Mechanisms of cell death in neurodegenerative and retinal diseases: common pathway?

Role of inflammation in a rat model of radiation retinopathy

Radiation retinopathy (RR) is a major side effect of ocular tumor treatment by plaque brachytherapy or proton beam therapy. RR manifests as delayed and progressive microvasculopathy, ischemia and macular edema, ultimately leading to vision loss, neovascular glaucoma, and, in extreme cases, secondary enucleation. Intravitreal anti-VEGF agents, steroids and laser photocoagulation have limited effects on RR. The role of retinal inflammation and its contribution to the microvascular damage occurring in RR remain incompletely understood. To explore cellular and vascular events after irradiation, we analyzed their time course at 1 week, 1 month and 6 months after rat eyes received 45 Gy X-beam photons. Müller glial cells, astrocytes and microglia were rapidly activated, and these markers of retinal inflammation persisted for 6 months after irradiation. This was accompanied by early cell death in the outer retina, which persisted at later time points, leading to retinal thinning. A delayed loss of small retinal capillaries and retinal hypoxia were observed after 6 months, indicating inner blood‒retinal barrier (BRB) alteration but without cell death in the inner retina. Moreover, activated microglial cells invaded the entire retina and surrounded retinal vessels, suggesting the role of inflammation in vascular alteration and in retinal cell death. Radiation also triggered early and persistent invasion of the retinal pigment epithelium by microglia and macrophages, contributing to outer BRB disruption. This study highlights the role of progressive and long-lasting inflammatory mechanisms in RR development and demonstrates the relevance of this rat model to investigate human pathology.

The blue light hazard and its use on the evaluation of photochemical risk for domestic lighting. An in vivo study

BACKGROUND

Nowadays artificial light highly increases human exposure to light leading to circadian rhythm and sleep perturbations. Moreover, excessive exposure of ocular structures to photons can induce irreversible retinal damage. Meta-analyses showed that sunlight exposure influences the age of onset and the progression of Age-related macular degeneration (AMD), the leading cause of blindness in people over fifty-year old. Currently, the blue-light hazard (BLH) curve is used in the evaluation of the phototoxicity of a light source for domestic lighting regulations.

OBJECTIVES

Here, we analyze the phototoxicity threshold in rats and investigate the role played by the light spectrum, assessing the relevance of the use of the BLH-weighting to define phototoxicity.

METHODS

We exposed albino rats to increasing doses of blue and white light, or to lights of different colors to evaluate the impact of each component of the white light spectrum on phototoxicity. Cellular mechanisms of cell death and cellular stress induced by light were analyzed.

RESULTS

Our results show that the phototoxicity threshold currently accepted for rats is overestimated by a factor of 50 when considering blue light and by a factor of 550 concerning white light. This is the result of the toxicity induced by green light that increases white light toxicity by promoting an inflammatory response. The content of green in white light induces 8 fold more invasion of macrophages in the retina than the content of blue light. Moreover, the use of BLH-weighting does not evaluate the amount of red radiations contained in white light that mitigates damage by inhibiting the nuclear translocation of L-DNase II and reducing by 33% the number of TUNEL-positive cells.

DISCUSSION

These findings question the current methods to determine the phototoxicity of a light source and show the necessity to take into account the entire emission spectrum. As current human phototoxicity thresholds were estimated with the same methods used for rats, our results suggest that they might need to be reconsidered.

Trolox aids coenzyme Q10 in neuroprotection against NMDA induced damage via upregulation of VEGF in rat model of glutamate excitotoxicity

