Retinal neurodegeneration may precede microvascular changes characteristic of diabetic retinopathy in diabetes mellitus

Effects of prolonged type 2 diabetes on changes in peripapillary retinal nerve fiber layer thickness in diabetic eyes without clinical diabetic retinopathy

To identify the effects of prolonged type 2 diabetes (T2DM) on changes in peripapillary retinal nerve fiber layer (pRNFL) thickness in patients without clinical diabetic retinopathy. Subjects were divided into two groups: controls and patients with T2DM (DM group). After the initial visits, the pRNFL thicknesses were measured three more times at 1-year intervals. Subgroup analyses were performed in patients with T2DM duration ≥ 10 years. The mean pRNFL thickness at each visit was 95.8 ± 8.1, 95.4 ± 8.3, 94.9 ± 8.1, and 94.5 ± 8.3 μm in the control group (P = 0.138) (n = 55); and 93.4 ± 9.1, 92.1 ± 9.3, 90.9 ± 9.3, and 89.5 ± 9.2 μm in the DM group (P < 0.001) (n = 85). The estimated rate of reduction in mean pRNFL thickness was - 0.45 μm/year in the control group and - 1.34 μm/year in the DM group, respectively. In the DM group, the BCVA and HbA1c (both P = 0.001) were significant factors associated with pRNFL reduction. In patients with T2DM duration ≥ 10 years, the estimated pRNFL reduction rate was - 1.61 μm/year, and hypertension was a significant factor affecting the pRNFL reduction (P = 0.046). We confirmed rapid pRNFL reduction over time in T2DM, and the reduction rate was higher in patients with T2DM ≥ 10 years. Additionally, BCVA and HbA1c levels were significantly associated with the change in pRNFL thickness in T2DM patients.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the main causes of vision loss in the working age population. It is characterized by a progressive deterioration of the retinal microvasculature, caused by long-term metabolic alterations inherent to diabetes, leading to a progressive loss of retinal integrity and function. The mammalian retina presents an orderly layered structure that executes initial but complex visual processing and analysis. Gap junction channels (GJC) forming electrical synapses are present in each retinal layer and contribute to the communication between different cell types. In addition, connexin hemichannels (HCs) have emerged as relevant players that influence diverse physiological and pathological processes in the retina. This article highlights the impact of diabetic conditions on GJC and HCs physiology and their involvement in DR pathogenesis. Microvascular damage and concomitant loss of endothelial cells and pericytes are related to alterations in gap junction intercellular communication (GJIC) and decreased connexin 43 (Cx43) expression. On the other hand, it has been shown that the expression and activity of HCs are upregulated in DR, becoming a key element in the establishment of proinflammatory conditions that emerge during hyperglycemia. Hence, novel connexin HCs blockers or drugs to enhance GJIC are promising tools for the development of pharmacological interventions for diabetic retinopathy, and initial in vitro and in vivo studies have shown favorable results in this regard.

Integrative Biology of Diabetic Retinal Disease: Lessons from Diabetic Kidney Disease

Diabetic retinal disease (DRD) remains the most common cause of vision loss in adults of working age. Progress on the development of new therapies for DRD has been limited by the complexity of the human eye, which constrains the utility of traditional research techniques, including animal and tissue culture models-a problem shared by those in the field of kidney disease research. By contrast, significant progress in the study of diabetic kidney disease (DKD) has resulted from the successful employment of systems biology approaches. Systems biology is widely used to comprehensively understand complex human diseases through the unbiased integration of genetic, environmental, and phenotypic aspects of the disease with the functional and structural manifestations of the disease. The application of a systems biology approach to DRD may help to clarify the molecular basis of the disease and its progression. Acquiring this type of information might enable the development of personalized treatment approaches, with the goal of discovering new therapies targeted to an individual's specific DRD pathophysiology and phenotype. Furthermore, recent efforts have revealed shared and distinct pathways and molecular targets of DRD and DKD, highlighting the complex pathophysiology of these diseases and raising the possibility of therapeutics beneficial to both organs. The objective of this review is to survey the current understanding of DRD pathophysiology and to demonstrate the investigative approaches currently applied to DKD that could promote a more thorough understanding of the structure, function, and progression of DRD.

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

Neurodegenerative retinal diseases, such as glaucoma and diabetic retinopathy, involve a gradual loss of neurons in the retina as the disease progresses. Central nervous system neurons are not able to regenerate in mammals, therefore, an often sought after course of treatment for neuronal loss follows a neuroprotective or regenerative strategy. Neuroprotection is the process of preserving the structure and function of the neurons that have survived a harmful insult; while regenerative approaches aim to replace or rewire the neurons and synaptic connections that were lost, or induce regrowth of damaged axons or dendrites. In order to test the neuroprotective effectiveness or the regenerative capacity of a particular agent, a robust experimental model of retinal neuronal damage is essential. Zebrafish are being used more often in this type of study because their eye structure and development is well-conserved between zebrafish and mammals. Zebrafish are robust genetic tools and are relatively inexpensive to maintain. The large array of functional and behavioral tests available in zebrafish makes them an attractive model for neuroprotection studies. Some common insults used to model retinal disease and study neuroprotection in zebrafish include intense light, chemical toxicity and mechanical damage. This review covers the existing retinal neuroprotection and regeneration literature in the zebrafish and highlights their potential for future studies.

Therapeutic potential of curcumin in diabetic retinopathy (Review)

Diabetic retinopathy (DR) is a type of retinal microangiopathy caused by diabetes mellitus. It has become the leading cause of blindness among working individuals worldwide. DR is becoming increasingly common among younger diabetic patients and there is a need for lifelong treatment. The pathogenic mechanisms of DR are influenced by a number of factors, such as hyperglycemia, hyperlipidemia, inflammatory response and oxidative stress, among others. Currently, the treatment methods for DR mainly include retinal photocoagulation, vitrectomy, or anti‑vascular endothelial growth factor (VEGF) therapy. However, these methods have some disadvantages and limitations. Therefore, it is a matter of great interest and urgency to discover drugs that can target the pathogenesis of DR. Since ancient times, traditional Chinese medicine practitioners have accumulated extensive experiences in the use of Chinese herbal medicine for the prevention and treatment of diseases. In the theory of traditional Chinese medicine, curcumin has the effects of promoting blood circulation and relieving pain. A number of studies have also demonstrated that curcumin has multiple biological activities, including exerting anti‑apoptotic, anti‑inflammatory, antioxidant and antitumor properties. In recent years, studies have also confirmed that curcumin can prevent a variety of diabetic complications, including diabetic nephropathy (DN). However, the preventive and curative effects of curcumin on DR and its mechanisms of action have not yet been fully elucidated. The present review aimed to explore the therapeutic potential of curcumin in diabetes mellitus and DR.

RETINAL TISSUE PERFUSION REDUCTION BEST DISCRIMINATES EARLY STAGE DIABETIC RETINOPATHY IN PATIENTS WITH TYPE 2 DIABETES MELLITUS

PURPOSE

To determine retinal microcirculation measured as retinal tissue perfusion (RTP) in patients with type 2 diabetes mellitus and mild nonproliferative diabetic retinopathy, and to compare its discrimination ability to that of retinal microvasculature and microstructure.

METHODS

Thirty eyes of 18 patients with mild nonproliferative diabetic retinopathy and 20 eyes of 20 age-matched and gender-matched normal controls were imaged. Retinal blood flow velocity and flow rate were measured using a retinal function imager. Retinal vessel density (Dbox) and intraretinal layer thicknesses were measured using optical coherence tomography angiography. Retinal tissue perfusion was measured as retinal blood flow divided by the volume of the inner retina.

RESULTS

Compared with normal control, RTP, vessel density, and the thickness of the retinal nerve fiber layer in patients with mild nonproliferative diabetic retinopathy showed significant reduction (P < 0.05). Retinal tissue perfusion had the best discrimination power (area under the curve = 0.97), with a sensitivity of 93.3% and specificity of 85.0%. In the eyes with mild nonproliferative diabetic retinopathy, RTP correlated with arteriolar blood flow velocity (r = 0.403, P = 0.027) but was unrelated to vessel density in any layer (r = -0.010 to 0.261, P > 0.05).

CONCLUSION

Because of its highest discrimination power, RTP may become a promising biomarker for detecting early-stage diabetic retinopathy.

Evidence for Structural and Functional Damage of the Inner Retina in Diabetes With No Diabetic Retinopathy

Purpose

To provide structural and functional evidence of inner retinal loss in diabetes prior to vascular changes and interpret the structure-function relationship in the context of an established neural model.

Methods

Data from one eye of 505 participants (134 with diabetes and no clinically evident vascular alterations of the retina) were included in this analysis. The data were collected as part of a large population-based study. Functional tests included best-corrected visual acuity, Pelli-Robson contrast sensitivity, mesopic microperimetry, and frequency doubling technology perimetry (FDT). Macular optical coherence tomography volume scans were collected for all participants. To interpret the structure-function relationship in the context of a neural model, ganglion cell layer (GCL) thickness was converted to local ganglion cell (GC) counts.

Results

The GCL and inner plexiform layer were significantly thinner in participants with diabetes (P < 0.05), with no significant differences in the macular retinal nerve fiber layer or the outer retina. All functional tests except microperimetry showed a significant loss in diabetic patients (P < 0.05). Both FDT and microperimetry showed a significant relationship with the GC count (P < 0.05), consistent with predictions from a neural model for partial summation conditions. However, the FDT captured additional significant damage (P = 0.03) unexplained by the structural loss.

