Tribbles Homolog 3 Mediates the Development and Progression of Diabetic Retinopathy

Comparative proteomic study of retinal ganglion cells undergoing various types of cellular stressors

Retinal ganglion cell (RGC) damage serves as a key indicator of various retinal degenerative diseases, including diabetic retinopathy (DR), glaucoma, retinal arterial and retinal vein occlusions, as well as inflammatory and traumatic optic neuropathies. Despite the growing body of data on the RGC proteomics associated with these conditions, there has been no dedicated study conducted to compare the molecular signaling pathways involved in the mechanism of neuronal cell death. Therefore, we launched the study using two different insults leading to RGC death: glutamate excitotoxicity and optic nerve crush (ONC). C57BL/6 mice were used for the study and underwent NMDA- and ONC-induced damage. Twenty-four hours after ONC and 1 h after NMDA injection, we collected RGCs using CD90.2 coupled magnetic beads, prepared protein extracts, and employed LC-MS for the global proteomic analysis of RGCs. Statistically significant changes in proteins were analyzed to identify changes to cellular signaling resulting from the treatment. We identified unique and common alterations in protein profiles in RGCs undergoing different types of cellular stresses. Our study not only identified both unique and shared proteomic changes but also laid the groundwork for the future development of a therapeutic platform for testing gene candidates for DR and glaucoma.

Integration of dietary nutrition and TRIB3 action into diabetes mellitus

Despite intensive studies for decades, the common mechanistic correlations among the underlying pathology of diabetes mellitus (DM), its complications, and effective clinical treatments remain poorly characterized. High-quality diets and nutrition therapy have played an indispensable role in the management of DM. More importantly, tribbles homolog 3 (TRIB3), a nutrient-sensing and glucose-responsive regulator, might be an important stress-regulatory switch, linking glucose homeostasis and insulin resistance. Therefore, this review aimed to introduce the latest research progress on the crosstalk between dietary nutrition intervention and TRIB3 in the development and treatment of DM. This study also summarized the possible mechanisms involved in the signaling pathways of TRIB3 action in DM, in order to gain an in-depth understanding of dietary nutrition intervention and TRIB3 in the pathogenesis of DM at the organism level.

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.

Proteomic analysis of diabetic retinas

Introduction

As a metabolic disease, diabetes often leads to health complications such as heart failure, nephropathy, neurological disorders, and vision loss. Diabetic retinopathy (DR) affects as many as 100 million people worldwide. The mechanism of DR is complex and known to impact both neural and vascular components in the retina. While recent advances in the field have identified major cellular signaling contributing to DR pathogenesis, little has been reported on the protein post-translational modifications (PTM) - known to define protein localization, function, and activity - in the diabetic retina overall. Protein glycosylation is the enzymatic addition of carbohydrates to proteins, which can influence many protein attributes including folding, stability, function, and subcellular localization. O-linked glycosylation is the addition of sugars to an oxygen atom in amino acids with a free oxygen atom in their side chain (i.e., threonine, serine). To date, more than 100 congenital disorders of glycosylation have been described. However, no studies have identified the retinal O-linked glycoproteome in health or disease. With a critical need to expedite the discovery of PTMomics in diabetic retinas, we identified both global changes in protein levels and the retinal O-glycoproteome of control and diabetic mice.

Methods

We used liquid chromatography/mass spectrometry-based proteomics and high throughput screening to identify proteins differentially expressed and proteins differentially O-glycosylated in the retinas of wildtype and diabetic mice.

Results

Changes in both global expression levels of proteins and proteins differentially glycosylated in the retinas of wild-type and diabetic mice have been identified. We provide evidence that diabetes shifts both global expression levels and O-glycosylation of metabolic and synaptic proteins in the retina.

Discussion

Here we report changes in the retinal proteome of diabetic mice. We highlight alterations in global proteins involved in metabolic processes, maintaining cellular structure, trafficking, and neuronal processes. We then showed changes in O-linked glycosylation of individual proteins in the diabetic retina.

