TXNIP regulates mitophagy in retinal Müller cells under high-glucose conditions: implications for diabetic retinopathy

Dysregulation of the NLRP3 Inflammasome in Diabetic Retinopathy and Potential Therapeutic Targets

Diabetic Retinopathy (DR) is an insidious neurovascular disorder secondary to chronic glycemic dysregulation in elderly diabetic patients. In the later stages of DR, the disease manifests as fluid infiltrating the macula, culminating in the leading cause of irreversible visual impairment in working age adults. With the current mainstay treatments preoccupied with slowing down the progression of DR, this presents an unsustainable solution from both an economic and quality of life perspective. Although the exact mechanisms by which hyperglycemia leads to retinal tissue insult are unknown, the evidence suggests that chronic low-grade inflammation in diabetic eye is in part driving the constellation of symptoms present in DR. Of the innate immune system within the eye, the NLR Family Pyrin Domain Containing 3 Inflammasome (NLRP3) has been identified in retinal cells as a causal factor in the pathogenesis of DR. Multiple pathways appear to be present in the diabetic eye that instigate prolonged activation of the NLRP3 which subsequently exerts its deleterious effects by upregulating the release of Interleukin-1Beta and Interleukin-18. In this review, we highlight the current understanding of the pathophysiology of DR, the dysregulation of the NLRP3 secondary to hyperglycemic stress in retinal cells, and novel therapeutic targets to alleviate overactivation of the inflammasome.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Diabetic Retinopathy: Mitochondria Caught in a Muddle of Homocysteine

Diabetic retinopathy is one of the most feared complications of diabetes. In addition to the severity of hyperglycemia, systemic factors also play an important role in its development. Another risk factor in the development of diabetic retinopathy is elevated levels of homocysteine, a non-protein amino acid, and hyperglycemia and homocysteine are shown to produce synergistic detrimental effects on the vasculature. Hyperhomocysteinemia is associated with increased oxidative stress, and in the pathogenesis of diabetic retinopathy, oxidative stress-mitochondrial dysfunction precedes the development of histopathology characteristic of diabetic retinopathy. Furthermore, homocysteine biosynthesis from methionine forms S-adenosyl methionine (SAM), and SAM is a co-substrate of DNA methylation. In diabetes, DNA methylation machinery is activated, and mitochondrial DNA (mtDNA) and several genes associated with mitochondrial homeostasis undergo epigenetic modifications. Consequently, high homocysteine, by further affecting methylation of mtDNA and that of genes associated with mtDNA damage and biogenesis, does not give any break to the already damaged mitochondria, and the vicious cycle of free radicals continues. Thus, supplementation of sensible glycemic control with therapies targeting hyperhomocysteinemia could be valuable for diabetic patients to prevent/slow down the development of this sight-threatening disease.

Ginseng-Sanqi-Chuanxiong (GSC) Extracts Ameliorate Diabetes-Induced Endothelial Cell Senescence through Regulating Mitophagy via the AMPK Pathway

Vascular endothelial senescence induced by high glucose and palmitate (HG/PA) contributes to endothelial dysfunction, which leads to diabetic cardiovascular complications. Reduction of endothelial senescence may attenuate these pathogenic processes. This study is aimed at determining whether Ginseng-Sanqi-Chuanxiong (GSC) extracts, traditional Chinese medicine, can ameliorate human aortic endothelial cell (HAEC) senescence under HG/PA-stressed conditions and further explore the underlying mechanism. We found that GSC extracts significantly increased antisenescent activity by reducing the HG/PA-induced mitochondrial ROS (mtROS) levels in senescent HAECs. GSC extracts also induced cellular mitophagy formation, which mediated the effect of GSC extracts on mtROS reduction. Apart from this, the data showed that GSC extracts stimulated mitophagy via the AMPK pathway, and upon inhibition of AMPK by pharmacological and genetic inhibitors, GSC extract-mediated mitophagy was abolished which further led to reverse the antisenescence effect. Taken together, these data suggest that GSC extracts prevent HG/PA-induced endothelial senescence and mtROS production by mitophagy regulation via the AMPK pathway. Thus, the induction of mitophagy by GSC extracts may provide a novel therapeutic candidate for cardiovascular protection in metabolic syndrome.

Metformin Protects ARPE-19 Cells from Glyoxal-Induced Oxidative Stress

The protective effects and mechanisms of metformin against oxidative stress were evaluated both in vivo and in vitro. ARPE-19 cells comprised the normal group, the glyoxal-treated group (0.5 mM glyoxal), and the glyoxal+metformin group (0.5 mM glyoxal and 0.1 mM metformin). In the in vitro model, differences in cell viability, ROS production, NO products, cellular apoptosis, and the expressions of phospho-AMPKα, total-AMPKα, Sirt1, Nrf2, TXNIP, ZO-1, and Occludin were assessed. In the glyoxal-treated group, cell viability and NO production were decreased, while ROS production and cell apoptosis were increased (p < 0.05), compared with the control group. These changes were prevented by metformin treatment. Protein expressions of phospho-AMPKα, Sirt1, TXNIP, ZO-1, and Occludin, but not Nrf2, were decreased significantly in the glyoxal-treated group compared to normal controls. Metformin treatment significantly increased the above protein expressions and slightly increased TXNIP expression. Immunofluorescence showed that metformin prevented the glyoxal-induced, disorganized tight junctions in ARPE-19 cells. To confirm metformin's protection, Sprague-Dawley rats were injected intravenously with sodium iodate (SI) to induce oxidative stress in the retinal pigment epithelium (RPE). Metformin was then delivered intraperitoneally or intravitreally. One day and three days after SI and metformin treatments, the RPE-Bruch's membrane-choriocapillaris complex was isolated and immune-stained with ZO-1 antibodies. The morphology of the RPE showed enlarged cellular bodies and disorganized ZO-1 staining in SI-treated rats. Metformin treatment prevented these changes. The results indicated that metformin maintained the barrier functions of RPE cells both in vivo and in vitro. Metformin exerted its protection against oxidative stress possibly via activating AMPK/Sirt1 and increasing TXNIP. Metformin has been proposed as a candidate drug for age-related macular degeneration (AMD) by both preclinical and clinical studies. The cellular and animal models used in this study might be useful for the interpretation of the molecular mechanisms involved in the drug activity.

