Neurodegeneration in diabetic retinopathy: Potential for novel therapies

ADAPTIVE OPTICS IMAGING IN DIABETIC RETINOPATHY: A Prospective Cohort Study

PURPOSE

To investigate the correlation between diabetic retinopathy (DR) severity and microscopic retinal and vascular alterations using adaptive optics imaging.

METHODS

In this single-center, prospective cohort study, adult participants with healthy eyes or DR underwent adaptive optics imaging. Participants were classified into control/mild nonproliferative DR, moderate/severe nonproliferative DR, and proliferative DR. Adaptive optics imaging using the RTX1 camera was obtained from 48 participants (87 eyes) for photoreceptor data and from 36 participants (62 eyes) for vascular data.

RESULTS

Photoreceptor parameters significantly differed between DR groups at 2° and 4° of retinal eccentricity. Wall-to-lumen ratio varied significantly at 2° eccentricity, while other vascular parameters remained nonsignificant. Cone density and dispersion were the strongest predictors for DR severity ( P < 0.001) in multivariable generalized estimating equation modeling, while other vascular parameters remained nonsignificant between DR severity groups. All photoreceptor parameters showed significant correlations with visual acuity overall and across most DR severity groups.

CONCLUSION

To date, this is one of the largest studies evaluating the use of adaptive optics imaging in DR. Adaptive optics imaging was demonstrated to differentiate between various levels of disease severity in DR. These results support the potential role in diagnostic and therapeutic microstructural evaluation in research and clinical practice.

Quantitative Analysis of the Vasculature and Cone Photoreceptors in Subjects With Diabetes Without Diabetic Retinopathy

PURPOSE

To characterize any differences in the vasculature and cone photoreceptor packing geometry (CPG) between subjects with diabetes without/no diabetic retinopathy (NDR) and healthy controls.

METHODS

Eight NDR and five controls were enrolled. Optical coherence tomography angiography (OCTA) taken at the macula was used to measure vessel density, vessel length density, and vessel density index (VDI) in three vascular plexuses, namely, the superficial vascular plexus, intermediate capillary plexus, and deep capillary plexus (DCP). The choriocapillaris (CC) flow deficit (FD) was also measured. OCTA images were binarized and processed to extrapolate the parafovea and parafoveal quadrants and the OCTA indices mentioned above. The CC was processed with six different radii to quantify FD. Adaptive optics - scanning laser ophthalmoscopy images were acquired and processed to extract CPG indices, i.e., cone density (CD), cone-to-cone spacing (CS), linear dispersion index, heterogeneity packing index and percent of cells with six neighbors at 3.6° in the temporal retina.

RESULTS

In all eyes, statistically significant differences were found (i) in parafoveal FD across the six radii (p < 0.001) and (ii) in the correlation between the parafoveal temporal quadrant (PTQ) DCP VDI and CS (r = 0.606, p = 0.048). No other significant correlations were found. For OCTA or CPG indices, no significant differences were found between the cohorts in the parafovea or parafoveal quadrants.

CONCLUSIONS

CS is the most sensitive CPG index for detecting alterations in the cone mosaic. The DCP and the cone photoreceptors are significantly correlated, indicating that alterations in the DCP can affect the cones. Future work elucidating the vascular alterations and neurodegeneration present in diabetic eyes should focus on the DCP and multiple CPG indices, not solely CD. Moreover, such alterations are highly localized, hence using larger regions e.g. parafovea versus smaller areas, such as the PTQ, will potentially mask significant correlations.

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Outer Retinal and Choroidal Changes in Adolescents with Long-Lasting Type 1 Diabetes

This study aimed to assess outer retinal layer (ORL), retinal pigment epithelium (RPE), choroid (Ch) and choriocapillaris (CC) modifications in adolescents with long-lasting (>10 years) type 1 diabetes (T1D) without (noDR) or with diabetic retinopathy (DR). ORL and RPE thickness were measured at optical coherence tomography (OCT) macular scans. Vascular parameters of Ch and CC were quantified after elaboration of macular OCT-angiography (OCTA) images. Insulin dose and auxological and metabolic parameters were correlated with OCT and OCTA findings in patients. ORL thickness was higher in DR eyes than in noDR and healthy controls (HC), and RPE thickness was higher in noDR and DR eyes than in HC, with statistical significance for some sectors in noDR versus HC. No OCTA parameters of CC and Ch differed among groups, and no significant correlation was observed with auxological and metabolic parameters. In conclusion, ORL and RPE were both increased in adolescents with long-lasting T1D. Such changes were not associated with insulin dose and glycemia control, nor to any choroid or choriocapillaris flow change clinically detectable at OCTA, and they could be potential imaging biomarkers of disease progression.

Agreement, repeatability, and reproducibility of quantitative retinal layer assessment using swept-source and spectral-domain optical coherence tomography in eyes with retinal diseases

Purpose

To evaluate the agreement and precision of retinal thickness measurements obtained using swept-source optical coherence tomography (SS-OCT) and spectral-domain OCT (SD-OCT) in healthy eyes and eyes with retinopathy.