Retinal Cell Damage in Diabetic Retinopathy

Diabetic retinopathy (DR), the most common microvascular complication that occurs in diabetes mellitus (DM), is the leading cause of vision loss in working-age adults. The prevalence of diabetic retinopathy is approximately 30% of the diabetic population and untreated DR can eventually cause blindness. For decades, diabetic retinopathy was considered a microvascular complication and clinically staged by its vascular manifestations. In recent years, emerging evidence has shown that diabetic retinopathy causes early neuronal dysfunction and neurodegeneration that may precede vascular pathology and affect retinal neurons as well as glial cells. This knowledge leads to new therapeutic strategies aiming to prevent dysfunction of retinal neurons at the early stage of DR. Early detection and timely treatment to protect retinal neurons are critical to preventing visual loss in DR. This review provides an overview of DR and the structural and functional changes associated with DR, and discusses neuronal degeneration during diabetic retinopathy, the mechanisms underlying retinal neurodegeneration and microvascular complications, and perspectives on current and future clinic therapies.

GADD34 Ablation Exacerbates Retinal Degeneration in P23H RHO Mice

The UPR is sustainably activated in degenerating retinas, leading to translational inhibition via p-eIF2α. Recent findings have demonstrated that ablation of growth arrest and DNA damage-inducible protein 34 (GADD34), a protein phosphatase 1 regulatory subunit permitting translational machinery operation through p-eIF2α elevation, does not impact the rate of translation in fast-degenerating rd16 mice. The current study aimed to validate whether P23H RHO mice degenerating at a slower pace manifest translational attenuation and whether GADD34 ablation impacts the rate of retinal degeneration via further suppression of retinal protein synthesis and apoptotic cell death. For this study, mice were examined with ERG and histological analyses. The molecular assessment was conducted in the naïve and LPS-challenged mice using Western blot and qRT-PCR analyses. Thus, this study demonstrates that the P23H RHO retinas manifest translational attenuation. However, GADD34 ablation resulted in a more prominent p-eIF2a increase without impacting the translation rate. GADD34 deficiency also led to a reduction in scotopic ERG amplitudes and an increased number of TUNEL-positive cells. Molecular analysis revealed that GADD34 deficiency reduces the expression of p-STAT3 and Il-6 while increasing the expression of Tnfa. Overall, the data indicate that GADD34 plays a multifunctional role. Under chronic UPR activation, GADD34 acts as a feedback player, dephosphorylating p-eIF2a, although this role does not seem to be critical. Additionally, GADD34 controls cytokine expression and STAT3 activation. Perhaps these molecular events are particularly important in controlling the pace of retinal degeneration.

TRIB3 promoter 33 bp VNTR is associated with the risk of cerebrovascular disease in type 2 diabetic patients

Previous studies have demonstrated that TRIB3 is closely related to insulin resistance, metabolic disorders and vascular diseases. Recently, it was reported that a 33 bp variable number of tandem repeats (VNTR) located in the TRIB3 promoter could considerably alter its transcriptional activity. Nonetheless, whether the shift of TRIB3 transcriptional activity has the effect of inducing diabetic vascular complications is still unclear. Therefore, in our study, we aimed to explore the relationship between the TRIB3 33bp VNTR and diabetic vascular complications. The TRIB3 33bp VNTR polymorphisms were determined by PCR and Sanger sequencing, a total of 798 eligible Chinese patients with type 2 diabetes (T2DM) were included in our study and then evaluated with clinical data. After adjusting for age, gender, BMI, smoking history, drinking history and duration of diabetes, we found that the high number of 33 bp tandem repeats (repeats>8) was significantly associated with an increase in the risk of cerebrovascular diseases compared with the low number of 33 bp tandem repeats (repeats≤6) in patients with T2DM(OR 2.66, 95% CI 1.29–5.47, p = 0.008). The intermediate number of 33bp tandem repeats (6 < repeat≤8) was markedly associated with a decreased risk of diabetic retinopathy compared with the low number of tandem repeats (OR 0.65, 95% CI 0.46–0.91, p = 0.012). Adjusting for gender, age and BMI, there was a significant difference in DBP levels among patients with the number of different 33 bp tandem repeats (Low vs. Intermediate vs. High, 81.6 ± 12.8 vs. 79.8 ± 12.4 vs. 78.7 ± 12.6 mmHg; p = 0.045). Subgroup analysis found that TRIB3 VNTR was significantly correlated with the difference in systolic blood pressure (SBP) in T2DM patients taking ACEI/ARB drugs (Low vs. Intermediate vs. High, 146.27 ± 18.23 vs. 140.01 ± 19.91 vs. 140.77 ± 18.64 mmHg; p = 0.018). Our results indicated that TRIB3 promoter 33bp VNTR is related to vascular diseases in T2DM patients, and may serve as a new biomarker for individualized prevention and therapy of T2DM.