Glutamate induced damage to retinal ganglion cells (RGCs) requires tight physiological regulation of the N-methyl-D-aspartate (NMDA) receptors. Previously, studies have demonstrated the neuroprotective abilities of antioxidants like coenzyme Q10 (CoQ10) and vitamin E analogs like α-tocopherol against neuropathies resulting from NMDA insult, but have failed to shed light on the effect of CoQ10 and trolox, a hydrophilic analog of vitamin E, on glaucomatous neurodegeneration. In the current study, we wanted to investigate whether the combined effect of trolox with CoQ10 could alleviate NMDA-induced death of retinal cells while also trying to elucidate the underlying mechanism in relation to the yet unexplained role of vascular endothelial growth factor (VEGF) in NMDA-mediated excitotoxicity. After successful NMDA-induced degeneration, we followed it up with the treatment of combination of Trolox and CoQ10. The structural damage by NMDA was repaired significantly and retina retained structural integrity comparable to levels of control in the treatment group of Trolox and CoQ10. Detection of ROS generation after NMDA insult showed that together, Trolox and CoQ10 could significantly bring down the high levels of free radicals while also rescuing mitochondrial membrane potential (MMP). A significant increase in NMDA receptor Grin2A by CoQ10 alone as well as by CoQ10 and trolox was accompanied by a lowered Grin2B receptor expression, suggesting neuroprotective action of Trolox and CoQ10. Subsequently, lowered VEGFR1 and VEGFR2 receptor expression by NMDA treatment also recovered when subjected to combined treatment of Trolox and CoQ10. Western blot analyses also indicated the same whereby Trolox and CoQ10 could increase the diminished levels of phosphorylated VEGFR2. Immunofluorescence studies also indicated a positive correlation between recovered VEGFR2 and NMDAR2A levels and diminished levels of NMDAR2D, confirming the results obtained by RT-PCR analysis. This is the first report in our knowledge that demonstrates the efficacy of trolox in combination with CoQ10 highlighting the importance of maintaining VEGF levels that are lowered in ocular diseases due to NMDA-related toxicities.

Progesterone alters the activation and typing of the microglia in the optic nerve crush model

Microglia have a protective effect on the central nervous system (CNS), but their over-proliferation can cause secondary injury to the retina following optic nerve crush (ONC). Progesterone as a steroid gonadal hormone has been used in some experimental animal models for its neuroprotective effect. However, there is limited attention on the interactions between progesterone and microglia in retinal diseases. This study investigated the proliferation, morphology changes, and cell types of microglia at 3 days and 7 days after ONC. We found that progesterone treatment in unilateral optic nerve injury mice significantly reduced densities and morphological change of microglia at 7 days in the ganglion cell layer (GCL), especially in the retinal central. Inhibition of the microglia proliferation and transformation of ramified microglia into ameboid macrophages also appeared in the inner plexiform layer (IPL). Moreover, progesterone also regulated the TNF signal pathway, which was similar to the specific elimination of the M1 phenotype. M1 marks such as tumor necrosis factor alpha (TNF-α), inducible NOS(iNOS), interleukin-6 (IL-6), and Fc receptor (CD16 and CD32) significantly downregulated by progesterone treatment whether at 3 days or 7 days after ONC. On the other hand, progesterone continuously increased the expression of the M2 marks, including interleukin-4 (IL-4), arginase 1 (Arg1), and mannose receptor (CD206) since the third day, while the expression levels of transforming growth factor (TGF-β) only increased at 7 days. In general, this study elucidated the mechanism that progesterone prevented further damage on the retina by inhibiting proliferation, activation, and changing the type of microglia.

Deoxyribonucleases and Their Applications in Biomedicine

Extracellular DNA, also called cell-free DNA, released from dying cells or activated immune cells can be recognized by the immune system as a danger signal causing or enhancing inflammation. The cleavage of extracellular DNA is crucial for limiting the inflammatory response and maintaining homeostasis. Deoxyribonucleases (DNases) as enzymes that degrade DNA are hypothesized to play a key role in this process as a determinant of the variable concentration of extracellular DNA. DNases are divided into two families-DNase I and DNase II, according to their biochemical and biological properties as well as the tissue-specific production. Studies have shown that low DNase activity is both, a biomarker and a pathogenic factor in systemic lupus erythematosus. Interventional experiments proved that administration of exogenous DNase has beneficial effects in inflammatory diseases. Recombinant human DNase reduces mucus viscosity in lungs and is used for the treatment of patients with cystic fibrosis. This review summarizes the currently available published data about DNases, their activity as a potential biomarker and methods used for their assessment. An overview of the experiments with systemic administration of DNase is also included. Whether low-plasma DNase activity is involved in the etiopathogenesis of diseases remains unknown and needs to be elucidated.