Conclusions

Functional and structural measurements support early neuronal loss in diabetes. The structure-function relationship follows the predictions from an established neural model. Functional tests could be improved to operate in total summation conditions in the macula, becoming more sensitive to early loss.

Overexpression of D-amino acid oxidase prevents retinal neurovascular pathologies in diabetic rats

AIMS/HYPOTHESIS

Diabetic retinopathy is characterised by retinal neurodegeneration and retinal vascular abnormalities, affecting one third of diabetic patients with disease duration of more than 10 years. Accumulated evidence suggests that serine racemase (SR) and D-serine are correlated with the pathogenesis of diabetic retinopathy and the deletion of the Srr gene reverses neurovascular pathologies in diabetic mice. Since D-serine content is balanced by SR synthesis and D-amino acid oxidase (DAAO) degradation, we examined the roles of DAAO in diabetic retinopathy and further explored relevant therapy.

METHODS

Rats were used as a model of diabetes by i.p. injection of streptozotocin at the age of 2 months and blood glucose was monitored with a glucometer. Quantitative real-time PCR was used to examine Dao mRNA and western blotting to examine targeted proteins in the retinas. Bisulphite sequencing was used to examine the methylation of Dao mRNA promoter in the retinas. Intravitreal injection of DAAO-expressing adenovirus (AAV8-DAAO) was conducted one week before streptozotocin administration. Brain specific homeobox/POU domain protein 3a (Brn3a) immunofluorescence was conducted to indicate retinal ganglion cells at 3 months after virus injection. The permeability of the blood-retinal barrier was examined by Evans blue leakage from retinal capillaries. Periodic acid-Schiff staining and haematoxylin counterstaining were used to indicate retinal vasculature, which was further examined with double immunostaining at 7 months after virus injection.

RESULTS

At the age of 12 months, DAAO mRNA and protein levels in retinas from diabetic animals were reduced to 66.2% and 70.4% of those from normal (control) animals, respectively. The Dao proximal promoter contained higher levels of methylation in diabetic than in normal retinas. Consistent with the observation, DNA methyltransferase 1 was increased in diabetic retinas. Injection of DAAO-expressing virus completely prevented the loss of retinal ganglion cells and the disruption of blood-retinal barrier in diabetic rats. Diabetic retinas contained retinal ganglion cells at a density of 54 ± 4/mm2, which was restored to 68 ± 9/mm2 by DAAO overexpression, similar to the levels in normal retinas. The ratio between the number of endothelial cells and pericytes in diabetic retinas was 6.06 ± 1.93/mm2, which was reduced to 3.42 ± 0.55/mm2 by DAAO overexpression; the number of acellular capillaries in diabetic retinas was 10 ± 5/mm2, which was restored to 6 ± 2/mm2 by DAAO overexpression, similar to the levels in normal retinas. Injection of the DAAO-expressing virus increased the expression of occludin and reduced gliosis, which were examined to probe the mechanism by which the disrupted blood-retinal barrier in diabetic rats was rescued and retinal neurodegeneration was prevented.

CONCLUSIONS/INTERPRETATION

Altogether, overexpression of DAAO before the onset of diabetes protects against neurovascular abnormalities in retinas from diabetic rats, which suggests a novel strategy for preventing diabetic retinopathy. Graphical abstract.

Interphotoreceptor Retinol-Binding Protein Ameliorates Diabetes-Induced Retinal Dysfunction and Neurodegeneration Through Rhodopsin

Patients with diabetes often experience visual defects before any retinal pathologies are detected. The molecular mechanism for the visual defects in early diabetes has not been elucidated. Our previous study reported that in early diabetic retinopathy (DR), rhodopsin levels were reduced due to impaired 11-cis-retinal regeneration. Interphotoreceptor retinol-binding protein (IRBP) is a visual cycle protein and important for 11-cis-retinal generation. IRBP levels are decreased in the vitreous and retina of DR patients and animal models. To determine the role of IRBP downregulation in the visual defects in early DR, we induced diabetes in transgenic mice overexpressing IRBP in the retina. IRBP overexpression prevented diabetes-induced decline of retinal function. Furthermore, IRBP overexpression also prevented decreases of rhodopsin levels and 11-cis-retinal generation in diabetic mice. Diabetic IRBP transgenic mice also showed ameliorated retinal oxidative stress, inflammation, apoptosis, and retinal degeneration compared with diabetic wild-type mice. These findings suggest that diabetes-induced IRBP downregulation impairs the regeneration of 11-cis-retinal and rhodopsin, leading to retinal dysfunction in early DR. Furthermore, increased 11-cis-retinal-free opsin constitutively activates the phototransduction pathway, leading to increased oxidative stress and retinal neurodegeneration. Therefore, restored IRBP expression in the diabetic retina may confer a protective effect against retinal degeneration in DR.

Sites of highest predictability for neurodegeneration in patients with type 2 diabetes mellitus without diabetic retinopathy

PURPOSE

Identifying earlier retinal thickness affection and predictability for diabetic retinal neurodegeneration (DRN) in patients with type 2 diabetes mellitus (DM2) without diabetic retinopathy (DR).

PATIENTS AND METHODS

This is a comparative cross-sectional study. Thirty-eight eyes of 19 patients with DM2 without any signs of DR and 38 eyes of 19 controls underwent retinal evaluation using optical coherence tomography. Macular ganglion cell layer (GCL), macular retinal nerve fiber layer (mRNFL), inner plexiform layer (IPL), total macular thickness, peripapillary retinal nerve fiber layer (pRNFL) and Bruch's membrane opening-minimum rim width (BMO-MRW) were evaluated.

RESULTS

GCL showed significant thickness reduction in the total, superior and inferior halves as well as the 9 ETDRS regions (except the nasal and lower outer regions). The mRNFL showed a significant reduction in the total, superior and inferior halves as well as the lower and nasal outer regions. The IPL showed significant reduction in the 4 inner regions only. The pRNFL showed significant reduction in the total, superotemporal and inferotemporal sectors values. The BMO-MRW did not show any significant thickness change.

CONCLUSION

The total, superior and inferior GCL and mRNFL, in addition to the global pRNFL were the most affected and predictive layers for DRN in patients with DM type 2 without DR. It appears that the GCL is the primary site of DRN and the rest of the changes represented a degeneration of the axonal path between the optic disk and the macular GCL.

βA1-crystallin regulates glucose metabolism and mitochondrial function in mouse retinal astrocytes by modulating PTP1B activity

βA3/A1-crystallin, a lens protein that is also expressed in astrocytes, is produced as βA3 and βA1-crystallin isoforms by leaky ribosomal scanning. In a previous human proteome high-throughput array, we found that βA3/A1-crystallin interacts with protein tyrosine phosphatase 1B (PTP1B), a key regulator of glucose metabolism. This prompted us to explore possible roles of βA3/A1-crystallin in metabolism of retinal astrocytes. We found that βA1-crystallin acts as an uncompetitive inhibitor of PTP1B, but βA3-crystallin does not. Loss of βA1-crystallin in astrocytes triggers metabolic abnormalities and inflammation. In CRISPR/cas9 gene-edited βA1-knockdown (KD) mice, but not in βA3-knockout (KO) mice, the streptozotocin (STZ)-induced diabetic retinopathy (DR)-like phenotype is exacerbated. Here, we have identified βA1-crystallin as a regulator of PTP1B; loss of this regulation may be a new mechanism by which astrocytes contribute to DR. Interestingly, proliferative diabetic retinopathy (PDR) patients showed reduced βA1-crystallin and higher levels of PTP1B in the vitreous humor.

Association between risk factors and retinal nerve fiber layer loss in early stages of diabetic retinopathy

AIM

To investigate the changes of retinal nerve fiber layer (RNFL) among normal individuals, diabetic patients without diabetic retinopathy (NDR) and non-proliferative diabetic retinopathy (NPDR), and explore the possible risk factors of early diabetic retinopathy (DR).

METHODS

In this cross-sectional study, 107 participants were divided in three groups. Totally 31 normal individuals (control group), 40 diabetic patients without DR (NDR group) and 36 patients with NPDR (NPDR group) were included. Optical coherence tomography (OCT) was used to detect RNFL thickness and other optic disc parameters among different groups. The potential association between RNFL loss and systemic risk factors were assessed for DR, including diabetes duration, body mass index (BMI), hemoglobin A1c (HbA1c), serum lipids, and blood pressure.

RESULTS

The average and each quadrant RNFL thickness were thinner in NPDR group compared to control group of the right (P=0.00, P=0.01, P=0.01, P=0.02, P=0.04) and left eyes (P=0.00, P=0.00, P=0.00, P=0.03, P=0.04). The average, superior and inferior RNFL thickness were thinner in NDR group compared to the NPDR group of the right (P=0.00, P=0.02, P=0.03) and left eyes (P=0.00, P=0.00, P=0.01). Diabetic duration was negatively correlated with the superior, inferior, and average RNFL thickness of the right (r=-0.385, P=0.001; r=-0.366, P=0.001; r=-0.503, P=0.000) and left eyes (r=-0.271, P=0.018; r=0.278, P=0.015; r=-0.260, P=0.023). HbA1c was negatively correlated with the superior, inferior, and average RNFL thickness of the right (r=-0.316 P=0.005; r=-0.414, P=0.000; r=-0.418, P=0.000) and left eyes (r=-0.367, P=0.001; r=-0.250, P=0.030; r=-0.393, P=0.000). Systolic pressure was negatively correlated with the inferior and average RNFL thickness of the right eye (r=-0.402, P=0.000; r=-0.371, P=0.001) and was negatively correlated with the superior and average RNFL thickness of the left eye (r=-0.264, P=0.021; r=-0.233, P=0.043).