Impairment of Autophagy Causes Superoxide Formation and Caspase Activation in 661 W Cells, a Cell Line for Cone Photoreceptors, under Hyperglycemic Conditions

Microvascular changes are the earliest adverse events in diabetic retinopathy, but recent studies have shown that oxidative stress induced by photoreceptors is associated with the development of the retinopathy. The purpose of this study was to determine the roles played by superoxides formed by photoreceptors under hyperglycemic conditions on autophagy. To accomplish this, we cultured 661 W cells, a transformed murine cone cell line, with 5.5 or 25 mM glucose in the presence or absence of 3 methyl adenine (3MA) or rapamycin. The superoxides were determined by flow cytometry using hydroethidine as a fluorescence probe. The autophagy activity was determined by changes in the expression of LC3B2 and P62 by immunoblotting. The degree of mitophagy was determined by the accumulation of mitochondria and lysosomes. Apoptotic changes of 661 W cells were determined by the caspase 3/7 activities. Our results showed higher levels of P62 and superoxides in cells cultured in 25 mM glucose than in 5.5 mM glucose. Addition of 3MA caused a significant increase of P62, superoxides, and caspase 3/7 activities in the 661 W cells cultured in high glucose but not in low glucose. These findings suggest that autophagy is important for the functioning and survival of 661 W cells under hyperglycemic conditions.

A prodrug of epigallocatechin-3-gallate alleviates high glucose-induced pro-angiogenic factor production by inhibiting the ROS/TXNIP/NLRP3 inflammasome axis in retinal Müller cells

Diabetic retinopathy (DR) is a neurovascular complication of diabetes mellitus that leads to blindness in the working-age population. Retinal Müller cells proliferate and produce pro-angiogenic factors, including vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF), via the reactive oxygen species (ROS)/thioredoxin interacting protein (TXNIP)/NACHT, LRR and PYD domain-containing protein 3 (NLRP3) inflammasome axis to promote proliferative DR. Epigallocatechin-3-gallate (EGCG) plays anti-oxidant, anti-inflammatory, anti-proliferative and anti-angiogenic roles in Müller cells. A prodrug of EGCG (pro-EGCG) enhances the bioavailability of EGCG. In an in vitro model of high glucose-stimulated Müller cells, pro-EGCG inhibited proliferation and pro-angiogenic factor production by down-regulating the activity of the ROS/TXNIP/NLRP3 inflammasome axis. In a mouse DR model, pro-EGCG reduced ROS accumulation, NLRP3 inflammasome activation, Müller cell proliferation, and production of the pro-angiogenic factors VEGF and HGF. In summary, pro-EGCG mitigated hyperglycaemia-challenged Müller cell proliferation and pro-angiogenic factor production by inhibiting ROS/TXNIP/NLRP3 inflammasome signalling, implying a potential therapeutic strategy for DR.

Tetrahydroxy stilbene glycoside alleviated inflammatory damage by mitophagy via AMPK related PINK1/Parkin signaling pathway

Alzheimer's disease (AD) is an irreversible neurodegenerative brain disorder with complex pathogenesis. The fibrillar peptide β-amyloid (Aβ) has a chief function in the pathogenesis of AD. Emerging evidence has indicated that there is a tight relationship between inflammation, mitochondrial dysfunction and Aβ formation. 2,3,5,4'-Tetrahydroxystilbene-2-O-β-D-glucoside (TSG) is one of the main active components extracted from Polygonum multiflorum. Recent research corroborated the beneficial roles of TSG in alleviating the learning and memory of AD models. Unfortunately, the underlying mechanism of TSG remains poorly elucidated. The purpose of the present study was to investigate the effects of TSG on LPS/ATP and Aβ_25-35-induced inflammation in microglia and neurons and its underlying molecular mechanisms. Our results found that treatment with TSG significantly attenuated the secretion of inflammatory cytokines, reduced NLRP3 inflammasome, and regulated mitophagy. TSG efficiently alleviated LPS-induced inflammatory response by inhibiting the NLRP3 signaling pathway both in microglia and neuron. Meanwhile, TSG promoted autophagy involved in the AMPK/PINK1/Parkin signaling pathway, which may contribute to the protective activity. Additional mechanistic investigations to evaluate the dependence of the neuroprotective role of TSG on PINK1 revealed that a lack of PINK1 inhibited autophagy, especially mitophagy in microglia. Importantly, knockdown of PINK1 or Parkin by siRNA or CRISPR/Cas9 system abolished the protective effects of TSG. In conclusion, these phenomena suggested that TSG prevented LPS/ATP and Aβ-induced inflammation via AMPK/PINK1/Parkin-dependent enhancement of mitophagy. We found the neuroprotective effect of TSG, suggesting it may be beneficial for AD prevention and treatment by suppressing the activation of inflammation.