The expression and role of PIDD in retina after optic nerve crush

To examine the expression of P53-induced protein with a death domain (PIDD) at retina in animal model of optic nerve crush (ONC) and to investigate the role of PIDD in retinal glial activation and NF-κB activation induced by optic nerve damage, ONC animal model was established in Sprague-Dawley rats. PIDD has three isoforms (Isof); Western blot was performed to examine the expression of PIDD (Isof-1, Isof-2, and Isof-3, respectively) in retina at different time points after ONC. Retinal glial activation is closely associated with retinal neuronal death and is monitored by the expression of GFAP+ glial cells and IBA1+ microglia, then activated microglia leads to inflammatory cytokine production. NF-kB activation in glial cells also can promote neuronal death. In our study, the role of PIDD in retinal glial activation and NF-kB activation was investigated with PIDD inhibition selectively. PIDD expression (Isof-1 and Isof-3) was dramatically increased, and peaked at 3 days after ONC, while Isof-2 did not show any difference. In the ONC animal model, the number of GFAP+ glial cells and IBA1+ microglia in retinal layers was increased significantly, inflammatory cytokine production was upregulated, and NF-κB in glial cell was also activated. Moreover, those responses induced by optic nerve damage were attenuated with PIDD inhibition, which indicated that PIDD could regulate retinal glial activation, neuro-inflammation, and NF-κB activation. These results provided the direct demonstration that the PIDD (Isof-1and Isof-3) was overexpressed in retina after ONC, and PIDD may be involved in retinal neurodegenerative diseases by regulating retinal glial activation and NF-κB activation.

Toxicity of blue led light and A2E is associated to mitochondrial dynamics impairment in ARPE-19 cells: implications for age-related macular degeneration

Age-related macular degeneration (AMD) is a multifactorial retinal disease characterized by a progressive loss of central vision. Retinal pigment epithelium (RPE) degeneration is a critical event in AMD. It has been associated to A2E accumulation, which sensitizes RPE to blue light photodamage. Mitochondrial quality control mechanisms have evolved to ensure mitochondrial integrity and preserve cellular homeostasis. Particularly, mitochondrial dynamics involve the regulation of mitochondrial fission and fusion to preserve a healthy mitochondrial network. The present study aims to clarify the cellular and molecular mechanisms underlying photodamage-induced RPE cell death with particular focus on the involvement of defective mitochondrial dynamics. Light-emitting diodes irradiation (445 ± 18 nm; 4.43 mW/cm²) significantly reduced the viability of both unloaded and A2E-loaded human ARPE-19 cells and increased reactive oxygen species production. A2E along with blue light, triggered apoptosis measured by MC540/PI-flow cytometry and activated caspase-3. Blue light induced mitochondrial fusion/fission imbalance towards mitochondrial fragmentation in both non-loaded and A2E-loaded cells which correlated with the deregulation of mitochondria-shaping proteins level (OPA1, DRP1 and OMA1). To our knowledge, this is the first work reporting that photodamage causes mitochondrial dynamics deregulation in RPE cells. This process could possibly contribute to AMD pathology. Our findings suggest that the regulation of mitochondrial dynamics may be a valuable strategy for treating retinal degeneration diseases, such as AMD.

Systems pathology analysis identifies neurodegenerative nature of age-related vitreoretinal interface diseases

Aging is a phenomenon that is associated with profound medical implications. Idiopathic epiretinal membrane (iEMR) and macular hole (MH) are the major vision-threatening vitreoretinal diseases affecting millions of aging people globally, making these conditions an important public health issue. iERM is characterized by fibrous tissue developing on the surface of the macula, which leads to biomechanical and biochemical macular damage. MH is a small breakage in the macula and is associated with many ocular conditions. Although several individual factors and pathways are suggested, a systems pathology level understanding of the molecular mechanisms underlying these disorders is lacking. Therefore, we performed mass spectrometry-based label-free quantitative proteomics analysis of the vitreous proteomes from patients with iERM and MH to identify the key proteins, as well as the multiple interconnected biochemical pathways, contributing to the development of these diseases. We identified a total of 1,014 unique proteins, many of which are linked to inflammation and the complement cascade, revealing the inflammation processes in retinal diseases. Additionally, we detected a profound difference in the proteomes of iEMR and MH compared to those of diabetic retinopathy with macular edema and rhegmatogenous retinal detachment. A large number of neuronal proteins were present at higher levels in the iERM and MH vitreous, including neuronal adhesion molecules, nervous system development proteins, and signaling molecules, pointing toward the important role of neurodegenerative component in the pathogenesis of age-related vitreoretinal diseases. Despite them having marked similarities, several unique vitreous proteins were identified in both iERM and MH, from which candidate targets for new diagnostic and therapeutic approaches can be provided.