Diabetic retinopathy: Involved cells, biomarkers, and treatments

Diabetic retinopathy (DR), a leading cause of vision loss and blindness worldwide, is caused by retinal neurovascular unit dysfunction, and its cellular pathology involves at least nine kinds of retinal cells, including photoreceptors, horizontal and bipolar cells, amacrine cells, retinal ganglion cells, glial cells (Müller cells, astrocytes, and microglia), endothelial cells, pericytes, and retinal pigment epithelial cells. Its mechanism is complicated and involves loss of cells, inflammatory factor production, neovascularization, and BRB impairment. However, the mechanism has not been completely elucidated. Drug treatment for DR has been gradually advancing recently. Research on potential drug targets relies upon clear information on pathogenesis and effective biomarkers. Therefore, we reviewed the recent literature on the cellular pathology and the diagnostic and prognostic biomarkers of DR in terms of blood, protein, and clinical and preclinical drug therapy (including synthesized molecules and natural molecules). This review may provide a theoretical basis for further DR research.

A novel Gboxin analog induces OXPHOS inhibition and mitochondrial dysfunction-mediated apoptosis in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is an aggressive B-cell non-Hodgkin's lymphoma. Currently, moderate efficacy and limitations of approved drugs still exist, and it is necessary to develop newer and more effective drugs. Gboxin is a promising inhibitor of OXPHOS, which specifically inhibits the growth of many kinds of cancer cell lines. In the present study, 21 Gboxin analogs incorporating amide and ester moieties were designed and synthesized. Preliminary screening results show that 5d also has specific selectivity for cancer cells, particularly on the DLBCL cells, which is weaker than that of Gboxin but still good. Thus, the effect and underlying mechanism of 5d on DLBCL cells were further studied. The results showed that 5d exhibits potent proliferation inhibition and cell cycle arrest effects, and its IC₅₀ to DLBCL cells is below 1 µM. In addition, 5d induces apoptosis of DLBCL cells in a time- and dose-dependent manner, and this effect is stronger than that of Gboxin and VP16. Mechanistically, 5d plays its role mainly through the stimulation of metabolic stress in DLBCL cell lines, which induces OXPHOS inhibition, inflammation, DNA damage and mitochondrial dysfunction. These data suggest that 5d has potential as a candidate agent for DLBCL alternative drug development.

NADH-Cytochrome B5 reductase 2 suppresses retinal vascular dysfunction through regulation of vascular endothelial growth factor A in diabetic retinopathy