Methods

This cross-sectional prospective study involved three DRI-OCT Triton (SS-OCT) and three 3D-OCT-1 Maestro (SD-OCT) devices. One of each device (Maestro and Triton) was paired with a single operator. Healthy subjects and patients with retinal diseases were recruited, with study eye and testing order randomized. At least 3 scans per eye were captured for wide scan (12 mm × 9 mm-Triton and Maestro) and macular cube scan (7 mm × 7 mm-Triton, 6 mm × 6 mm-Maestro). Thickness of the full retina, ganglion cell layer + inner plexiform layer (GCL+), and ganglion cell complex (GCL++) were obtained from wide scan and cube scans. Agreement of the measurements between the Triton and Maestro was evaluated by Bland-Altman analysis and Deming regression for each group. Repeatability and reproducibility were assessed using a two-way random effect analysis of variance (ANOVA) model for each parameter by group.

Results

Twenty-five healthy subjects (25 eyes) and 26 patients with retinal diseases (26 eyes), including, but not limited to, age-related macular degeneration, macular hole, and diabetic retinopathy were recruited. Overall, the measurement differences between Triton and Maestro were <6 μm (mean differences of full retina, GCL++, and GCL+ thickness were ≤5.5 μm, 1.3 μm, and 2.8 μm, respectively) and not statistically significant across the parameters. The repeatability and reproducibility estimates indicate high precision in both devices and groups. Across all the parameters, the repeatability limit was ≤7.6 μm for Triton and ≤12.7 μm for Maestro; reproducibility limit was ≤9.2 μm for Triton and ≤14.4 μm for Maestro. In eyes with retinal pathology, the repeatability coefficient of variation (CV)% was ≤2.6% for Triton and ≤3.4% for Maestro; reproducibility CV% was ≤3.3% for Triton and ≤3.5% for Maestro.

Conclusion

Both Triton SS-OCT and Maestro SD-OCT provide reliable measurements of retinal thickness in healthy eyes and eyes with retinal diseases. Excellent agreement between the two devices indicates interoperability when testing healthy eyes or eyes with retinal pathology. These findings support the use of thickness measurements from Triton SS-OCT and Maestro SD-OCT in clinical practice.

Exendin-4 promotes retinal ganglion cell survival and function by inhibiting calcium channels in experimental diabetes

Progressive damage of retinal ganglion cells (RGCs) is observed in early diabetic retinopathy. Intracellular Ca2+ overload mediated by Ca2+ influx through voltage-gated Ca2+ channels (VGCCs) is involved in neurodegeneration, whereas glucagon-like peptide-1 (GLP-1) provides neuroprotection. However, whether GLP-1 plays a neuroprotective role in diabetic retinas by modulating VGCCs remains unknown. We found that eye drops of exendin-4, a long-acting GLP-1 receptor (GLP-1R) agonist, prevented the increase of L-type Ca2+ current (ILCa) densities of RGCs induced by 4-week hyperglycemia and promoted RGC survival by suppressing L-type VGCC (L-VGCC) activity in streptozotocin-induced diabetic rats. Moreover, exendin-4-induced suppression of ILCa in RGCs may be mediated by a GLP-1R/Gs/cAMP-PKA/ryanodine/Ca2+/calmodulin/calcineurin/PP1 signaling pathway. Furthermore, exendin-4 functionally improved the light-evoked spiking ability of diabetic RGCs. These results suggest that GLP-1R activation enhances cAMP to PP1 signaling and that PP1 inactivates L-VGCCs by dephosphorylating them, thereby reducing Ca2+ influx, which could protect RGCs against excitotoxic Ca2+ overload.

Visual Dysfunction in Diabetes

Although diabetic retinopathy (DR) is clinically diagnosed as a vascular disease, many studies find retinal neuronal and visual dysfunction before the onset of vascular DR. This suggests that DR should be viewed as a neurovascular disease. Prior to the onset of DR, human patients have compromised electroretinograms that indicate a disruption of normal function, particularly in the inner retina. They also exhibit reduced contrast sensitivity. These early changes, especially those due to dysfunction in the inner retina, are also seen in rodent models of diabetes in the early stages of the disease. Rodent models of diabetes exhibit several neuronal mechanisms, such as reduced evoked GABA release, increased excitatory glutamate signaling, and reduced dopamine signaling, that suggest specific neuronal deficits. This suggests that understanding neuronal deficits may lead to early diabetes treatments to ameliorate neuronal dysfunction.

Diabetic rats with high levels of endogenous dopamine do not show retinal vascular pathology

Purpose

Limited research exists on the time course of long-term retinal and cerebral deficits in diabetic rodents. Previously, we examined short term (4-8 weeks) deficits in the Goto-Kakizaki (GK) rat model of Type II diabetes. Here, we investigated the long-term (1-8 months) temporal appearance of functional deficits (retinal, cognitive, and motor), retinal vascular pathology, and retinal dopamine levels in the GK rat.

Methods

In GK rats and Wistar controls, retinal neuronal function (electroretinogram), cognitive function (Y-maze), and motor function (rotarod) were measured at 1, 2, 4, 6, and 8 months of age. In addition, we evaluated retinal vascular function (functional hyperemia) and glucose and insulin tolerance. Retinas from rats euthanized at ≥8 months were assessed for vascular pathology. Dopamine and DOPAC levels were measured via HPLC in retinas from rats euthanized at 1, 2, 8, and 12 months.