Elevated microRNA-20b-3p and reduced thioredoxin-interacting protein ameliorate diabetic retinopathy progression by suppressing the NLRP3 inflammasomes

Over the years, microRNA-20b-3p (miR-20b-3p) has been found to play an essential role in human diseases; we aimed to investigate the effect of miR-20b-3p on the progression of diabetic retinopathy (DR). The DR rat models were established by streptozotocin injection and treated with miR-20b-3p mimics, silenced, or overexpressed thioredoxin-interacting protein (TXNIP). Afterward, the expression of miR-20b-3p and TXNIP, visual function, inflammatory factors, microvascular injury, vascular permeability, cell apoptosis, and angiogenesis in rats' retinal tissues were assessed. The target relation between miR-20b-3p and TXNIP was confirmed by dual luciferase reporter gene assay. MiR-20b-3p was poorly expressed while TXNIP was highly expressed in DR rats' retinal tissues. Elevated miR-20b-3p and inhibited TXNIP promoted the visual function, and restricted the inflammatory reaction, microvascular injury, vascular permeability, cell apoptosis, and angiogenesis in DR rats, thereby decelerating the development of DR. Furthermore, TXNIP was targeted by miR-20b-3p. We have found in this study that elevated miR-20b-3p could repress the levels of inflammatory factors by inhibiting TXNIP, thus attenuating the pathology of retina in DR rats, which provided new candidates for DR treatment.

NOD-like Receptors in the Eye: Uncovering Its Role in Diabetic Retinopathy

Diabetic retinopathy (DR) is an ocular complication of diabetes mellitus (DM). International Diabetic Federations (IDF) estimates up to 629 million people with DM by the year 2045 worldwide. Nearly 50% of DM patients will show evidence of diabetic-related eye problems. Therapeutic interventions for DR are limited and mostly involve surgical intervention at the late-stages of the disease. The lack of early-stage diagnostic tools and therapies, especially in DR, demands a better understanding of the biological processes involved in the etiology of disease progression. The recent surge in literature associated with NOD-like receptors (NLRs) has gained massive attraction due to their involvement in mediating the innate immune response and perpetuating inflammatory pathways, a central phenomenon found in the pathogenesis of ocular diseases including DR. The NLR family of receptors are expressed in different eye tissues during pathological conditions suggesting their potential roles in dry eye, ocular infection, retinal ischemia, cataract, glaucoma, age-related macular degeneration (AMD), diabetic macular edema (DME) and DR. Our group is interested in studying the critical early components involved in the immune cell infiltration and inflammatory pathways involved in the progression of DR. Recently, we reported that NLRP3 inflammasome might play a pivotal role in the pathogenesis of DR. This comprehensive review summarizes the findings of NLRs expression in the ocular tissues with special emphasis on its presence in the retinal microglia and DR pathogenesis.

Mitochondrial Defects Drive Degenerative Retinal Diseases

Mitochondrial dysfunction is involved in the pathology of two major blinding retinal diseases, diabetic retinopathy (DR) and age-related macular degeneration (AMD). These diseases accumulate mitochondrial defects in distinct retinal subcellular structures, the vascular/neural network in DR and the retinal pigment epithelium (RPE) in AMD. These mitochondrial defects cause a metabolic crisis that drives disease. With no treatments to stop these diseases, coupled with an increasing population suffering from AMD and DR, there is an urgent need to develop new therapeutics targeting the mitochondria to prevent or reverse disease-specific pathology.

Uncoupled turnover disrupts mitochondrial quality control in diabetic retinopathy

Mitochondrial quality control (MQC) is crucial for regulating CNS homeostasis, and its disruption has been implicated in the pathogenesis of some of the most common neurodegenerative diseases. In healthy tissues, the maintenance of MQC depends upon an exquisite balance between mitophagy (removal of damaged mitochondria by autophagy) and biogenesis (de novo synthesis of mitochondria). Here, we show that mitophagy is disrupted in diabetic retinopathy (DR) and decoupled from mitochondrial biogenesis during the progression of the disease. Diabetic retinas from human postmortem donors and experimental mice exhibit a net loss of mitochondrial contents during the early stages of the disease process. Using diabetic mitophagy-reporter mice (mitoQC-Ins2^Akita) alongside pMitoTimer (a molecular clock to address mitochondrial age dynamics), we demonstrate that mitochondrial loss arose due to an inability of mitochondrial biogenesis to compensate for diabetes-exacerbated mitophagy. However, as diabetes duration increases, Pink1-dependent mitophagy deteriorates, leading to the build-up of mitochondria primed for degradation in DR. Impairment of mitophagy during prolonged diabetes is linked with the development of retinal senescence, a phenotype that blunted hyperglycemia-induced mitophagy in mitoQC primary Müller cells. Our findings suggest that normalizing mitochondrial turnover may preserve MQC and provide therapeutic options for the management of DR-associated complications.

Uncoupled mitophagy and mitochondrial biogenesis leads to mitochondrial damage in the retina during the progression of diabetes.