Drug delivery devices for retinal diseases

Retinal degenerative diseases are a leading cause of irreversible blindness and visual impairment, affecting millions of people worldwide. Although intravitreal injection can directly deliver drugs to the posterior segment of the eye, it is invasive and associated with serious side effects. The design of drug delivery systems targeting the posterior segment of the eye in a less invasive manner has still been challenging because of various anatomical and physiological barriers. In this review, we provide an overview of the current implant device-based approaches used for treating retinal degenerative diseases. We then offer our perspectives on future directions and challenges that remain for developing more effective device-based therapies for retinal diseases.

Effects of white light-emitting diode (LED) exposure on retinal pigment epithelium in vivo

Ageing and alteration of the functions of the retinal pigment epithelium (RPE) are at the origin of lost of vision seen in age-related macular degeneration (AMD). The RPE is known to be vulnerable to high-energy blue light. The white light-emitting diodes (LED) commercially available have relatively high content of blue light, a feature that suggest that they could be deleterious for this retinal cell layer. The aim of our study was to investigate the effects of "white LED" exposure on RPE. For this, commercially available white LEDs were used for exposure experiments on Wistar rats. Immunohistochemical stain on RPE flat mount, transmission electron microscopy and Western blot were used to exam the RPE. LED-induced RPE damage was evaluated by studying oxidative stress, stress response pathways and cell death pathways as well as the integrity of the outer blood-retinal barrier (BRB). We show that white LED light caused structural alterations leading to the disruption of the outer blood-retinal barrier. We observed an increase in oxidized molecules, disturbance of basal autophagy and cell death by necrosis. We conclude that white LEDs induced strong damages in rat RPE characterized by the breakdown of the BRB and the induction of necrotic cell death.

The hidden side of SERPINB1/Leukocyte Elastase Inhibitor

SERPINB1, also called Leukocyte Elastase Inhibitor (LEI) is a member of the clade B of SERPINS. It is an intracellular protein and acts primarily to protect the cell from proteases released into the cytoplasm during stress. Its role in inflammation is clear due to its involvement in the resolution of chronic inflammatory lung and bowel diseases. LEI/SERPINB1 intrinsically possesses two enzymatic activities: an antiprotease activity dependent on its reactive site loop, which is analogous to the other proteins of the family and an endonuclease activity which is unveiled by the cleavage of the reactive site loop. The conformational change induced by this cleavage also unveils a bipartite nuclear localization signal allowing the protein to translocate to the nucleus. Recent data indicate that it has also a role in cell migration suggesting that it could be involved in diverse processes like wound healing and malignant metastases.

Mitochondria mediates caspase-dependent and independent retinal cell death in Staphylococcus aureus endophthalmitis

Bacterial endophthalmitis, a vision-threatening complication of ocular surgery or trauma, is characterized by increased intraocular inflammation and retinal tissue damage. Although significant vision loss in endophthalmitis has been linked to retinal cell death, the underlying mechanisms of cell death remain elusive. In this study, using a mouse model of Staphylococcus aureus endophthalmitis and cultured human retinal Müller glia (MIO-M1 cell line), we demonstrate that S. aureus caused significant apoptotic cell death in the mouse retina and Müller glia, as evidenced by increased number of terminal dUTP nick end labeling and Annexin V and propidium iodide-positive cells. Immunohistochemistry and western blot studies revealed the reduction in mitochondrial membrane potential (JC-1 staining), release of cytochrome c into the cytosol, translocation of Bax to the mitochondria and the activation of caspase-9 and -3 in S. aureus-infected retina/retinal cells. In addition, the activation of PARP-1 and the release of apoptosis inducing factor from mitochondria was also observed in S. aureus-infected retinal cells. Inhibition studies using pan-caspase (Q-VD-OPH) and PARP-1 (DPQ) inhibitors showed significant reduction in S. aureus-induced retinal cell death both in vivo and in vitro. Together, our findings demonstrate that in bacterial endophthalmitis, retinal cells undergo apoptosis in the both caspase-dependent and independent manners, and mitochondria have a central role in this process. Hence, targeting the identified signaling pathways may provide the rationale to design therapeutic interventions to prevent bystander retinal tissue damage in bacterial endophthalmitis.