CONCLUSION

RNFL loss, especially in the superior and inferior quadrants, may be the earliest structural change of the retina in diabetic patients, and is also associated with diabetic duration, HbA1c, and systolic pressure.

Targeted pharmacotherapy against neurodegeneration and neuroinflammation in early diabetic retinopathy

Diabetic retinopathy (DR), the most frequent complication of diabetes, is one of the leading causes of irreversible blindness in working-age adults and has traditionally been regarded as a microvascular disease. However, increasing evidence has revealed that synaptic neurodegeneration of retinal ganglion cells (RGCs) and activation of glial cells may represent some of the earliest events in the pathogenesis of DR. Upon diabetes-induced metabolic stress, abnormal glycogen synthase kinase-3β (GSK-3β) activation drives tau hyperphosphorylation and β-catenin downregulation, leading to mitochondrial impairment and synaptic neurodegeneration prior to RGC apoptosis. Moreover, glial cell activation triggers enhanced inflammation and oxidative stress, which may accelerate the deterioration of diabetic RGCs neurodegeneration. These findings have opened up opportunities for therapies, such as inhibition of GSK-3β, glial cell activation, glutamate excitotoxicity and the use of neuroprotective drugs targeting early neurodegenerative processes in the retina and halting the progression of DR before the manifestation of microvascular abnormalities. Such interventions could potentially remedy early neurodegeneration and help prevent vision loss in people suffering from DR.

Associations Between Capillary Diameter, Capillary Density, and Microaneurysms in Diabetic Retinopathy: A High-Resolution Confocal Microscopy Study

Purpose

To use high-resolution histology to define the associations between microaneurysms, capillary diameter and capillary density alterations in diabetic retinopathy (DR).

Methods

Quantitative comparisons of microaneurysm number, capillary density and capillary diameter were performed between eight human donor eyes with nonproliferative DR and six age- and eccentricity-matched normal donor eyes after retinal vascular perfusion labelling. The parafovea, 3-mm, 6-mm, and 9-mm retinal eccentricities were analyzed and associations between microvascular alterations defined.

Results

Mean capillary density was reduced in all retina regions in the DR group (P = 0.013). Microaneurysms occurred in all retina regions in the DR group, but the association between decreased capillary density and microaneurysm number was only significant in the 3-mm (P = 0.040) and 6-mm (P = 0.007) eccentricities. The mean capillary diameter of the DR group (8.9 ± 0.53 µm) was greater than the control group (7.60 ± 0.40 µm; P = 0.033). There was no association between capillary diameter increase and capillary density decrease (P = 0.257) and capillary diameter increase and microaneurysm number (P = 0.147) in the DR group. Within the parafovea of the DR group, capillary density was significantly reduced, and capillary diameter was significantly increased in the deep capillary plexus compared with the superficial and intermediate plexuses (all P < 0.05).

Conclusions

In DR, capillary density reduction occurs across multiple retina eccentricities with a predilection for the deep capillary plexus. The association between microaneurysm number and capillary density is specific to retina eccentricity. Capillary diameter increase may be an early biomarker of DR. These findings may refine the application of optical coherence tomography angiography techniques for the management of DR.

Prevalence of Focal Inner, Middle, and Combined Retinal Thinning in Diabetic Patients and Its Relationship With Systemic and Ocular Parameters

Purpose

To determine the prevalence of focal inner, middle, and combined inner/middle retinal thinning (FIRT, FMRT, and FCRT, respectively) in different stages of diabetic retinopathy (DR) without diabetic macular edema and to assess the relationship between such findings with ocular and systemic parameters.

Methods

This was a cross-sectional, comparative study comprising healthy participants and diabetic patients with different stages of DR. Forty-nine horizontal macular B-scans from the selected eye were obtained using spectral-domain optical coherence tomography (SD-OCT) and analyzed for the presence of FIRT, FMRT, or FCRT and any relationship with systemic and ocular parameters. Focal retinal thinning (FRT) was subjectively defined as any evidence of inner and/or middle retinal thinning.

Results

A total of 190 participants (52 healthy participants and 138 diabetic patients) were included. A higher prevalence of FRT was observed in eyes with advanced DR versus healthy eyes and versus diabetic eyes with no DR or mild DR. FIRT and FCRT were significantly greater in eyes with proliferative DR treated with pan-retinal photocoagulation, and FMRT was significantly more common in eyes with severe nonproliferative DR. FRT was significantly more common in patients with coronary artery disease and was positively correlated with diabetes duration, serum creatinine, and glycosylated hemoglobin and negatively correlated with age, estimated glomerular filtration rate, and visual acuity.

Conclusions

FRT occurs in all stages of DR and is increasingly prevalent with increasing severity of DR.

Translational Relevance

OCT identification of FRT may provide a surrogate biomarker of retinal and systemic disease in diabetic patients.

Retinal Vascular Endothelial Cell Dysfunction and Neuroretinal Degeneration in Diabetic Patients

Diabetes mellitus (DM) has become a vital societal problem as epidemiological studies demonstrate the increasing incidence of type 1 and type 2 diabetes. Lesions observed in the retina in the course of diabetes, referred to as diabetic retinopathy (DR), are caused by vascular abnormalities and are ischemic in nature. Vascular lesions in diabetes pertain to small vessels (microangiopathy) and involve precapillary arterioles, capillaries and small veins. Pericyte loss, thickening of the basement membrane, and damage and proliferation of endothelial cells are observed. Endothelial cells (monolayer squamous epithelium) form the smooth internal vascular lining indispensable for normal blood flow. Breaking its continuity initiates blood coagulation at that site. The endothelium controls the process of exchange of chemical substances (nutritional, regulatory, waste products) between blood and the retina, and blood cell passing through the vascular wall. Endothelial cells produce biologically active substances involved in blood coagulation, regulating vascular wall tension and stimulating neoangiogenesis. On the other hand, recent studies have demonstrated that diabetic retinopathy may be not only a microvascular disease, but is a result of neuroretinal degeneration. Neuroretinal degeneration appears structurally, as neural apoptosis of amacrine and Muller cells, reactive gliosis, ganglion cell layer/inner plexiform (GCL) thickness, retinal thickness, and retinal nerve fiber layer thickness, and a reduction of the neuroretinal rim in minimum rim width (MRW) and functionally as an abnormal electroretinogram (ERG), dark adaptation, contrast sensitivity, color vision, and microperimetric test. The findings in early stages of diabetic retinopathy may precede microvascular changes of this disease. Furthermore, the article's objective is to characterize the factors and mechanisms conducive to microvascular changes and neuroretinal apoptosis in diabetic retinopathy. Only when all the measures preventing vascular dysfunction are determined will the risk of complications in the course of diabetes be minimized.

Gypenoside XVII alleviates early diabetic retinopathy by regulating Müller cell apoptosis and autophagy in db/db mice

Diabetic retinopathy (DR) is a widespread vision-threatening disease in working people. Müller cells are important glial cells that participate in the blood retinal barrier and promote the maintenance of retinal physiological and structural homeostasis. Müller cell apoptosis and autophagy play an important role in the pathogenesis of DR. Gypenoside XVII (Gyp-17) exerts strong antiapoptotic and autophagic activities. However, the effect of Gyp-17 on DR and its mechanism of action have not been elucidated. This study explored the effect of Gyp-17 on early DR and Müller cell injury in db/db mice. Blood glucose and blood lipids were measured. Optical coherence tomography and fundus fluorescein angiography were applied to detect retinal thickness and vascular leakage, respectively. Hematoxylin eosin staining assessed the pathological changes of the retina. Retinal oxidative environment and cell apoptosis and autophagy were monitored using commercial kits, immunofluorescence, and Western blot assays. Results showed that Gyp-17 exerted no significant effect on blood glucose and lipid levels but maintained normal retinal permeability, physiological structure, high anti-oxidative enzyme expression, and the thickness of the inner nuclear layer compared with the model group. Moreover, Western blot analysis and TUNEL assay indicated that Gyp-17 significantly decreased pro-apoptotic-related protein expression and increased pro-autophagy-related protein expression compared with the model group. Immunofluorescence colocalization exhibited that the regulating action of Gyp-17 may focus on Müller cells. These data strongly demonstrate that Gyp-17 prevents early DR by decreasing apoptosis and increasing autophagy in Müller cells. Gyp-17 may be a candidate drug for early DR therapy.