Mechanism of Placenta Damage in Gestational Diabetes Mellitus by Investigating TXNIP of Patient Samples and Gene Functional Research in Cell Line

INTRODUCTION

Gestational diabetes mellitus (GDM) is a gestational complication that affects maternal and child health. The placenta provides the fetus with the necessary nutrition and oxygen and takes away the metabolic waste. Patients with GDM are diagnosed and treated merely on the basis of the blood glucose level; this approach does nothing to help evaluate the status of the placenta, which is worth noting in GDM. The purpose of this research was to clarify the relation between thioredoxin-interacting protein (TXNIP) and reactive oxygen species (ROS) in the placenta of patients with GDM, which has thus far remained unclear.

METHODS

The expression of TXNIP in the placentas of 10 patients with GDM and 10 healthy puerperae (control group) was investigated via immunofluorescence. The relation among TXNIP, ROS, and the function of mitochondria was explored in HTR-8/SVneo cells stimulated by high glucose (HG).

RESULTS

The results showed the expression of TXNIP in the placentas of patients with GDM was higher than that in the control group, and the expression of TXNIP in HTR-8/SVneo cells treated with HG was higher than that in the control group, causing the accumulation of ROS and changes of mitochondria, promoting apoptosis and inhibition of migration.

CONCLUSIONS

High expression of TXNIP caused by HG mediates the increasing ROS and the mitochondria dysfunction in GDM; this impairs the function of the placenta and is the basis for the prediction of perinatal outcome.

Auranofin Mediates Mitochondrial Dysregulation and Inflammatory Cell Death in Human Retinal Pigment Epithelial Cells: Implications of Retinal Neurodegenerative Diseases

Purpose

Photoreceptor degeneration occurs in various retinal diseases including age-related macular degeneration (AMD), Retinitis pigmentosa (RP), and diabetic retinopathy (DR). However, molecular mechanisms are not fully understood yet. The retinal pigment epithelium (RPE) forms the outer blood retinal barrier (oBRB) and supplies glucose, oxygen and nutrients from the fenestrated choriocapillaris to photoreceptors for visual function. Therefore, RPE dysfunction leads to photoreceptor injury/death and progression of blinding eye diseases. This study aims to understand the role of the thioredoxin (Trx) and its reductase (TrxR) redox signaling in human RPE dysfunction and cell death mechanism(s) in an in vitro system.

Methods

A human RPE cell line (APRE-19) was cultured in DMEM/F12 medium and treated with auranofin (AF - 4 μM, an inhibitor of TrxR) for 4 and 24 h. Mitochondrial and lysosomal function, cellular oxidative stress and NLRP3 inflammasome activity were measured using cell assays, Western blotting, and confocal microscopy. Antioxidants and anti-inflammatory compounds were tested for blocking AF effects on RPE damage. Cell death mechanisms (LDH release to culture media) were determined using necroptosis, ferroptosis and pyroptosis inhibitors. P < 0.05 was considered significant in statistical analysis.

Results

Auranofin causes mitochondrial dysfunction (Δψm↓ and ATP↓), oxidative stress (H₂O₂↑) and mitophagic flux to lysosomes. Furthermore, the lysosomal enzyme (cathepsin L) activity is reduced while that of pro-inflammatory caspase-1 (NLRP3 inflammasome) is enhanced in ARPE-19. These effects of AF on ARPE-19 are inhibited by antioxidant N-acetylcysteine (5 mM, NAC) and significantly by a combination of SS31 (mitochondrial antioxidant) and anti-inflammatory drugs (amlexanox and tranilast). AF also causes cell death as measured by cytosolic LDH release/leakage, which is not inhibited by either ferrostatin-1 or necrostatin-1 (ferroptosis and necroptosis inhibitors, respectively). Conversely, AF-induced LDH release is significantly reduced by MCC950 and Ac-YVAD-cmk (NLRP3 and Caspase-1 inhibitors, respectively), suggesting a pro-inflammatory cell death by pyroptosis.

Conclusion

The Trx/TrxR redox system is critical for RPE function and viability. We previously showed that thioredoxin-interacting protein (TXNIP) is strongly induced in DR inhibiting the Trx/TrxR system and RPE dysfunction. Therefore, our results suggest that the TXNIP-Trx-TrxR redox pathway may participate in RPE dysfunction in DR and other retinal neurodegenerative diseases.

Mitochondrial Structural Changes in the Pathogenesis of Diabetic Retinopathy

At the core of proper mitochondrial functionality is the maintenance of its structure and morphology. Physical changes in mitochondrial structure alter metabolic pathways inside mitochondria, affect mitochondrial turnover, disturb mitochondrial dynamics, and promote mitochondrial fragmentation, ultimately triggering apoptosis. In high glucose condition, increased mitochondrial fragmentation contributes to apoptotic death in retinal vascular and Müller cells. Although alterations in mitochondrial morphology have been detected in several diabetic tissues, it remains to be established in the vascular cells of the diabetic retina. From a mechanistic standpoint, our current work supports the notion that increased expression of fission genes and decreased expression of fusion genes are involved in promoting excessive mitochondrial fragmentation. While mechanistic insights are only beginning to reveal how high glucose alters mitochondrial morphology, the consequences are clearly seen as release of cytochrome c from fragmented mitochondria triggers apoptosis. Current findings raise the prospect of targeting excessive mitochondrial fragmentation as a potential therapeutic strategy for treatment of diabetic retinopathy. While biochemical and epigenetic changes have been reported to be associated with mitochondrial dysfunction, this review focuses on alterations in mitochondrial morphology, and their impact on mitochondrial function and pathogenesis of diabetic retinopathy.