Diabetic retinopathy (DR) is a progressive vascular complication of diabetes mellitus (DM) and is related to retinal vascular abnormalities. NADH-Cytochrome B5 Reductase 2 (CBR2) has been implicated in angiogenesis, but the effect of CBR2 on angiogenesis and endothelial cell biological behavior in DR remains unclear. Here, we aimed to explore the effect of CBR2 on retinal vascular dysfunction under diabetic conditions. The histological analyses were performed to explore the effect of CBR2 on pathological change in streptozotocin (STZ)-induced diabetic rat retinas. The effect of CBR2 on endothelial cell function was explored by CCK-8, scratch wound, transwell, tube formation, and immunofluorescence assays in high glucose (HG)-stimulated human retinal microvascular endothelial cells (HRMECs). CBR2 expression was significantly downregulated in DM rat retinas and HG-stimulated HRMECs. Intravitreal injection of CBR2-expressing lentivirus under diabetic conditions reduced retinal angiogenesis, acellular capillary formation, and pericyte loss, along with decreased expression of hypoxia-inducible factor-1α (HIF-1α), cluster of differentiation 31 (CD31), and vascular endothelial growth factor A (VEGFA) in vivo. Moreover, CBR2 overexpression inhibited cell growth and tube formation and led to decreased expression of HIF-1α and VEGFA in HG-induced HRMECs. Interestingly, the repressive effects of CBR2 on cell proliferation, migration, and tube formation under HG conditions were strongly reversed when VEGFA was overexpressed. Overall, the key findings of our study suggested that CBR2 might alleviate retinal vascular dysfunction and abnormal endothelial proliferation during the process of DR by regulating VEGFA, providing a piece of potent evidence for DR therapy.

Diabetic Retinopathy: From Animal Models to Cellular Signaling

Diabetic retinopathy (DR) is an ocular complication of diabetes mellitus (DM), a metabolic disorder characterized by elevation in blood glucose level. The pathogenesis of DR includes vascular, neuronal, and inflammatory components leading to activation of complex cellular molecular signaling. If untreated, the disease can culminate in vision loss that eventually leads to blindness. Animal models mimicking different aspects of DM complications have been developed to study the development and progression of DR. Despite the significant contribution of the developed DR models to discovering the mechanisms of DR and the recent achievements in the research field, the sequence of cellular events in diabetic retinas is still under investigation. Partially, this is due to the complexity of molecular mechanisms, although the lack of availability of models that adequately mimic all the neurovascular pathobiological features observed in patients has also contributed to the delay in determining a precise molecular trigger. In this review, we provide an update on the status of animal models of DR to help investigators choose an appropriate system to validate their hypothesis. We also discuss the key cellular and physiological events of DR in these models.

A Novel Tree Shrew Model of Diabetic Retinopathy

Existing animal models with rod-dominant retinas have shown that hyperglycemia injures neurons, but it is not yet clearly understood how blue cone photoreceptors and retinal ganglion cells (RGCs) deteriorate in patients because of compromised insulin tolerance. In contrast, northern tree shrews (Tupaia Belangeri), one of the closest living relatives of primates, have a cone-dominant retina with short wave sensitivity (SWS) and long wave sensitivity (LWS) cones. Therefore, we injected animals with a single streptozotocin dose (175 mg/kg i.p.) to investigate whether sustained hyperglycemia models the features of human diabetic retinopathy (DR). We used the photopic electroretinogram (ERG) to measure the amplitudes of A and B waves and the photopic negative responses (PhNR) to evaluate cone and RGC function. Retinal flat mounts were prepared for immunohistochemical analysis to count the numbers of neurons with antibodies against cone opsins and RGC specific BRN3a proteins. The levels of the proteins TRIB3, ISR-1, and p-AKT/p-mTOR were measured with western blot. The results demonstrated that tree shrews manifested sustained hyperglycemia leading to a slight but significant loss of SWS cones (12%) and RGCs (20%) 16 weeks after streptozotocin injection. The loss of BRN3a-positive RGCs was also reflected by a 30% decline in BRN3a protein expression. These were accompanied by reduced ERG amplitudes and PhNRs. Importantly, the diabetic retinas demonstrated increased expression of TRIB3 and level of p-AKT/p-mTOR axis but reduced level of IRS-1 protein. Therefore, a new non-primate model of DR with SWS cone and RGC dysfunction lays the foundation to better understand retinal pathophysiology at the molecular level and opens an avenue for improving the research on the treatment of human eye diseases.