Novel Short-Chain Quinones to Treat Vision Loss in a Rat Model of Diabetic Retinopathy

Diabetic retinopathy (DR), one of the leading causes of blindness, is mainly diagnosed based on the vascular pathology of the disease. Current treatment options largely focus on this aspect with mostly insufficient therapeutic long-term efficacy. Mounting evidence implicates mitochondrial dysfunction and oxidative stress in the central etiology of DR. Consequently, drug candidates that aim at normalizing mitochondrial function could be an attractive therapeutic approach. This study compared the mitoprotective compounds, idebenone and elamipretide, side-by-side against two novel short-chain quinones (SCQs) in a rat model of DR. The model effectively mimicked type 2 diabetes over 21 weeks. During this period, visual acuity was monitored by measuring optokinetic response (OKR). Vision loss occurred 5–8 weeks after the onset of hyperglycemia. After 10 weeks of hyperglycemia, visual function was reduced by 65%. From this point, the right eyes of the animals were topically treated once daily with the test compounds. The left, untreated eye served as an internal control. Only three weeks of topical treatment significantly restored vision from 35% to 58–80%, while visual acuity of the non-treated eyes continued to deteriorate. Interestingly, the two novel SCQs restored visual acuity better than idebenone or elamipretide. This was also reflected by protection of retinal pathology against oxidative damage, retinal ganglion cell loss, reactive gliosis, vascular leakage, and retinal thinning. Overall, mitoprotective and, in particular, SCQ-based compounds have the potential to be developed into effective and fast-acting drug candidates against DR.

The neuroscience of diabetic retinopathy

Diabetic retinopathy remains a leading cause of blindness despite recent advance in therapies. Traditionally, this complication of diabetes was viewed predominantly as a microvascular disease but research has pointed to alterations in ganglion cells, glia, microglia, and photoreceptors as well, often occurring without obvious vascular damage. In neural tissue, the microvasculature and neural tissue form an intimate relationship with the neural tissue providing signaling cues for the vessels to form a distinct barrier that helps to maintain the proper neuronal environment for synaptic signaling. This relationship has been termed the neurovascular unit (NVU). Research is now focused on understanding the cellular and molecular basis of the neurovascular unit and how diabetes alters the normal cellular communications and disrupts the cellular environment contributing to loss of vision in diabetes.

Non-invasive multimodal imaging of Diabetic Retinopathy: A survey on treatment methods and Nanotheranostics

Diabetes Retinopathy (DR) is one of the most prominent microvascular complications of diabetes. It is one of the pre-eminent causes for vision impairment followed by blindness among the working-age population worldwide. The de facto cause for DR remains challenging, despite several efforts made to unveil the mechanism underlying the pathology of DR. There is quite less availability of the low cost pre-emptive theranostic imaging tools in terms of in-depth resolution, due to the multiple factors involved in the etiology of DR. This review work comprehensively explores the various reports and research works on all perspectives of diabetic retinopathy (DR), and its mechanism. It also discusses various advanced non-destructive imaging modalities, current, and future treatment approaches. Further, the application of various nanoparticle-based drug delivery strategies used for the treatment of DR are also discussed. In a nutshell, the present review work bolsters the pursuit of the development of an advanced non-invasive optical imaging modal with a nano-theranostic approach for the future diagnosis and treatment of DR and its associated ocular complications.

The innate immune system in diabetic retinopathy

The prevalence of diabetes has been rising steadily in the past half-century, along with the burden of its associated complications, including diabetic retinopathy (DR). DR is currently the most common cause of vision loss in working-age adults in the United States. Historically, DR has been diagnosed and classified clinically based on what is visible by fundoscopy; that is vasculature alterations. However, recent technological advances have confirmed pathology of the neuroretina prior to any detectable vascular changes. These, coupled with molecular studies, and the positive impact of anti-inflammatory therapeutics in DR patients have highlighted the central involvement of the innate immune system. Reminiscent of the systemic impact of diabetes, immune dysregulation has become increasingly identified as a key element of the pathophysiology of DR by interfering with normal homeostatic systems. This review uses the growing body of literature across various model systems to demonstrate the clear involvement of all three pillars of the immune system: immune-competent cells, mediators, and the complement system. It also demonstrates how the relative contribution of each of these requires more extensive analysis, including in human tissues over the continuum of disease progression. Finally, although this review demonstrates how the complex interactions of the immune system pose many more questions than answers, the intimately connected nature of the three pillars of the immune system may also point to possible new targets to reverse or even halt reverse retinopathy.

Optogenetic Stimulation of Cholinergic Amacrine Cells Improves Capillary Blood Flow in Diabetic Retinopathy

Purpose

Disruption in blood supply to active retinal circuits is the earliest hallmark of diabetic retinopathy (DR) and has been primarily attributed to vascular deficiency. However, accumulating evidence supports an early role for a disrupted neuronal function in blood flow impairment. Here, we tested the hypothesis that selectively stimulating cholinergic neurons could restore neurovascular signaling to preserve the capillary circulation in DR.

Methods

We used wild type (wt) and choline acetyltransferase promoter (ChAT)-channelrhodopsin-2 (ChR2) mice expressing ChR2 exclusively in cholinergic cells. Mice were made diabetic by streptozotocin (STZ) injections. Two to 3 months after the last STZ injection, the rate of capillary blood flow was measured in vivo within each retinal vascular layer using high speed two-photon imaging. Measurements were done at baseline and following ChR2-driven activation of retinal cholinergic interneurons, the sole source of the vasodilating neurotransmitter acetylcholine. After recordings, retinas were collected and assessed for physiological and structural features.

Results

In retinal explants from ChAT-ChR2 mice, we found that channelrhodopsin2 was selectively expressed in all cholinergic amacrine cells. Its direct activation by blue light led to dilation of adjacent retinal capillaries. In living diabetic ChAT-ChR2 animals, basal capillary blood flow was significantly higher than in diabetic mice without channelrhodopsin. However, optogenetic stimulation with blue light did not result in flickering light-induced functional hyperemia, suggesting a necessity for a concerted neurovascular interaction.

Conclusions

These findings provide direct support to the utility and efficacy of an optogenetic approach for targeting selective retinal circuits to treat DR and its complications.

Association of Myopia with Peripapillary Retinal Nerve Fiber Layer Thickness in Diabetic Patients Without Diabetic Retinopathy

Purpose

To investigate the association between myopia and peripapillary retinal nerve fiber layer (pRNFL) thickness in diabetic patients without diabetic retinopathy (DR).

Methods

A total of 271 eyes of 271 participants were included. They were divided into four groups according to the presence of myopia (≤ -3 diopters [D]) and diabetes without DR: (1) control group (n = 76), (2) myopia group (n = 57), (3) diabetes group (n = 82), and (4) diabetes + myopia group (n = 56). The peripapillary average and sector RNFL thicknesses were measured and compared among the four groups to determine the effects of myopia and diabetes. Covariates were adjusted using analyses of covariance. Linear regression analyses were fitted to evaluate the factors associated with pRNFL.

Results

Spherical equivalents were 0.12 ± 1.31 D in the control group, -4.00 ± 1.47 D in the myopia group, 0.00 ± 1.05 D in the diabetes group, and -4.33 ± 1.70 D in the diabetes + myopia group (P < 0.001). The respective axial lengths (ALs) were 23.91 ± 0.99 mm, 25.16 ± 0.94 mm, 23.68 ± 0.77 mm, and 25.34 ± 1.33 mm (P < 0.001). The average pRNFL showed a progressive decrease from the control group (97.16 ± 8.73 µm) to the myopia group (94.04 ± 9.13 µm) to the diabetes group (93.33 ± 9.07 µm) to the diabetes + myopia group (91.25 ± 9.72 µm) (P = 0.009). Age, diabetes, hypertension, and AL were significantly correlated with the pRNFL. The rate of reduction of pRNFL with increasing age was higher in the diabetes + myopia group than in the other groups, and pRNFL in the diabetes groups decreased more steeply with increasing AL compared to the non-diabetic groups.

Conclusions

Myopia and diabetes are important factors affecting pRNFL thickness, and the simultaneous presence of diabetes and myopia results in greater pRNFL damage than observed with either pathology alone.

Angiotensin-(1-7) Expressed From Lactobacillus Bacteria Protect Diabetic Retina in Mice

Purpose

A multitude of animal studies substantiates the beneficial effects of Ang-(1–7), a peptide hormone in the protective axis of the renin angiotensin system, in diabetes and its associated complications including diabetic retinopathy (DR). However, the clinical application of Ang-(1–7) is limited due to unfavorable pharmacological properties. As emerging evidence implicates gut dysbiosis in pathogenesis of diabetes and supports beneficial effects of probiotics, we sought to develop probiotics-based expression and delivery system to enhance Ang-(1–7) and evaluate the efficacy of engineered probiotics expressing Ang-(1–7) in attenuation of DR in animal models.

Methods

Ang-(1–7) was expressed in the Lactobacillus species as a secreted fusion protein with a trans-epithelial carrier to allow uptake into circulation. To evaluate the effects of Ang-(1–7) expressed from Lactobacillus paracasei (LP), adult diabetic eNOS^−/− and Akita mice were orally gavaged with either 1 × 10⁹ CFU of LP secreting Ang-(1–7) (LP-A), LP alone or vehicle, 3 times/week, for 8 and 12 weeks, respectively.

Results

Ang-(1–7) is efficiently expressed from different Lactobacillus species and secreted into circulation in mice fed with LP-A. Oral administration of LP-A significantly reduced diabetes-induced loss of retinal vascular capillaries. LP-A treatment also prevented loss of retinal ganglion cells, and significantly decreased retinal inflammatory cytokine expression in both diabetic eNOS^−/− and Akita mice.