Imaging and Biomarkers in Diabetic Macular Edema and Diabetic Retinopathy

PURPOSE OF REVIEW

Diabetic retinopathy (DR) is the leading cause of acquired vision loss in adults across the globe. Early identification and treatment of patients with DR is paramount for vision preservation. The aim of this review paper is to outline current and new imaging techniques and biomarkers that are valuable for clinical diagnosis and management of DR.

RECENT FINDINGS

Ultrawide field imaging and automated deep learning algorithms are recent advancements on traditional fundus photography and fluorescein angiography. Optical coherence tomography (OCT) and OCT angiography are techniques that image retinal anatomy and vasculature and OCT is routinely used to monitor response to treatment. Many circulating, vitreous, and genetic biomarkers have been studied to facilitate disease detection and development of new treatments. Recent advancements in retinal imaging and identification of promising new biomarkers for DR have the potential to increase detection, risk stratification, and treatment for patients with DR.

Type 2 diabetes-induced hyposalivation of the submandibular gland through PINK1/Parkin-mediated mitophagy

Diabetes is often accompanied by dysfunction of salivary glands. However, the molecular mechanism remains unclear. The mechanisms that underlie diabetic hyposalivation were studied by db/db mice and SMG‐C6 cells. We found morphological changes and decreased stimulated salivary flow rates of the submandibular gland (SMG) in diabetic mice. We observed structural changes and dysfunction of mitochondria. More mitophagosomes and higher expression of autophagy‐related proteins were detected. Increased levels of proteins PINK1 and Parkin indicate that PINK1/Parkin‐mediated mitophagy was activated in diabetic SMG. Consistently, high glucose (HG) induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy in cultivated SMG‐C6 cells. HG also increased reactive oxygen species (ROS) and lessened activation of antioxidants in SMG‐C6 cells. In addition, HG lowered ERK1/2 phosphorylation and HG‐induced mitophagy was decreased after ERK1/2 was activated by LM22B‐10. Altogether, these data suggest that ROS played a crucial role in diabetes‐induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy and ERK1/2 was required in HG‐induced mitophagy in SMG.

TXNIP mediates high glucose-induced mitophagic flux and lysosome enlargement in human retinal pigment epithelial cells

Thioredoxin-interacting protein (TXNIP) plays a critical role in oxidative stress, inflammation, apoptosis and the pathogenesis of diabetic retinopathy (DR). However, the role of TXNIP in high glucose-induced retinal pigment epithelium (RPE) dysfunction is still unknown. Here, we show that high glucose (HG; 25 mM,) significantly increases TXNIP expression at both the mRNA and protein levels when compared to low glucose (LG; 5.5 mM) in a human RPE cell line (ARPE-19) and primary human RPE (HRPE) cells. TXNIP upregulation is associated with mitochondrial membrane depolarization, fragmentation and mitophagic flux to lysosomes. We used confocal live-cell imaging of RPE cells expressing mt-Keima, a coral protein that emits green light in mitochondria (alkaline or neutral pH) and red light in the acidic lysosome, to measure mitophagic flux. We observed an elongated mitochondrial network of green mt-Keima under LG, which is fragmented in HG. Red mt-Keima accumulates in lysosomes as small punctate aggregations under LG in both ARPE-19 and HRPE cells, whereas they are significantly enlarged (two- to threefold) under HG. Lysosomal enlargement under HG is further illustrated by lysosomal membrane protein LAMP1-mCherry expression in both ARPE-19 and HRPE cells. Furthermore, HG causes lysosomal cathepsin L inactivation and pro-inflammatory caspase-1 activation in ARPE-19 cells. TXNIP knockdown by shRNA prevents mitochondrial fragmentation, mitophagic flux and lysosome enlargement under HG. In addition, antioxidant N-acetylcysteine (NAC) and Amlexanox (Amlx), an inhibitor of protein kinase TBK1 and of the mitophagic adaptors Optineurin (Optn) and Sequestosome 1 (p62/SQSTM1), prevent mitophagic flux and lysosome enlargement. These results suggest that TXNIP mediates several deleterious effects of high glucose on RPE, which may be implicated in the development of DR.

Early impairments in the retina of rats fed with high fructose/high fat diet are associated with glucose metabolism deregulation but not dyslipidaemia

Way of life changes such as high consumption of processed foods rich in fat and sugar and sedentary lifestyle are associated with the increasing prevalence of metabolic syndrome (MetS) that affects about 35% in the American population. MetS is the main risk factor for diabetes mellitus, which is associated with vascular changes in the retina. However, the early consequences of MetS in the retina are not well described. We therefore aimed at characterizing the early effects of a high fructose and high fat diet (HFHF) on the function and structure of the rat retina, and evaluate the associations with metabolic changes. Brown Norway rats of 6 weeks of age were fed for 8 days, 5 weeks or 13 weeks with HFHF diet, or a standard chow. After only 4 weeks of this diet, rats exhibited a reduction in cone photoreceptor sensitivity to light. Moreover, we observed that MetS significantly exacerbated laser-induced choroidal neovascularization by 72% and 67% 2 weeks and 3 weeks post laser treatment, respectively. These retinal abnormalities were associated with deregulation of glucose metabolism but not lipid metabolism. These data showed retinal modifications in HFHF-induced MetS in the rat, at very early stage of the disease.