Results

Goto-Kakizaki rats exhibited significant glucose intolerance beginning at 4 weeks and worsening over time (p < 0.001). GK rats also showed significant delays in flicker and oscillatory potential implicit times (p < 0.05 to p < 0.001) beginning at 1 month. Cognitive deficits were observed beginning at 6 months (p < 0.05), but no motor deficits. GK rats showed no deficits in functional hyperemia and no increase in acellular retinal capillaries. Dopamine levels were twice as high in GK vs. Wistar retinas at 1, 2, 8, and 12 months (p < 0.001).

Conclusion

As shown previously, retinal deficits were detectable prior to cognitive deficits in GK rats. While retinal neuronal function was compromised, retinal vascular pathology was not observed, even at 12+ months. High endogenous levels of dopamine in the GK rat may be acting as an anti-angiogenic and providing protection against vascular pathology.

Association of Placental Growth Factor and Angiopoietin in Human Retinal Endothelial Cell-Pericyte co-Cultures and iPSC-Derived Vascular Organoids

PURPOSE

Placental growth factor (PlGF) and Angiopoietin (Ang)-1 are two proteins that are involved in the regulation of endothelial cell (EC) growth and vasculature formation. In the retina and endothelial cells, pericytes are the major source of both molecules. The purpose of this study is to examine the association of PlGF and Ang-1 with human EC/pericyte co-cultures and iPSC-derived vascular organoids.

METHODS

In this study, we used co-cultures of human primary retinal endothelial cells (HREC) and primary human retinal pericytes (HRP), western blotting, immunofluorescent analysis, TUNEL staining, LDH-assays, and RNA seq analysis, as well as human-induced pluripotent stem cells (iPSC), derived organoids (VO) to study the association between PlGF and Ang-1.

RESULTS

Inhibition of PlGF by PlGF neutralizing antibody in HREC-HRP co-cultures resulted in the increased expression of Ang-1 and Tie-2 in a dose-dependent manner. This upregulation was not observed in HREC and HRP monocultures but only in co-cultures suggesting the association of pericytes and endothelial cells. Furthermore, Vascular endothelial growth factor receptor 1 (VEGFR1) inhibition abolished the Ang-1 and Tie-2 upregulation by PlGF inhibition. The pericyte viability in high-glucose conditions was also reduced by VEGFR1 neutralization. Immunofluorescent analysis showed that Ang-1 and Ang-2 were expressed mainly by perivascular cells in the VO. RNA seq analysis of the RNA isolated from VO in high glucose conditions indicated increased PlGF and Ang-2 expressions in the VO. PlGF inhibition increased the expression of Ang-1 and Tie-2 in VO, increasing the pericyte coverage of the VO microvascular network.

CONCLUSION

Combined, these results suggest PlGF's role in the regulation of Ang-1 and Tie-2 expression through VEGFR1. These findings provide new insights into the neovascularization process in diabetic retinopathy and new targets for potential therapeutic intervention.

Rod photoreceptor activation and deactivation in early-stage diabetic eye disease

PURPOSE

To infer rod phototransduction activation and deactivation characteristics in diabetics who have mild or no clinically-apparent retinopathy.

METHODS

Fifteen non-diabetic controls, 15 diabetics with no clinically-apparent diabetic retinopathy (NDR), and 15 diabetics with mild non-proliferative diabetic retinopathy (MDR) participated. Dark-adapted flash electroretinograms (3.2 to 4.4 log scot td-s) were recorded to assess rod activation. The a-waves were fit with a Gaussian model to derive R_mp3 (maximum photoreceptor response amplitude) and S (phototransduction sensitivity). Rod deactivation was assessed with a paired flash paradigm, in which a-waves were measured for two flashes separated by inter-stimulus intervals (ISIs) of 0.125 to 16 s. The ISI needed for the a-wave amplitude of the second flash to recover to 50% of the first flash (t₅₀) was determined. The effect of stimulus retinal illuminance on activation and deactivation was evaluated in a subset of control subjects.

RESULTS

Analysis of variance indicated that both diabetic groups had significant log S reductions compared to controls (p < 0.001). Mean S was reduced by approximately 49% and 78% for the NDR and MDR groups, respectively. In contrast, log R_mp3 and log t₅₀ did not differ significantly among the groups (both p > 0.08). Reducing stimulus retinal illuminance significantly reduced S, but did not significantly affect R_max or t₅₀.