Conclusions

These results provide proof-of-concept for feasibility and efficacy of using engineered probiotic species as live vector for delivery of Ang-(1–7) with enhanced bioavailability.

Translational Relevance

Probiotics-based delivery of Ang-(1–7) may hold important therapeutic potential for the treatment of DR and other diabetic complications.

Correlation Analysis between Nerve Fiber Layer Thickness and Peripapillary Vessel Density and Influencing Factors of Peripapillary Vessel Density in Preclinical Diabetic Retinopathy

Purpose

To observe the changes of the retinal nerve fiber layer (RNFL) thickness and the optic disc vessel density (VD) in preclinical diabetic retinopathy (DR) and the relationship between RNFL changes and VD, as well as to investigate the influencing factors on peripapillary vessel density.

Methods

This was a cross-sectional study. Thirty-four eyes of 34 type 2 diabetes mellitus (T2DM) patients diagnosed with preclinical diabetic retinopathy (DR) were included in our study, with twenty-three eyes of 23 healthy subjects set up as normal controls. History of diabetes, hypertension, and dyslipidemia was recorded in detail. All participants underwent color fundus photography (CFP), RNFL around the optic disc, and OCT angiography (OCTA) over the optic disc. The 4.5 mm × 4.5 mm Angio Disc scan mode was performed with all participants by using the OCTA instrument. The relationship between changes of RNFL in the four quadrants (superior, inferior, temporal, and nasal) and VD changes was analyzed.

Results

Vessel density was significantly lower in the superior (t = −2.27) and temporal (t = −2.02) peripapillary sectors of diabetic eyes compared to normal eyes (P < 0.05). The retinal nerve fiber layer (RNFL) was significantly thinner in the temporal quadrant (P < 0.001) of diabetic eyes compared to normal eyes. Pearson correlation coefficient analysis showed a significant positive correlation between vessel density and RNFL thickness in the peripapillary region in the temporal (r = 0.468, P < 0.01) and superior (r = 0.612, P < 0.01) sectors. Multiple linear regression analysis showed that glycated hemoglobin (HbA1c) (β = −1.50, P < 0.01) and the duration of diabetes (β = −0.33, P=0.03) were associated with peripapillary vessel density.

Conclusions

Preclinical DR presented optic disc microcirculation changes. Temporal RNFL thinning is an early sign of retinal neurodegeneration and is associated with temporal peripapillary vessel density reduction. The duration of diabetes and HbA1c are risk factors for peripapillary vessel density reduction in patients with preclinical DR.

Longitudinal analysis of microvascular perfusion and neurodegenerative changes in early type 2 diabetic retinal disease

AIM

To prospectively monitor subclinical changes in capillary perfusion and retinal layer thickness in patients with type 2 diabetes and early diabetic retinal disease over 2 years.

METHODS

In this longitudinal study we performed biannual retinal vascular imaging using optical coherence tomography angiography (RTVue) to analyse the foveal avascular zone (FAZ) area, perimeter, acircularity index (AI) and parafoveal superficial/deep vessel density (VD). Spectral-domain optical coherence tomography (Spectralis) was used to measure the thickness of nine macular layers and the peripapillary nerve fibre layer.

RESULTS

Among 117 eyes (58 left) of 59 patients (21 female), 105 had no diabetic retinopathy (DR), 6 mild and 6 moderate non-proliferative DR at baseline. We found DR progression in 13 eyes at year 2. The FAZ area (+0.008±0.002 mm²/year, p<0.0001), perimeter (+0.036±0.010 mm/year, p=0.006) and AI (+0.005±0.002/year, p=0.0280) increased significantly. A pronounced decrease was found in the superficial (-1.425±0.290%/year, p<0.0001) but not the deep VD. Inner neuroretinal loss was confined to the ganglion cell (-0.539±0.150 µm/year, p=0.0004) and the inner plexiform layer (-0.361±0.127 µm/year, p=0.0045). In the outer retina, we observed a statistically significant decrease in thickness in the outer plexiform, photoreceptor layer and pigment epithelium of -0.921±0.161 µm/year, -0.325±0.139 µm/year and -0.385±0.084 µm/year, respectively.

CONCLUSION

Subclinical signs of microangiopathy and neurodegeneration appear in parallel and are highly progressive even in the earliest stages of diabetic retinal disease.Trial registration number EudraCT20156000239634.

Vessel Density around Foveal Avascular Zone as a Potential Imaging Biomarker for Detecting Preclinical Diabetic Retinopathy: An Optical Coherence Tomography Angiography Study

Purpose: The purpose of this study was to investigate the changes of optical coherence tomography angiography (OCTA) parameters in diabetic retinopathy (DR) using an updated software with 3D projection artifact removal. Methods: In this cross-sectional observational study, 192 eyes of 111 patients with diabetes mellitus (DM) and 55 eyes of 34 age-matched healthy subjects were included. Diabetic patients were divided into three subgroups: without DR, with mild non-proliferative DR, and with moderate-to-severe non-proliferative DR. All eyes underwent dilated fundoscopy along with 3x3mm and 6x6mm OCTA image acquisition. Vessel density (VD), retinal thickness and foveal avascular zone (FAZ) parameters were analyzed. Correlation analyses between OCTA parameters and DR severity were also performed. Results: There was a statistically significant difference in all OCTA parameters among groups, except for superficial foveal VD in 6x6mm scan and whole image retinal thickness in both 3x3mm and 6x6mm scans, while 3x3mm scan parameters were found to be diagnostically superior to the corresponding ones of 6x6mm scan. As the DR stage progressed, the mean VD values decreased. FD-300, which is the VD of a 300-μm width annulus surrounding FAZ, demonstrated the strongest inverse correlation with DR severity (r = -0.590/r_s = -0.562, p < .001) and showed the highest area under the ROC curve (AUROC = 0.833 ± 0.030, p < .001) in scan 3 × 3. Conclusion: OCTA shows progressive decrease of VD parameters with increasing DR severity. Foveal VD, FAZ area, and perimeter are not very useful indexes due to the high interindividual variability of FAZ size. OCTA and specifically FD-300 may serve as a promising DR screening tool for detecting preclinical microvascular alterations.

Retinal capillary degeneration and blood-retinal barrier disruption in murine models of Alzheimer's disease

Extensive effort has been made studying retinal pathology in Alzheimer’s disease (AD) to improve early noninvasive diagnosis and treatment. Particularly relevant are vascular changes, which appear prominent in early brain pathogenesis and could predict cognitive decline. Recently, we identified platelet-derived growth factor receptor beta (PDGFRβ) deficiency and pericyte loss associated with vascular Aβ deposition in the neurosensory retina of mild cognitively impaired (MCI) and AD patients. However, the pathological mechanisms of retinal vascular changes and their possible relationships with vascular amyloidosis, pericyte loss, and blood-retinal barrier (BRB) integrity remain unknown. Here, we evaluated the retinas of transgenic APP_SWE/PS1_ΔE9 mouse models of AD (ADtg mice) and wild-type mice at different ages for capillary degeneration, PDGFRβ expression, vascular amyloidosis, permeability and inner BRB tight-junction molecules. Using a retinal vascular isolation technique followed by periodic acid-Schiff or immunofluorescent staining, we discovered significant retinal capillary degeneration in ADtg mice compared to age- and sex-matched wild-type mice (P < 0.0001). This small vessel degeneration reached significance in 8-month-old mice (P = 0.0035), with males more susceptible than females. Degeneration of retinal capillaries also progressively increased with age in healthy mice (P = 0.0145); however, the phenomenon was significantly worse during AD-like progression (P = 0.0001). A substantial vascular PDGFRβ deficiency (~ 50% reduction, P = 0.0017) along with prominent vascular Aβ deposition was further detected in the retina of ADtg mice, which inversely correlated with the extent of degenerated capillaries (Pearson’s r = − 0.8, P = 0.0016). Importantly, tight-junction alterations such as claudin-1 downregulation and increased BRB permeability, demonstrated in vivo by retinal fluorescein imaging and ex vivo following injection of FITC-dextran (2000 kD) and Texas Red-dextran (3 kD), were found in ADtg mice. Overall, the identification of age- and Alzheimer’s-dependent retinal capillary degeneration and compromised BRB integrity starting at early disease stages in ADtg mice could contribute to the development of novel targets for AD diagnosis and therapy.

Retinal neurovascular changes in chronic kidney disease

PURPOSE

To examine retinal neurovascular changes in patients with chronic kidney disease (CKD).

METHODS

Case-control study. A total of 171 CKD cases and 40 controls were recruited (mean age 62.9 ± 10.3 versus 60.8 ± 9.2, p = 0.257). Retinal neural parameters, including parafoveal retinal thickness (PfRT), macular ganglion cell complex thickness (GCCt), global loss volume (GLV), focal loss volume (FLV) and peripapillary retinal nerve fibre layer thickness (RNFLt), were measured using optical coherence tomography (OCT). Microvascular parameters, including foveal avascular zone size, vessel density over the parafoveal superficial vascular plexus (SVP-VD), parafoveal deep vascular plexus (DVP-VD) and radial peripapillary capillary (RPC-VD), were measured using OCT angiography.