Novel Triapine Derivative Induces Copper-Dependent Cell Death in Hematopoietic Cancers

Triapine, an iron chelator that inhibits ribonucleotide reductase, has been evaluated in clinical trials for cancer treatment. Triapine in combination with other chemotherapeutic agents shows promising efficacy in certain hematologic malignancies; however, it is less effective against many advanced solid tumors, probably due to the unsatisfactory potency and pharmacokinetic properties. In this report, we developed a triapine derivative IC25 (10) with potent antitumor activity. 10 Preferentially inhibited the proliferation of hematopoietic cancers by inducing mitochondria reactive oxygen species production and mitochondrial dysfunction. Unlike triapine, 10 executed cytotoxic action in a copper-dependent manner. 10-Induced up-expression of thioredoxin-interacting protein resulted in decreased thioredoxin activity to permit c-Jun N-terminal kinase and p38 activation and ultimately led to the execution of the cell death program. Remarkedly, 10 showed good bioavailability and inhibited tumor growth in mouse xenograft models. Taken together, our study identifies compound 10 as a copper-dependent antitumor agent, which may be applied to the treatment of hematopoietic cancers.

Notoginsenoside R1 Ameliorates Diabetic Retinopathy through PINK1-Dependent Activation of Mitophagy

Accumulating evidence has indicated that inflammation, oxidative stress, apoptosis, and autophagy in retinal Müller cells are involved in diabetic retinopathy (DR). Notoginsenoside R1 (NGR1), a novel saponin extracted from Panax notoginseng, posesses pharmacological properties, including treating diabetic encephalopathy and improving microcirculatory disorders. Nevertheless, its beneficial effects on DR and the potential mechanism remain to be elucidated. In this study, we found retinal vascular degeneration, reduced retinal thickness, and impaired retinal function in db/db mice were all dramatically attenuated by oral treatment with NGR1 (30 mg/kg) for 12 weeks. NGR1 pretreatment also significantly inhibited apoptosis, markedly suppressed the VEGF expression, markedly increased PEDF expression and markedly inhibited oxidative stress and inflammation in rat retinal Müller cells (rMC-1) subjected to high glucose (HG) and in the retinas of db/db mice. Furthermore, NGR1 pre-treatment upregulated the level of PINK1 and Parkin, increased the LC3-II/LC3-I ratio, and downregulated the level of p62/SQSTM1 in rMC-1 cells induced by HG and in the retinas of db/db mice. Moreover, NGR1 administration enhanced the co-localization of GFP-LC3 puncta and MitoTracker in rMC-1 cells. Importantly, knockdown of PINK1 abolished the protective effects of NGR1. In conclusion, these phenomena suggested that NGR1 prevented DR via PINK1-dependent enhancement of mitophagy.

Mitochondrial dysfunctions, endothelial progenitor cells and diabetic retinopathy

AIM

Diabetic retinopathy (DR) is the leading cause of vision loss in the working age population. Endothelial progenitor cells (EPC) play a vital role in vascular damage repair. This article will review recent progress regarding mitochondrial and EPC dysfunction associated with DR.

RESULTS

EPCs represent a limited population of adult stem cells possessing vasculogenic potential postnatally; their number and function are changed in DR. Among all the function changes, mitochondrial dysfunction plays an important role in the dysregulation of EPCs, as mitochondria regulate energy balance, and cell fate determination.

CONCLUSIONS

Although the mechanism for the role of mitochondria dysregulation in EPC function remains elusive, mitochondria of EPCs represent a promising target for the treatment of the vasculopathy presented within DR.

Deletion of the stress-response protein REDD1 promotes ceramide-induced retinal cell death and JNK activation

The role of dyslipidemia in the development of retinal dysfunction remains poorly understood. Using an animal model of diet-induced obesity/pre-type 2 diabetes, we investigated molecular defects in the retina arising from consumption of a diet high in saturated fats and sugars ( i.e., a Western diet). We found that feeding mice a Western diet increased the abundance of retinal sphingolipids, attenuated protein kinase B (Akt) phosphorylation, enhanced JNK activation, and increased retinal cell death. When we used palmitate or C6-ceramide (Cer) to assess sphingolipid-mediated signaling in cultured murine and human cells, we observed similar effects on Akt, JNK, and cell death. Furthermore, both Western diet and C6-Cer exposure enhanced expression of the stress-response protein regulated in development and DNA damage response 1 (REDD1) and loss of REDD1 increased C6-Cer-induced JNK activation and cell death. Exogenous REDD1 expression repressed JNK-mediated phosphorylation in cultured cells. We found that thioredoxin-interacting protein (TXNIP) expression was elevated in REDD1-deficient cell lines and C6-Cer promoted TXNIP expression in both wild-type and REDD1-deficient cells. Likewise, TXNIP knockdown attenuated JNK activation and caspase 3 cleavage after either C6-Cer exposure or REDD1 deletion. The results support a model wherein Cer-induced REDD1 expression attenuates TXNIP-dependent JNK activation and retinal cell death.-Dai, W., Miller, W. P., Toro, A. L., Black, A. J., Dierschke, S. K., Feehan, R. P., Kimball, S. R., Dennis, M. D. Deletion of the stress-response protein REDD1 promotes ceramide-induced retinal cell death and JNK activation.