CONCLUSIONS

Only phototransduction sensitivity was abnormal in this sample of diabetic subjects. The normal deactivation kinetics suggests that circulating rod current is normal. These findings begin to constrain possible explanations for abnormal rod function in early diabetic retinal disease.

miRNA-124 Prevents Rat Diabetic Retinopathy by Inhibiting the Microglial Inflammatory Response

Diabetic retinopathy (DR) is characterized by vasoregression and glial activation. miRNA-124 (miR-124) reduces retinal microglial activation and alleviates vasoregression in a neurodegenerative rat model. Our aim was to determine whether miR-124 affects vascular and neural damage in the early diabetic retina. Diabetes was induced in 8-week-old Wistar rats by streptozotocin (STZ) injection. At 16 and 20 weeks, the diabetic rats were intravitreally injected with miR-124 mimic, and retinae were analyzed at 24 weeks. Microvascular damage was identified by evaluating pericyte loss and acellular capillary (AC) formation. Müller glial activation was assessed by glial fibrillary acidic protein (GFAP) immunofluorescence staining. Microglial activation was determined by immunofluorescent staining of ionized calcium-binding adaptor molecule 1 (Iba1) in whole mount retinae. The neuroretinal function was assessed by electroretinography. The expression of inflammation-associated genes was evaluated by qRT-PCR. A wound healing assay was performed to quantitate the mobility of microglial cells. The results showed that miR-124 treatment alleviated diabetic vasoregression by reducing AC formation and pericyte loss. miR-124 blunted Müller glial- and microglial activation in diabetic retinae and ameliorated neuroretinal function. The retinal expression of inflammatory factors including Tnf-α, Il-1β, Cd74, Ccl2, Ccl3, Vcam1, Tgf-β1, Arg1, and Il-10 was reduced by miR-124 administration. The elevated mobility of microglia upon high glucose exposure was normalized by miR-124. The expression of the transcription factor PU.1 and lipid raft protein Flot1 was downregulated by miR-124. In rat DR, miR-124 prevents vasoregression and glial activation, improves neuroretinal function, and modulates microglial activation and inflammatory responses.

Diabetic Retinopathy: Soluble and Imaging Ocular Biomarkers

Diabetic retinopathy (DR), the most common microvascular complication of diabetes mellitus, represents the leading cause of acquired blindness in the working-age population. Due to the potential absence of symptoms in the early stages of the disease, the identification of clinical biomarkers can have a crucial role in the early diagnosis of DR as well as for the detection of prognostic factors. In particular, imaging techniques are fundamental tools for screening, diagnosis, classification, monitoring, treatment planning and prognostic assessment in DR. In this context, the identification of ocular and systemic biomarkers is crucial to facilitate the risk stratification of diabetic patients; moreover, reliable biomarkers could provide prognostic information on disease progression as well as assist in predicting a patient's response to therapy. In this context, this review aimed to provide an updated and comprehensive overview of the soluble and anatomical biomarkers associated with DR.

Vitreous humor proteome: unraveling the molecular mechanisms underlying proliferative and neovascular vitreoretinal diseases

Proliferative diabetic retinopathy (PDR), proliferative vitreoretinopathy (PVR), and neovascular age-related macular degeneration (nAMD) are among the leading causes of blindness. Due to the multifactorial nature of these vitreoretinal diseases, omics approaches are essential for a deeper understanding of the pathophysiologic processes underlying the evolution to a proliferative or neovascular etiology, in which patients suffer from an abrupt loss of vision. For many years, it was thought that the function of the vitreous was merely structural, supporting and protecting the surrounding ocular tissues. Proteomics studies proved that vitreous is more complex and biologically active than initially thought, and its changes reflect the physiological and pathological state of the eye. The vitreous is the scenario of a complex interplay between inflammation, fibrosis, oxidative stress, neurodegeneration, and extracellular matrix remodeling. Vitreous proteome not only reflects the pathological events that occur in the retina, but the changes in the vitreous itself play a central role in the onset and progression of vitreoretinal diseases. Therefore, this review offers an overview of the studies on the vitreous proteome that could help to elucidate some of the pathological mechanisms underlying proliferative and/or neovascular vitreoretinal diseases and to find new potential pharmaceutical targets.

Transcriptomic Analysis Reveals That Retinal Neuromodulation Is a Relevant Mechanism in the Neuroprotective Effect of Sitagliptin in an Experimental Model of Diabetic Retinopathy

Synaptic dysfunction and neuronal damage have been extensively associated with diabetic retinopathy (DR). Our group evidenced that chronic hyperglycemia reduces the retinal expression of presynaptic proteins, which are crucial for proper synaptic function. The aim of the study was to explore the effect of topically administered sitagliptin, an inhibitor of the enzyme dipeptidyl peptidase-4, on the retinal expression patterns of an experimental model of DR. Transcriptome analysis was performed, comparing the retinas of 10 diabetic (db/db) mice randomly treated with sitagliptin eye drops (10 mg/mL) twice daily and the retinas of 10 additional db/db mice that received vehicle eye drops. Ten non-diabetic mice (db/+) were used as a control group. The Gene Ontology (GO) and Reactome databases were used to perform the gene set enrichment analysis (GSEA) in order to explore the most enriched biological pathways among the groups. The most differentiated genes of these pathways were validated through quantitative RT-PCR. Transcriptome analysis revealed that sitagliptin eye drops have a significant effect on retinal expression patterns and that neurotransmission is the most enriched biological process. Our study evidenced enriched pathways that contain genes involved in membrane trafficking, transmission across chemical synapses, vesicle-mediated transport, neurotransmitter receptors and postsynaptic signal transmission with negative regulation of signaling as a consequence of neuroprotector treatment with sitagliptin. This improves the modulation of the macromolecule biosynthetic process with positive regulation of cell communication, which provides beneficial effects for the neuronal metabolism. This study suggests that topical administration of sitagliptin ameliorates the abnormalities on presynaptic and postsynaptic signal transmission during experimental DR and that this improvement is one of the main mechanisms behind the previously demonstrated beneficial effects.

