct photoreceptor dysfunction in diabetes, the hypothesis that exogenous visual chromophores have antioxidant activity in the retina of diabetic mice in vivo was tested.

METHODS

Rod function in 2-month-old diabetic mice was evaluated using transretinal electrophysiology in excised retinas and apparent diffusion coefficient (ADC) MRI to measure light-evoked expansion of subretinal space (SRS) in vivo. Optokinetic tracking was used to evaluate cone-based visual performance. Retinal production of superoxide free radicals, generated mostly in rod cells, was biochemically measured with lucigenin. Diabetic mice were systemically treated with a single injection of either 11-cis-retinaldehyde, 9-cis-retinaldehyde (a chromophore surrogate), or all-trans-retinaldehyde (the photoisomerization product of 11-cis-retinaldehyde).

RESULTS

Consistent with previous reports, diabetes significantly reduced (1) dark-adapted rod photo responses (transretinal recording) by ∼18%, (2) rod-dominated light-stimulated SRS expansion (ADC MRI) by ∼21%, and (3) cone-dominated contrast sensitivity (using optokinetic tracking [OKT]) by ∼30%. Both 11-cis-retinaldehyde and 9-cis-retinaldehyde largely corrected these metrics of photoreceptor dysfunction. Higher-than-normal retinal superoxide production in diabetes by ∼55% was also significantly corrected following treatment with 11-cis-retinaldehyde, 9-cis-retinaldehyde, or all-trans-retinaldehyde.

CONCLUSIONS

Collectively, data suggest that retinaldehydes improve photoreceptor dysfunction in diabetic mice, independent of the visual cycle, via an antioxidant mechanism.