The Function of Thioredoxin-Binding Protein-2 (TBP-2) in Different Diseases

Thioredoxin-binding protein-2 (TBP-2) has an important role in the redox system, but it plays a different role in many different diseases (e.g., various cancers, diabetes mellitus (DM), cardiovascular disease, and cataracts) by influencing cell proliferation, differentiation, apoptosis, autophagy, and metabolism. Distinct transcription factors (TFs) stimulated by different factors combine with binding sites or proteins to upregulate or downregulate TBP-2 expression, in order to respond to the change in the internal environment. Most research disclosed that the main function of TBP-2 is associating with thioredoxin (Trx) to inhibit the antioxidant capacity of Trx. Furthermore, the TBP-2 located in tissues, whether normal or abnormal, has the ability to cause the dysfunctioning of cells and even death through different pathways, such as shortening the cell cycle and inducing apoptosis or autophagy. Through these studies, we found that TBP-2 promoted the development of diseases which are involved in inflammatory and oxidative damage. To a certain extent, we believe that there is some hidden connection between the biological functions which TBP-2 participates in and some distinct diseases. This review presents only a summary of the roles that TBP-2 plays in cancer, DM, cataracts, and so on, as well as its universal mechanisms. Further investigations are needed for the cell signaling pathways of the effects caused by TBP-2. A greater understanding of the mechanisms of TBP-2 could produce potential new targets for the treatment of diseases, including cancer and diabetes, cardiovascular disease, and cataracts.

Long non-coding RNA nuclear paraspeckle assembly transcript 1 inhibits the apoptosis of retina Müller cells after diabetic retinopathy through regulating miR-497/brain-derived neurotrophic factor axis

BACKGROUND

The role of long non-coding RNA in diabetic retinopathy, a serious complication of diabetes mellitus, has attracted increasing attention in recent years. The purpose of this study was to explore whether long non-coding RNA nuclear paraspeckle assembly transcript 1 was involved in the context of diabetic retinopathy and its underlying mechanisms.

RESULTS

Our results revealed that nuclear paraspeckle assembly transcript 1 was significantly downregulated in the retina of diabetes mellitus rats. Meanwhile, miR-497 was significantly increased in diabetes mellitus rats' retina and high glucose-treated Müller cells, but brain-derived neurotrophic factor was increased. We also found that high glucose-induced apoptosis of Müller cells was accompanied by the significant downregulation of nuclear paraspeckle assembly transcript 1 in vitro. Further study demonstrated that high glucose-promoted Müller cells apoptosis through downregulating nuclear paraspeckle assembly transcript 1 and downregulated nuclear paraspeckle assembly transcript 1 mediated this effect via negative regulating miR-497. Moreover, brain-derived neurotrophic factor was negatively regulated by miR-497 and associated with the apoptosis of Müller cells under high glucose.

CONCLUSION

Our results suggested that under diabetic conditions, downregulated nuclear paraspeckle assembly transcript 1 decreased the expression of brain-derived neurotrophic factor through elevating miR-497, thereby promoting Müller cells apoptosis and aggravating diabetic retinopathy.

Mitophagic Flux Deregulation, Lysosomal Destabilization and NLRP3 Inflammasome Activation in Diabetic Retinopathy: Potentials of Gene Therapy Targeting TXNIP and The Redox System

The retina being a part of the central nervous system consumes large amounts of glucose and oxygen to generate ATP for its visual function. During ATP generation in the mitochondrial electron transport chain, mitochondrial Reactive Oxygen Species (mtROS) is generated as a byproduct. Although anti-oxidants are present in the mitochondrion to counter free radicals, excess mtROS causes damage to mitochondrial proteins, mtDNA, and membrane lipids. Furthermore, damaged mitochondria are inefficient in ATP production but continue to release ROS. Mitochondrial components, when released into the cytosol, are recognized as Danger-Associated Molecular Patterns (DAMPS) by pattern recognition NOD-like receptors including the NLRP3 inflammasome. NLRP3 inflammasomes process inactive pro-caspase-1 to an active caspase-1, which cleaves pro-inflammatory IL-1β to mature IL-1β causing inflammation and premature cell death. To counter the damaging action of mtROS and inflammasomes in fully differentiated retinal cells, the removal of dysfunctional mitochondria is needed by mitophagy, a specific form of lysosomal degradation via autophagy. Nonetheless, mitophagy deregulation, lysosome destabilization and NLRP3 inflammasome activations occur in Diabetic Retinopathy (DR) causing chronic inflammation and disease progression. Recently, the Thioredoxin-interacting protein, TXNIP, has been shown to be induced strongly by high glucose and diabetes inhibiting the anti-oxidant function of Thioredoxin. Subsequently, TXNIP causes mitochondrial dysfunction, oxidative stress, mitophagy deregulation, lysosome destabilization and inflammation in DR. Therefore, gene therapies targeting TXNIP, NLRP3 and/or the redox system have potentials to prevent/slow down retinal damages in DR.

Therapeutic targets for altering mitochondrial dysfunction associated with diabetic retinopathy

INTRODUCTION

Retinopathy remains as one of the most feared blinding complications of diabetes, and with the prevalence of this life-long disease escalating at an alarming rate, the incidence of retinopathy is also climbing. Although the cutting edge research has identified many molecular mechanisms associated with its development, the exact mechanism how diabetes damages the retina remains obscure, limiting therapeutic options for this devastating disease. Areas covered: This review focuses on the central role of mitochondrial dysfunction/damage in the pathogenesis of diabetic retinopathy, and how damaged mitochondria initiates a self-perpetuating vicious cycles of free radicals. We have also reviewed how mitochondria could serve as a therapeutic target, and the challenges associated with the complex double mitochondrial membranes and a well-defined blood-retinal barrier for optimal pharmacologic/molecular approach to improve mitochondrial function. Expert opinion: Mitochondrial dysfunction provides many therapeutic targets for ameliorating the development of diabetic retinopathy including their biogenesis, DNA damage and epigenetic modifications. New technology to enhance pharmaceuticals uptake inside the mitochondria, nanotechnology to deliver drugs to the retina, and maintenance of mitochondrial homeostasis via lifestyle changes and novel therapeutics to prevent epigenetic modifications, could serve as some of the welcoming avenues for a diabetic patient to target this sight-threatening disease.