Activation of the GABA-alpha receptor by berberine rescues retinal ganglion cells to attenuate experimental diabetic retinopathy

Purpose

The aim of this study was to investigate the role and mechanism of berberine (BBR) in the protection of injured retinal ganglion cells (RGCs) in diabetic retinopathy (DR).

Methods

Experimental diabetic retinopathy rat model was successfully induced by a single intraperitoneal injection of streptozotocin (STZ, 60 mg/kg) in male SD rats with sufficient food and water for 8 weeks. Animals were randomly divided into four groups: (1) non-diabetic, (2) diabetic, (3) diabetic + BBR + PBS, and (4) diabetic + BBR + SR95531. BBR (100 mg/kg) was given daily by gavage to rats in the group (3) and group (4) for 8 weeks, and weekly intravitreal injections were conducted to rats in the group (3) with 5 μL of 1×PBS and rats in the group (4) with 5 μL of GABA-alpha receptor antagonist SR95531 to investigate the underlying mechanisms. The survival and apoptosis of RGCs were observed by fluorescence gold labeling technology and TUNEL staining. Visual function was evaluated by visual electrophysiological examination. Western blotting and immunofluorescence staining were used to analyze the expression of GABA-alpha receptors in RGCs.

Results

In an animal model, BBR can increase the survival of RGCs, reduce RGCs apoptosis, and significantly improve the visual function. The reduction of GABA, PKC-α, and Bcl-2 protein expression caused by DR can be considerably increased by BBR. SR95531 inhibits BBR's protective effect on RGC and visual function, as well as its upregulation of PKC-α and Bcl-2.

Conclusion

BBR is a promising preventive or adjuvant treatment for DR complications, and its key protective effect may involve the regulation of RGC apoptosis through the GABA-alpha receptor/protein kinase C-alpha (GABAAR/PKC-α) pathway.

Unconventional avenues to decelerated diabetic retinopathy

Diabetic retinopathy (DR) is an important microvascular complication of diabetes mellitus (DM), causing significant visual impairment worldwide. Current gold standards for retarding the progress of DR include blood sugar control and regular fundus screening. Despite these measures, the incidence and prevalence of DR and vision-threatening DR remain high. Given its slowly progressive course and long latent period, opportunities to contain or slow DR before it threatens vision must be explored. This narrative review assesses the recently described unconventional strategies to retard DR progression. These include gut-ocular flow, gene therapy, mitochondrial dysfunction-oxidative stress, stem cell therapeutics, neurodegeneration, anti-inflammatory treatments, lifestyle modification, and usage of phytochemicals. These therapies impact DR directly, while some of them also influence DM control. Most of these strategies are currently in the preclinical stage, and clinical evidence remains low. Nevertheless, our review suggests that these approaches have the potential for human use to prevent the progression of DR.

mTOR inhibition as a novel gene therapeutic strategy for diabetic retinopathy

In addition to laser photocoagulation, therapeutic interventions for diabetic retinopathy (DR) have heretofore consisted of anti-VEGF drugs, which, besides drawbacks inherent to the treatments themselves, are limited in scope and may not fully address the condition’s complex pathophysiology. This is because DR is a multifactorial condition, meaning a gene therapy focused on a target with broader effects, such as the mechanistic target of rapamycin (mTOR), may prove to be the solution in overcoming these concerns. Having previously demonstrated the potential of a mTOR-inhibiting shRNA packaged in a recombinant adeno-associated virus to address a variety of angiogenic retinal diseases, here we explore the effects of rAAV2-shmTOR-SD in a streptozotocin-induced diabetic mouse model. Delivered via intravitreal injection, the therapeutic efficacy of the virus vector upon early DR processes was examined. rAAV2-shmTOR-SD effectively transduced mouse retinas and therein downregulated mTOR expression, which was elevated in sham-treated and control shRNA-injected (rAAV2-shCon-SD) control groups. mTOR inhibition additionally led to marked reductions in pericyte loss, acellular capillary formation, vascular permeability, and retinal cell layer thinning, processes that contribute to DR progression. Immunohistochemistry showed that rAAV2-shmTOR-SD decreased ganglion cell loss and pathogenic Müller cell activation and proliferation, while also having anti-apoptotic activity, with these effects suggesting the therapeutic virus vector may be neuroprotective. Taken together, these results build upon our previous work to demonstrate the broad ability of rAAV2-shmTOR-SD to address aspects of DR pathophysiology further evidencing its potential as a human gene therapeutic strategy for DR.