RESULTS

Chronic kidney disease (CKD) patients showed reduced PfRT, GCCt and RNFLt and increased GLV and FLV compared with the controls (all p < 0.005). Among patients with CKD, estimated glomerular filtration rate was an independent factor associated with PfRT (coefficient 0.19, p = 0.015), GCCt (coefficient 0.10, p = 0.006), GLV (coefficient - 0.08, p = 0.001), FLV (coefficient - 0.02, p = 0.006) and RNFLt (coefficient 0.15, p = 0.002). Parafoveal retinal thickness (PfRT), GCCt, GLV, FLV and RNFLt were correlated with SVP-VD (all p < 0.001) but not with DVP-VD (all p > 0.1).

CONCLUSIONS

Chronic kidney disease (CKD) patients demonstrated a significant reduction in macular thickness and changes in retinal neural parameters. These changes were associated with the severity of CKD and correlated with the microvascular rarefaction in the parafoveal SVP.

Reversed Neurovascular Coupling on Optical Coherence Tomography Angiography Is the Earliest Detectable Abnormality before Clinical Diabetic Retinopathy

Diabetic retinopathy (DR) has traditionally been viewed as either a microvasculopathy or a neuropathy, though neurovascular coupling deficits have also been reported and could potentially be the earliest derangement in DR. To better understand neurovascular coupling in the diabetic retina, we investigated retinal hemodynamics by optical coherence tomography angiography (OCTA) in individuals with diabetes mellitus (DM) but without DR (DM no DR) and mild non-proliferative DR (mild NPDR) compared to healthy eyes. Using an experimental design to monitor the capillary responses during transition from dark adaptation to light, we examined 19 healthy, 14 DM no DR and 11 mild NPDR individuals. We found that the only structural vascular abnormality in the DM no DR group was increased superficial capillary plexus (SCP) vessel density (VD) compared to healthy eyes, while mild NPDR eyes showed significant vessel loss in the SCP at baseline. There was no significant difference in inner retinal thickness between the groups. During dark adaptation, the deep capillary plexus (DCP) VD was lower in mild NPDR individuals compared to the other two groups, which may leave the photoreceptors more susceptible to ischemia in the dark. When transitioning from dark to ambient light, both diabetic groups showed a qualitative reversal of VD trends in the SCP and middle capillary plexus (MCP), with significantly decreased SCP at 5 min and increased MCP VD at 50 s compared to healthy eyes, which may impede metabolic supply to the inner retina during light adaptation. Mild NPDR eyes also demonstrated DCP dilation at 50 s and 5 min and decreased adjusted flow index at 5 min in light. Our results show altered neurovascular responses in all three macular vascular plexuses in diabetic subjects in the absence of structural neuronal changes on high resolution imaging, suggesting that neurovascular uncoupling may be a key mechanism in the early pathogenesis of DR, well before the clinical appearance of vascular or neuronal loss.

Diabetic Retinopathy: Important Biochemical Alterations and the Main Treatment Strategies

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by impaired glucose homeostasis, insulin resistance and hyperglycemia. Among its serious multisystemic complications is diabetic retinopathy (DR), which develops slowly and often insidiously. This disorder-the most common cause of vision loss in working-age adults-is characterized by functional and morphological changes in the retina. It results from the exacerbation of ischemic and inflammatory conditions prompted by alterations in the blood vessels, such as the development of leukostasis, thickening of the basement membrane, retinal neovascularization and fibrovascular tissue formation at the vitreoretinal interface. The pathogenic alterations are usually triggered at the biochemical level, involving a greater activity in 4 pathways: the polyol pathway, the hexosamine pathway, the formation of advanced glycation end-products and the activation of protein kinase C isoforms. When acting together, these pathways give rise to increased levels of reactive oxygen species and decreased levels of endogenous antioxidant agents, thus generating oxidative stress. All current therapies are aimed at the later stages of DR, and their application implies side effects. One possible strategy for preventing the complications of DM is to counteract the elevated superoxide production stemming from a high level of blood glucose. Accordingly, some treatments are under study for their capacity to reduce vascular leakage and avoid retinal ischemia, retinal neovascularization and macular edema. The present review summarizes the biochemical aspects of DR and the main approaches for treating it.

Analysis of retinal neurodegeneration in gestational and type 2 diabetes using swept-source optical coherence tomography

OBJECTIVE

This study aimed to compare the retina and choroid thickness in age-matched pregnant individuals with gestational diabetes mellitus, nonpregnant diabetic females, and healthy nonpregnant females.

METHOD

This cross-sectional study included 2 study groups, 1 composed of pregnant women with gestational diabetes mellitus and 1 consisting of nonpregnant type 2 diabetic patients without diabetic retinopathy, and a control group of healthy nonpregnant subjects. Swept-source optical coherence tomography was used to measure the retinal and choroidal thickness. The measurements were compared between the study groups and between the study groups and the control group.

RESULTS

All groups had similar mean ages, best-corrected visual acuity, and intraocular pressure (p = 0.122, p = 0.158, and p = 0.186, respectively). The mean central macular thickness of the gestational diabetes, type 2 diabetes, and control groups was 215.3 ± 10.83, 220.58 ± 21.62, and 230.03 ± 21.24 μm, respectively (p = 0.002). The retinal nerve fibre layer was slightly thinner only in the inferior zone of the study groups (p = 0.058) compared with the control group. We observed statistically significant differences in the thickness of all sectors of the ganglion cell layer between all groups (all p < 0.05), with the nonpregnant type 2 diabetic group exhibiting the lowest values. A similar mean subfoveal choroidal thickness was observed in all 3 groups (p = 0.247).

CONCLUSION

Swept-source optical coherence tomography plays an important role in detecting retinal neurodegenerative changes and choroidal thickness induced by gestational and type 2 diabetes before the development of diabetic retinopathy.

The cells involved in the pathological process of diabetic retinopathy

Diabetic retinopathy(DR) is an expanding global health problem, the exact mechanism of which has not yet been clarified clearly, new insights into retinal physiology indicate that diabetes-induced retinal dysfunction may be viewed as an impairment of the retinal neurovascular unit, including retinal ganglion cells, glial cells, endothelial cells, pericytes, and retinal pigment epithelium. Different retinal cells have unique structure and functions, while the interactions among which are less known. Cells are the basic unit of organism structure and function, their impairment could lead to abnormal physiological functions and even organ disorder. Considering the body is multi-dimension and the complexity of DR, one point or a single type of cell can't be used to illustrate the mechanism of occurrence and development of DR. In this review, we provided a systematic and comprehensive elaboration of the cells that are involved in the process of DR. We underlined the importance of considering the neurovascular unit, not just retinal vascular and neural cells, in understanding the pathophysiology of DR. Our studies provided a better understanding of the pathological process in DR and provide a theoretical basis for further research.

Diabetic photoreceptors: Mechanisms underlying changes in structure and function

Based on clinical findings, diabetic retinopathy (DR) has traditionally been defined as a retinal microvasculopathy. Retinal neuronal dysfunction is now recognized as an early event in the diabetic retina before development of overt DR. While detrimental effects of diabetes on the survival and function of inner retinal cells, such as retinal ganglion cells and amacrine cells, are widely recognized, evidence that photoreceptors in the outer retina undergo early alterations in diabetes has emerged more recently. We review data from preclinical and clinical studies demonstrating a conserved reduction of electrophysiological function in diabetic retinas, as well as evidence for photoreceptor loss. Complementing in vivo studies, we discuss the ex vivo electroretinography technique as a useful method to investigate photoreceptor function in isolated retinas from diabetic animal models. Finally, we consider the possibility that early photoreceptor pathology contributes to the progression of DR, and discuss possible mechanisms of photoreceptor damage in the diabetic retina, such as enhanced production of reactive oxygen species and other inflammatory factors whose detrimental effects may be augmented by phototransduction.

Retinoprotective effect of donepezil in diabetic mice involves mitigation of excitotoxicity and activation of PI3K/mTOR/BCl2 pathway

Donepezil (DNPZ) has shown neuroprotective effect in many disorders. The current study tested the putative retinoprotection provided by donepezil in mouse diabetic retinopathy. Swiss albino mice were allocated to, 1] saline control, 2] diabetic, 3&4] diabetic+DNPZ (1 or 4 mg/kg). After induction of diabetes, mice were maintained for 8 weeks then DNPZ therapy was launched for 28 days. Retinas were isolated and used for histopathology and immunohistochemistry for caspase 3 and the anti-apoptotic protein, B-cell lymphoma 2 (BCl2). Retinas were examined for glutamate, acetylcholine and oxidation markers. Western blot analysis measured inflammatory cytokines, N-methyl-d-aspartate receptors (NMDARs), phosphorylated and total phosphatidylinositol-3 kinase and mTOR, BCl2 and cleaved caspase 3. Significant histopathological changes and decreased thickness were found in diabetic retinas (125.52 ± 2.85 vs. 157.15 ± 7.55 in the saline group). In addition, retinal glutamate (2.39-fold), inflammatory cytokines and NMDARs proteins (4.9-fold) were higher in the diabetic retinas. Western blot analysis revealed low ratio of phosphorylated/total PI3K (0.21 ± 0.043 vs. 1 ± 0.005) and mTOR (0.18 ± 0.04 vs. 1 ± 0.005), low BCl2 (0.28 ± 0.06 vs. 1 ± 0.005) and upregulated cleaved caspase 3 (5.18 ± 1.27 vs. 1 ± 0.05 in the saline group) versus the saline control. DNPZ ameliorated the histopathologic manifestations and to prevent the decrease in retinal thickness. DNPZ (4 mg/kg) improved phosphorylation of PI3K (0.76 ± 0.12 vs. 0.21 ± 0.04) and mTOR (0.59 ± 0.09 vs. 0.18 ± 0.04) and increased BCl2 (0.75 ± 0.08 vs. 0.28 ± 0.06) versus the diabetic control group. This study explained the retinoprotective effect of DNPZ in mouse diabetic retinopathy and highlighted that mitigation of excitotoxicity, improving phosphorylation of PI3K/mTOR and increasing BCl2 contribute to this effect.