Potentials of Gene Therapy for Diabetic Retinopathy: The Use of Nucleic Acid Constructs Containing a TXNIP Promoter

Diabetic retinopathy (DR) is considered as a chronic eye disease leading to blindness. DR is associated with hyperglycemia-induced oxidative stress, chronic low-grade inflammation and premature cell death. DR affects retinal capillaries, neuroretina and the retinal pigment epithelium. Recently, the thioredoxin-interacting protein TXNIP has been shown as a pro-oxidative stress, pro-inflammatory and pro-apoptotic protein, highly induced by diabetes and high glucose in all cells examined including the retina. TXNIP's actions involve binding to and inhibition of anti-oxidant and thiol-reducing capacities of thioredoxins (Trx) causing cellular oxidative stress and apoptosis. Trx1 is found in the cytosol and nucleus while Trx2 is the mitochondrial isoform. Several studies provided evidence that knockdown of TXNIP by siRNA or chemical blockade ameliorates early abnormalities of DR including endothelial dysfunction, pericyte apoptosis, Müller cell gliosis and neurodegeneration. Therefore, TXNIP is considered a potential target for preventing or slowing down the progression of DR. We recently proposed that nucleic acid constructs containing a proximal TXNIP promoter linked to a redox gene or shRNA that reduces oxidative stress and inflammation may be used to treat DR. The TXNIP promoter is sensitive to hyperglycemia therefore can drive expression of the linked gene or shRNA under high glucose environment such as seen in diabetes while remaining unresponsive at physiological glucose levels. Such a TXNIP-promoter linked gene or shRNA construct can be delivered to the retina by using adeno-associated viral vectors including AAV2 and AAV2/8 or an appropriate carrier via the intravitreal or sub retinal delivery for long-term gene therapies in DR.

Gene and Tissue Engineering For the Treatment of Diabetes and Its Retinal Complications: The Use of Nucleic Acid Constructs Bearing A TXNIP Gene Promoter

Diabetes is a chronic disease in which insulin production is deficient (Type 1) or resistant (Type 2) leading to organ complications including the heart, kidney, retina, and peripheral nerves. About 10% of diabetics are Type 1 while ~90 percent are Type 2 associated with life style changes and obesity. Whether it is Type 1 or Type 2, chronic hyperglycemia prevails and associated oxidative stress and low grade inflammation are considered to play critical roles in diabetes and its complications including diabetic retinopathy (DR). Thioredoxin-Interacting Protein, TXNIP, is strongly induced by diabetes and high glucose in all tissues examined including the pancreatic beta cells and the retina. TXNIP binds to and inhibits the anti-oxidant and thiol reducing capacity of thioredoxins and causes cellular oxidative stress, inflammation and premature cell death. TXNIP is induced strongly by high glucose and its metabolites with minutes and remains elevated as long as hyperglycemia persists. Therefore, the TXNIP gene promoter linked with insulin or a gene of interest may be used to induce gene expression or suppression and in tissue engineering for adipose or tissue-derived autologous stem cells producing insulin for the treatment of diabetes and its complications such as DR as well as age-related neurodegenerative diseases.

The Role of Txnip in Mitophagy Dysregulation and Inflammasome Activation in Diabetic Retinopathy: A New Perspective

Mitochondria are responsible for bioenergetics, metabolism and apoptosis signals in health and disease. The retina being a part of the central nervous system consumes large amounts of glucose and oxygen to generate ATP via the mitochondrial oxidative phosphorylation for its phototransduction and visual function. During ATP generation, electrons leak from the mitochondrial electron transport chain, which is captured by molecular oxygen to produce reactive oxygen species (ROS). These mtROS damage mitochondrial proteins, mtDNA, and membrane lipids and release them in the cytosol. Mitochondrial components are recognized as danger-associated molecular patterns (DAMPS) by cytosolic pattern recognition receptors such as NOD-like receptors, NLRP3 inflammasomes. They process pro-caspase-1 to active caspase-1, which cleaves pro-inflammatory IL-1β o mature IL-1β causing inflammation and cell death by pyroptosis. To counter the damaging action of mtROS and inflammasomes in fully differentiated cells in the retina, the removal of the damaged and dysfunctional mitochondria by a double-membrane autophagic process via lysosomal degradation called mitophagy is critical for mitochondrial homeostasis and cell survival. Nonetheless, under chronic diseases including diabetic retinopathy (DR), mitophagy dysregulation and NLRP3 inflammasome activation exist, which cause premature cell death and disease progression. Recently, the thioredoxin-interacting protein TXNIP, which is strongly induced by diabetes and inhibits anti-oxidant function of thioredoxin, has been implicated in mitochondrial dysfunction, mitophagic dysregulation and NLRP3 inflammasome activation in DR. Therefore, TXNIP silencing or pharmacological inhibition may normalize mitophagic flux and NLRP3 inflammasome activation, which will prevent or slow down the progression of DR.