Preventive Efficacy of an Antioxidant Compound on Blood Retinal Barrier Breakdown and Visual Dysfunction in Streptozotocin-Induced Diabetic Rats

In diabetic retinopathy (DR), high blood glucose drives chronic oxidative stress and inflammation that trigger alterations of the neurovascular balance finally resulting in vascular abnormalities and retinal cell death, which converge towards altered electroretinogram (ERG). In the last years, a growing body of preclinical evidence has suggested that nutrients with anti-inflammatory/antioxidant properties can be able to hamper DR progression since its very early stages. In the present study, we used a streptozotocin-induced rat model of DR, which mimics most aspects of the early stages of human DR, to test the preventive efficacy of a novel compound containing cyanidin-3-glucoside (C3G), verbascoside and zinc as nutrients with antioxidant and anti-inflammatory properties. Western blot, immunofluorescence and electroretinographic analyses demonstrated a dose-dependent inhibition of oxidative stress- and inflammation-related mechanisms, with a significant counterpart in preventing molecular mechanisms leading to DR-associated vasculopathy and its related retinal damage. Preventive efficacy of the compound on dysfunctional a- and b-waves was also demonstrated by electroretinography. The present demonstration that natural compounds, possibly as a consequence of vascular rescue following ameliorated oxidative stress and inflammation, may prevent the apoptotic cascade leading to ERG dysfunction, adds further relevance to the potential application of antioxidants as a preventive therapy to counteract DR progression.

Neurodegeneration of the cornea and retina in patients with type 1 diabetes without clinical evidence of diabetic retinopathy

AIM

Diabetic retinopathy (DR) is widely considered the earliest and most common microvascular complication of diabetes. However, recent studies have shown that retinal nerve fiber layer and corneal nerve abnormalities may be present in diabetic patients without retinopathy. This preliminary study aimed to establish if structural and functional changes in the nerve fiber layer of the retina and cornea occur in patients with type 1 diabetes (T1DM) without retinopathy.

METHODS

Twenty patients with T1DM, without clinical evidence of retinopathy (Age: 47.0 ± 2.5 years; Duration diabetes: 27.0 ± 3 years) and 15 age-matched healthy control subjects underwent detailed medical neurological examinations. Ophthalmic examinations using Spectral Domain Optical coherence tomography (SD-OCT), Standard Automated Perimetry (SAP), Flicker Defined Form High Edge Perimetry (FDF), Corneal Confocal Microscopy (CCM) and Non-contact corneal Aesthesiometry (NCCA) were performed to quantify the structure and function of the nerves in the retina and cornea, respectively.

RESULTS

At the structural level, retinal nerve fiber layer thickness (RNFL) was significantly reduced in the superior nasal (p=0.001) and inferior temporal (p=0.004) sectors, in diabetic patients. Retinal ganglion layer function was reduced in the patient group when assessed using Flicker Defined Form Perimetry (FDF), but this was not significant. The function of the cornea assessed by corneal sensitivity, using a non-contact corneal aesthesiometer (NCCA), was significantly reduced (p=0.001). Structural assessment of corneal nerves using corneal confocal microscopy (CCM) showed reduction at corneal nerve fiber density (CNFD) (p=0.01), branch density (CNBD) (p=0.006) and length (CNFL) (p=0.01) in patients with diabetes. Compared to control subjects, the percentage of abnormality in patients with T1DM for RNFL was 32% while the FDF was abnormal in 61% of patients. Corneal abnormality was observed in 47% for NCCA, 28% for CNFD, and 17% for CNFL. There was no correlation between neuronal damage in the retina and cornea.

CONCLUSIONS

Neuronal abnormalities were observed in both the retina and cornea of diabetic patients without evidence of retinopathy. The prevalence of structural and functional changes was higher in the retina compared to the cornea. This preliminary study suggests that structural neuronal changes may occur in parallel and correlate with functional changes. The assessment of corneal and retinal nerve structure may be clinically useful for detecting and monitoring the earliest stages of diabetic microvascular abnormalities.

Common pathways in dementia and diabetic retinopathy: understanding the mechanisms of diabetes-related cognitive decline

Type 2 diabetes (T2D) is associated with multiple comorbidities, including diabetic retinopathy (DR) and cognitive decline, and T2D patients have a significantly higher risk of developing Alzheimer's disease (AD). Both DR and AD are characterized by a number of pathological mechanisms that coalesce around the neurovascular unit, including neuroinflammation and degeneration, vascular degeneration, and glial activation. Chronic hyperglycemia and insulin resistance also play a significant role, leading to activation of pathological mechanisms such as increased oxidative stress and the accumulation of advanced glycation end-products (AGEs). Understanding these common pathways and the degree to which they occur simultaneously in the brain and retina during diabetes will provide avenues to identify T2D patients at risk of cognitive decline.

Long Noncoding RNAs and Mitochondrial Homeostasis in the Development of Diabetic Retinopathy

Retinopathy is one of the most devastating complications of diabetes, which a patient fears the most. Hyperglycemic environment results in many structural, functional, molecular and biochemical abnormalities in the retina, and overproduction of mitochondrial superoxide, induced by hyperglycemic milieu, is considered to play a central role in the development of diabetic retinopathy. Expression of many genes associated with maintaining mitochondrial homeostasis is also altered. Recent research has shown that several long noncoding RNAs, RNAs with more than 200 nucleotides but without any reading frames, are aberrantly expressed in diabetes, and altered expression of these long noncoding RNAs is now being implicated in the development of diabetes and its complications including retinopathy. This review focuses the role of long noncoding RNAs in the development of diabetic retinopathy, with a special emphasis on the maintenance of mitochondrial homeostasis.