Neuropathic damage in the diabetic eye: clinical implications

In recent years, emerging evidence support that the eye is target of diabetes neuropathy. There are two components of the eye that are mainly involved in the neurodegenerative process induced by diabetes: the retina and the cornea. The study of functional and structural changes in these components of the eye will provide useful information to identify subjects with diabetes at risk of diabetic peripheral neuropathy and dementia. In this review the state of the art regarding the evidence and clinical implications of this emerging concept will be provided. In addition, the relationship between retinal and corneal neurodegeneration with peripheral neuropathy and cognitive decline will be analyzed. Finally, the scientific gaps than need to be covered and will be critically examined.

Vascular changes precede tomographic changes in diabetic eyes without retinopathy and improve artificial intelligence diagnostics

The purpose of this study was to evaluate early vascular and tomographic changes in the retina of diabetic patients using artificial intelligence (AI). The study included 74 age-matched normal eyes, 171 diabetic eyes without retinopathy (DWR) eyes and 69 mild non-proliferative diabetic retinopathy (NPDR) eyes. All patients underwent optical coherence tomography angiography (OCTA) imaging. Tomographic features (thickness and volume) were derived from the OCTA B-scans. These features were used in AI models. Both OCT and OCTA features showed significant differences between the groups (P < .05). However, the OCTA features indicated early retinal changes in DWR eyes better than OCT (P < .05). In the AI model using both OCT and OCTA features simultaneously, the best area under the curve of 0.91 ± 0.02 was obtained (P < .05). Thus, the combined use of AI, OCT and OCTA significantly improved the early diagnosis of diabetic changes in the retina.

A critical review: Psychophysical assessments of diabetic retinopathy

Diabetic retinal disease remains a leading cause of vision loss despite currently available screening methods, ocular treatments, and efforts to control metabolic dysfunction. It is now understood that diabetes damages the entire retina and the cellular components of the neurovascular unit. Multiple studies have demonstrated impairment of various aspects of retinal function across the spectrum of retinopathy severity. Here we review these tests, the principles underlying their use, clinical data from multiple publications, the strengths and limitations of the studies, and prospects for their application to understand the pathophysiology of diabetic retinal disease and monitor its response to therapy. We focus on visual acuity, contrast sensitivity, color vision, visual field, and dark adaptation and their use to understand the pathophysiology of diabetic retinopathy and as potential endpoints for clinical trials.

Peripapillary Retinal Nerve Fiber Layer Changes in Patients with Diabetes Mellitus: A Case-control Study

PURPOSE

To evaluate peripapillary retinal nerve fiber layer (RNFL) changes in patients with diabetes mellitus (DM) and compare them with those of normal population. In addition, this study aims to determine potential factors, affecting RNFL changes in patients with DM.

METHODS

Participants in this study were 107 patients (211 eyes) with DM and 100 healthy controls (200 eyes). Diabetic patients were further classified into four groups depending on severity of diabetic retinopathy (no retinopathy, mild, moderate, severe nonproliferative diabetic retinopathy, and proliferative diabetic retinopathy, while presence of macular edema was also assessed. All participants underwent spectral-domain optical coherence tomography (SD-OCT) to measure RNFL thickness, while demographic and clinical characteristics of the participants were also recorded.

RESULTS

Patients with DM with or without DR presented significantly decreased peripapillary RNFL thickness in all quadrants. In the diabetic group, the multivariate analysis showed a significant association between decreased average RNFL thickness and increased HbA1c (p < 0.001), longer DM duration (p = 0.007), and more severe diabetic retinopathy status (p = 0.016), while presence of DME, age, gender, hypertension, and hyperlipidemia were not found to be associated with RNFL thickness decrease.

CONCLUSION

Diabetes mellitus seems to affect RNFL thickness, suggesting that early neurodegenerative changes may occur, potentially before microvascular alterations. Since duration of disease, DR severity, and HbA1c levels were associated with RNFL thinning, modifying glycemic control seems to be important to potentially prevent the development of DM complications.

Rod phototransduction and light signal transmission during type 2 diabetes

INTRODUCTION

Diabetic retinopathy is a major complication of diabetes recently associated with compromised photoreceptor function. Multiple stressors in diabetes, such as hyperglycemia, oxidative stress and inflammatory factors, have been identified, but systemic effects of diabetes on outer retina function are incompletely understood. We assessed photoreceptor physiology in vivo and in isolated retinas to better understand how alterations in the cellular environment compared with intrinsic cellular/molecular properties of the photoreceptors, affect light signal transduction and transmission in the retina in chronic type 2 diabetes.

RESEARCH DESIGN AND METHODS

Photoreceptor function was assessed in BKS.Cs-Dock7^m+/+Lepr ^db/J mice, using homozygotes for Lepr^db as a model of type 2 diabetes and heterozygotes as non-diabetic controls. In vivo electroretinogram (ERG) was recorded in dark-adapted mice at both 3 and 6 months of age. For ex vivo ERG, isolated retinas were superfused with oxygenated Ames' media supplemented with 30 mM glucose or mannitol as iso-osmotic control and electrical responses to light stimuli were recorded.

RESULTS

We found that both transduction and transmission of light signals by rod photoreceptors were compromised in 6-month-old (n=9-10 eyes from 5 animals, ***p<0.001) but not in 3-month-old diabetic mice in vivo (n=4-8 eyes from 2 to 4 animals). In contrast, rod signaling was similar in isolated retinas from 6-month-old control and diabetic mice under normoglycemic conditions (n=11). Acutely elevated glucose ex vivo increased light-evoked rod photoreceptor responses in control mice (n=11, ***p<0.001), but did not affect light responses in diabetic mice (n=11).

CONCLUSIONS

Our data suggest that long-term diabetes does not irreversibly change the ability of rod photoreceptors to transduce and mediate light signals. However, type 2 diabetes appears to induce adaptational changes in the rods that render them less sensitive to increased availability of glucose.

Dysfunctional Nurr1 promotes high glucose-induced Müller cell activation by up-regulating the NF-κB/NLRP3 inflammasome axis

Diabetic retinopathy (DR) is the most common microvascular complication of diabetes mellitus (DM). During DR, high glucose levels induce Müller cell gliosis, and the dysfunction of Müller cells further promotes the pathogenesis of DR. Transcription factor nuclear receptor subfamily 4 group A member 2 (Nurr1) inhibits the inflammatory response by suppressing nuclear factor-kappa B (NF-κB) and downregulating the downstream NACHT, LRR and PYD domain-containing protein 3 (NLRP3) inflammasome. This study aimed to investigate whether Nurr1 dysfunction in Müller cells promoted the NF-κB/NLRP3 inflammasome axis during DR. In vitro, Nurr1 expression and nuclear translocation decreased in Müller cells exposed to high glucose levels; therefore, p65 was activated, and the downstream NLRP3 inflammasome was up-regulated via the interaction of p65 with its promoter. These phenomena promoted Müller cell activation and proliferation. Moreover, in vivo, gavage of the Nurr1 agonist C-DIM12 reduced retinal ganglion cell (RGC) loss in a mouse model of streptozotocin (STZ)-induced diabetes. Together, these results showed that Nurr1 played important anti-inflammatory and neuroprotective roles in Müller cells during DR, suggesting that Nurr1 may be a potential molecular target for the treatment of DR.

The eye, the kidney, and cardiovascular disease: old concepts, better tools, and new horizons

Chronic kidney disease (CKD) is common, with hypertension and diabetes mellitus acting as major risk factors for its development. Cardiovascular disease is the leading cause of death worldwide and the most frequent end point of CKD. There is an urgent need for more precise methods to identify patients at risk of CKD and cardiovascular disease. Alterations in microvascular structure and function contribute to the development of hypertension, diabetes, CKD, and their associated cardiovascular disease. Homology between the eye and the kidney suggests that noninvasive imaging of the retinal vessels can detect these microvascular alterations to improve targeting of at-risk patients. Retinal vessel-derived metrics predict incident hypertension, diabetes, CKD, and cardiovascular disease and add to the current renal and cardiovascular risk stratification tools. The advent of optical coherence tomography (OCT) has transformed retinal imaging by capturing the chorioretinal microcirculation and its dependent tissue with near-histological resolution. In hypertension, diabetes, and CKD, OCT has revealed vessel remodeling and chorioretinal thinning. Clinical and preclinical OCT has linked retinal microvascular pathology to circulating and histological markers of injury in the kidney. The advent of OCT angiography allows contrast-free visualization of intraretinal capillary networks to potentially detect early incipient microvascular disease. Combining OCT's deep imaging with the analytical power of deep learning represents the next frontier in defining what the eye can reveal about the kidney and broader cardiovascular health.