Retinal Protein O-GlcNAcylation and the Ocular Renin-angiotensin System: Signaling Cross-roads in Diabetic Retinopathy

It is well established that diabetes and its associated hyperglycemia negatively impact retinal function, yet we know little about the role played by augmented flux through the Hexosamine Biosynthetic Pathway (HBP). This offshoot of the glycolytic pathway produces UDP-Nacetyl- glucosamine, which serves as the substrate for post-translational O-linked modification of proteins in a process referred to as O-GlcNAcylation. HBP flux and subsequent protein O-GlcNAcylation serve as nutrient sensors, enabling cells to integrate metabolic information to appropriately modulate fundamental cellular processes including gene expression. Here we summarize the impact of diabetes on retinal physiology, highlighting recent studies that explore the role of O-GlcNAcylation- induced variation in mRNA translation in retinal dysfunction and the pathogenesis of Diabetic Retinopathy (DR). Augmented O-GlcNAcylation results in wide variation in the selection of mRNAs for translation, in part, due to O-GlcNAcylation of the translational repressor 4E-BP1. Recent studies demonstrate that 4E-BP1 plays a critical role in regulating O-GlcNAcylation-induced changes in the translation of the mRNAs encoding Vascular Endothelial Growth Factor (VEGF), a number of important mitochondrial proteins, and CD40, a key costimulatory molecule involved in diabetes-induced retinal inflammation. Remarkably, 4E-BP1/2 ablation delays the onset of diabetes- induced visual dysfunction in mice. Thus, pharmacological interventions to prevent the impact of O-GlcNAcylation on 4E-BP1 may represent promising therapeutics to address the development and progression of DR. In this regard, we discuss the potential interplay between retinal O-GlcNAcylation and the ocular renin-angiotensin system as a potential therapeutic target of future interventions.

Retinal Neurodegeneration in Diabetes: an Emerging Concept in Diabetic Retinopathy

PURPOSE OF REVIEW

Diabetic retinopathy (DR), the leading cause of blindness in working-aged adults, remains clinically defined and staged by its vascular manifestations. However, early retinal neurodegeneration may precede vascular pathology, suggesting that this neuronal damage may contribute to disease pathogenesis and represent an independent target for intervention. This review will discuss the evidence and implications for diabetic retinal neurodegeneration.

RECENT FINDINGS

A growing body of literature has identified progressive retinal thinning and visual dysfunction in patients with diabetes even prior to the onset of DR, though advances in retinal vascular imaging suggest that vascular remodeling and choroidal changes occur during these early stages as well. Animal models of diabetes and in vitro studies have also suggested that diabetes may directly affect the retinal neural and glial tissue, providing support to the concept that diabetic retinal neurodegeneration occurs early in the disease and suggesting potentially relevant molecular pathways. Diabetic retinal neurodegeneration may represent a "preclinical" manifestation of diabetic retinal disease and remains an active area of investigation. As the natural history and molecular mechanisms become increasingly understood, it may lead to upcoming developments in not only the treatment options but also the clinical definition of DR.

Potential of Human Neural Precursor Cells in Diabetic Retinopathy Therapeutics - Preclinical Model

Purpose: This study aimed to evaluate a cell therapy strategy with human neural precursor cells (hNPCs) to treat diabetic retinopathy (DR) in Wistar rats induced to diabetes by injecting streptozotocin.

Material and methods: Wharton's Jelly Mesenchymal stem cells (WJ-MSCs) were isolated, expanded, and seeded onto a biopolymer substrate to develop neurospheres and obtain the hNPCs. The animals were divided into three groups; non-diabetic (ND) n = four; diabetic without treatment (DM) n = nine; and diabetic with cell therapy (DM + hNPCs) n = nine. After eight weeks of diabetes induction and DR characteristics installed, intravitreal injection of hNPCs (1 x 10⁶ cel/µL) was performed in the DM + hNPCs group. Optical Coherence Tomography (OCT) and Electroretinography (ERG) evaluations were before and during diabetes and after cell therapy. Four weeks post-treatment, histopathological and immunohistochemistry analyses were performed.

Results: The repair of the retinal structures in the treated group (DM + hNPCs) was observed by increased thickness of neuroretinal layers, especially in the ganglion cell and photoreceptor layers, higher ERG oscillatory potentials (OPs) amplitudes, and transplanted hNPCs integration into the Retinal Pigment Epithelium.

Conclusions: The results indicate that hNPCs reduced DR progression by a neuroprotective effect and promoted retinal repair, making them potential candidates for regenerating the neuroretinal tissue.