The Complex Relationship between Diabetic Retinopathy and High-Mobility Group Box: A Review of Molecular Pathways and Therapeutic Strategies

High-mobility group box 1 (HMGB1) is a protein that is part of a larger family of non-histone nuclear proteins. HMGB1 is a ubiquitary protein with different isoforms, linked to numerous physiological and pathological pathways. HMGB1 is involved in cytokine and chemokine release, leukocyte activation and migration, tumorigenesis, neoangiogenesis, and the activation of several inflammatory pathways. HMGB1 is, in fact, responsible for the trigger, among others, of nuclear factor-κB (NF-κB), tumor necrosis factor-α (TNF-α), toll-like receptor-4 (TLR-4), and vascular endothelial growth factor (VEGF) pathways. Diabetic retinopathy (DR) is a common complication of diabetes mellitus (DM) that is rapidly growing in number. DR is an inflammatory disease caused by hyperglycemia, which determines the accumulation of oxidative stress and cell damage, which ultimately leads to hypoxia and neovascularization. Recent evidence has shown that hyperglycemia is responsible for the hyperexpression of HMGB1. This protein activates numerous pathways that cause the development of DR, and HMGB1 levels are constantly increased in diabetic retinas in both proliferative and non-proliferative stages of the disease. Several molecules, such as glycyrrhizin (GA), have proven effective in reducing diabetic damage to the retina through the inhibition of HMGB1. The main focus of this review is the growing amount of evidence linking HMGB1 and DR as well as the new therapeutic strategies involving this protein.

Updates on the Management of Ocular Vasculopathies with VEGF Inhibitor Conbercept

Purpose: To provide a detailed review on the therapeutic efficacy of conbercept for the management of ocular vasculopathies. Methods: A comprehensive literature search of various electronic databases was performed. Results: Ocular vasculopathy is one of the major causes of visual impairment and blindness which includes a range of disorders. Vascular endothelial growth factor (VEGF) regulates angiogenesis, enhances vascular permeability, and drives the formation of neovascularization. Anti-VEGF therapy has been shown to prevent vision loss or potentially improve vision in patients with exudative or neovascular retinal disease. The most recent anti-VEGF drug in China is conbercept. In the USA and Europe, bevacizumab is the most recently approved anti-VEGF agent. Conclusions: Conbercept serves as another anti-VEGF option for patients with neovascular AMD and other retinal vascular disorders. There have not been many clinical trials that study conbercept as compared with other currently available anti-VEGF drugs. There is a need for large-scale, well-designed, randomized clinical trials to ensure its long-term safety and efficacy and to determine if it has any advantages over other anti-VEGF agents.

Intraocular pressure fluctuation and neurodegeneration in the diabetic rat retina

BACKGROUND AND PURPOSE

Early retinal neurodegeneration occurs as one of the complications of diabetes even before clinically detectable diabetic vascular retinopathy. The pathogenesis of retinal diabetic neuropathy is still not well understood. We investigated the serial changes or fluctuations in intraocular pressure (IOP) and examined their roles in the pathogenesis of neuronal degeneration in diabetic retina.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats with streptozotocin-induced diabetes were treated with ophthalmic preparations of brinzolamide, latanoprost, both drugs (combined treatment) or saline for 8 weeks. IOP was measured daily under general anaesthesia using a rebound tonometer. Antegrade axoplasmic flow in the optic nerve was assessed with a fluorescent substrate. Immunohistochemical staining, TUNEL assays and western blots were also used.

KEY RESULTS

The fluctuation of IOP was higher in the diabetes group than in the normal control or the combined treatment group. Diabetes-induced apoptosis of retinal ganglion cells was decreased by combined treatment. Increased expression of glial fibrillary acidic protein or Iba-1 in the retina or optic nerve head, induced by diabetes, was attenuated only by the combined treatment. Intercellular adhesion molecule-1 was increased in diabetic rats but not in the combined treatment group. Diabetes-induced loss of antegrade axoplasmic transport was partially relieved with combined treatment.

CONCLUSION AND IMPLICATIONS

Elevated IOP fluctuations seemed to be associated with the gliosis, neuroinflammation, and neurodegeneration induced by diabetes. The loss of retinal ganglion cells might be relieved by IOP-lowering medication. The improvement of unstable perfusion pressure could play a role in neuroprotection in the diabetic retina.

Long-term lutein administration attenuates retinal inflammation and functional deficits in early diabetic retinopathy using the Ins2Akita/+ mice

INTRODUCTION

Lutein is a carotenoid whose protective effects in the retina have been reported in various studies. The effect of lutein has not been reported in the retina of the Ins2^Akita/+ mouse, a well-characterized genetic model for diabetic retinopathy (DR) in which the etiology of diabetes is better defined than the chemically induced diabetes. The objective of the present study is to investigate the effect of long-term administration of lutein in early stages of DR using the Ins2^Akita/+ mouse.

RESEARCH DESIGN AND METHODS

Heterozygous male Ins2^Akita/+ and age-matched wild-type mice were used. Lutein was administered to the mice in drinking water starting 6 weeks old daily until analysis at 4.5, 6.5 or 9 months of age. Plain water served as non-treatment control. Microglia were immunostained with ionized calcium-binding adapter molecule 1 (Iba-1) and cluster of differentiation 68 (CD68) in retinal flat-mounts. Vascular endothelial growth factor (VEGF) level in the retina was assessed by enzyme-linked immunosorbent assay (ELISA). Vascular permeability was analyzed in retinal flat-mounts after fluorescein isothiocyanate (FITC)-dextran perfusion. Retinal occludin expression was assessed via Western blots. Retinal function was examined by electroretinography (ERG).

RESULTS

Increased microglial reactivity was detected in the Ins2^Akita/+ mouse retina and was suppressed by lutein. Lutein administration also reduced the upregulation of VEGF in the Ins2^Akita/+ mouse retina. Increased vascular leakage and decreased occludin expression were observed in the Ins2^Akita/+ mouse retina, and these alterations were attenuated by lutein treatment. ERG recordings showed reduced a-wave and b-wave amplitudes in the Ins2^Akita/+ mice. With lutein treatment, the ERG deficits were significantly alleviated.

CONCLUSIONS

We showed beneficial effects of long-term lutein administration in the Ins2^Akita/+ mouse retina, including suppression of retinal inflammation, protection of retinal vasculature and preservation of retinal function. These results point to lutein's potential as a long-term therapeutic intervention for prevention of inflammation and retinal degeneration in patients with early DR.

Mitochondrial Dysfunction as a Novel Target for Neuroprotective Nutraceuticals in Ocular Diseases

The eyes require a rich oxygen and nutrient supply; hence, the high-energy demand of the visual system makes it sensitive to oxidative stress. Excessive free radicals result in mitochondrial dysfunction and lead to retinal neurodegeneration, as an early stage of retinal metabolic disorders. Retinal cells are vulnerable because of their coordinated interaction and intricate neural networks. Nutraceuticals are believed to target multiple pathways and have shown neuroprotective benefits by scavenging free radicals and promoting mitochondrial gene expression. Furthermore, encouraging results demonstrate that nutraceuticals improve the organization of retinal cells and visual functions. This review discusses the mitochondrial impairments of retinal cells and the mechanisms underlying the neuroprotective effects of nutraceuticals. However, some unsolved problems still exist between laboratory study and clinical therapy. Poor bioavailability and bioaccessibility strongly limit their development. A new delivery system and improved formulation may offer promise for health care applications.

Assessment of retinal neurodegeneration with spectral-domain optical coherence tomography: a systematic review and meta-analysis

OBJECTIVES

To comprehensively assess diabetic retinopathy neurodegeneration (DRN) as quantified by retinal neuronal and axonal layers measured with spectral-domain optical coherence tomography (SD-OCT) in subjects with diabetes mellitus (DM).

METHODS

Articles on the topic of examining macular ganglion cell-inner plexiform layer (m-GCIPL), macular retinal nerve fibre layer (m-RNFL), macular ganglion cell complex (m-GCC), and peripapillary RNFL (p-RNFL) measured with SD-OCT in DM subjects without DR (NDR) or with non-proliferative DR (NPDR) were searched in PubMed and Embase up to November 31, 2019. Standardized mean difference (SMD) as effect size were pooled using random-effects model.

RESULTS

Thirty-six studies searched from online databases and the CUHK DM cohort were included in the meta-analysis. In the comparison between NDR and control, macular measures including mean m-GCIPL (SMD = -0.26, p = 0.003), m-RNFL (SMD = -0.26, p = 0.046), and m-GCC (SMD = -0.28; p = 0.009) were significantly thinner in the NDR group. In the comparison between NPDR and NDR, only mean p-RNFL was significantly thinner in the NPDR group (SMD = -0.27; p = 0.03), but not other macular measures.

CONCLUSIONS

Thinning of retinal neuronal and axonal layers at macula as measured by SD-OCT are presented in eyes with NDR, supporting DRN may be the early pathogenesis in the DM patients without the presence of clinical signs of DR. In the future, these SD-OCT measures may be used as surrogates of DRN to stratify DM patients with a high risk of DR, and may be used as a therapeutic target if neuroprotection treatment for DR is available.