Current status and future possibilities of retinal imaging in diabetic retinopathy care applicable to low- and medium-income countries

Diabetic retinopathy (DR) is a leading cause of blindness among adults and the numbers are projected to rise. There have been dramatic advances in the field of retinal imaging since the first fundus image was captured by Jackman and Webster in 1886. The currently available imaging modalities in the management of DR include fundus photography, fluorescein angiography, autofluorescence imaging, optical coherence tomography, optical coherence tomography angiography, and near-infrared reflectance imaging. These images are obtained using traditional fundus cameras, widefield fundus cameras, handheld fundus cameras, or smartphone-based fundus cameras. Fluorescence lifetime ophthalmoscopy, adaptive optics, multispectral and hyperspectral imaging, and multicolor imaging are the evolving technologies which are being researched for their potential applications in DR. Telemedicine has gained popularity in recent years as remote screening of DR has been made possible. Retinal imaging technologies integrated with artificial intelligence/deep-learning algorithms will likely be the way forward in the screening and grading of DR. We provide an overview of the current and upcoming imaging modalities which are relevant to the management of DR.

[Modern concepts of pathogenesis of diabetic retinopathy]

This literature review presents modern view on the pathogenesis of diabetic retinopathy (DR) paying particular attention to the molecular mechanisms leading to its development, as well as the manifestations of retinal neurodegeneration in such patients. Assessment of this condition and its clinical manifestations makes it possible to diagnose DR at the stage of absent initial vascular changes. Investigating the neurodegeneration mechanisms could supplement the existing understanding of the disease pathogenesis and could possibly help find new ways of treatment and prevention of DR.

A Nature-Inspired Nrf2 Activator Protects Retinal Explants from Oxidative Stress and Neurodegeneration

Oxidative stress (OS) plays a key role in retinal dysfunctions and acts as a major trigger of inflammatory and neurodegenerative processes in several retinal diseases. To prevent OS-induced retinal damage, approaches based on the use of natural compounds are actively investigated. Recently, structural features from curcumin and diallyl sulfide have been combined in a nature-inspired hybrid (NIH1), which has been described to activate transcription nuclear factor erythroid-2-related factor-2 (Nrf2), the master regulator of the antioxidant response, in different cell lines. We tested the antioxidant properties of NIH1 in mouse retinal explants. NIH1 increased Nrf2 nuclear translocation, Nrf2 expression, and both antioxidant enzyme expression and protein levels after 24 h or six days of incubation. Possible toxic effects of NIH1 were excluded since it did not alter the expression of apoptotic or gliotic markers. In OS-treated retinal explants, NIH1 strengthened the antioxidant response inducing a massive and persistent expression of antioxidant enzymes up to six days of incubation. These effects resulted in prevention of the accumulation of reactive oxygen species, of apoptotic cell death, and of gliotic reactivity. Together, these data indicate that a strategy based on NIH1 to counteract OS could be effective for the treatment of retinal diseases.

PET Imaging of Vesicular Monoamine Transporter 2 in Early Diabetic Retinopathy Using [18F]FP-(+)-DTBZ

PURPOSE

Diabetic retinopathy (DR) is characterized by dopaminergic neuron loss in the retina of the eyes. [¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) positron emission tomography (PET) has been shown to detect dopaminergic neuron loss. The study is to investigate the feasibility of PET imaging with [¹⁸F]FP-(+)-DTBZ for early diagnosis of diabetic retinopathy (DR) in diabetes mellitus (DM) rat models.

METHODS

The DM rat model was established by a single intraperitoneal injection of streptozotocin (STZ) (65 mg/kg). After 4 weeks, 8 weeks, and 12 weeks of STZ injection, the retinas of the rats were evaluated by electroretinogram (ERG), color fundus photography (CFP), fundus fluorescein angiography (FFA), and small animal PET scan with [¹⁸F]FP-(+)-DTBZ by targeting vesicular monoamine transporter 2 (VMAT2). [¹⁸F]FP-(+)-DTBZ uptake in retina was quantified as standardized uptake value (SUV). Immunofluorescence staining and Western blot were also performed to confirm the expression level of VMAT2 in retina.

RESULTS

ERG dysfunction was observed at 8 weeks in STZ-diabetic rats, evidenced by smaller amplitudes of oscillatory potentials (OPs) when compared with OPs in normal rats. CFP and FFA showed no significant difference in vascular leakage and neovascularization between STZ-diabetic retinas and normal ones until 8 weeks. PET imaging revealed that the SUV of [¹⁸F]FP-(+)-DTBZ was significantly lower in the STZ-diabetic retinas compared with the normal ones as early as of week 4. The results from immunofluorescence staining and Western blots confirmed the early findings in PET imaging studies.

CONCLUSIONS

Early DR can be non-invasively detected with PET imaging using [¹⁸F]FP-(+)-DTBZ targeting VMAT2. The expression level of VMAT2 in retina may act as a new biomarker for early DR diagnosis.

The potential role of m6A RNA methylation in diabetic retinopathy

Diabetic retinopathy (DR), a major microvascular complication of diabetes, affects most diabetic individuals and has become the leading cause of vision loss. Metabolic memory associated with diabetes retains the risk of disease occurrence even after the termination of glycemic insult. Further, various limitations associated with its current diagnostic and treatment strategies like unavailability of early diagnostic and treatment methods, variation in treatment response from patient to patient, and cost-effectiveness have driven the need to find alternative solutions. Post-transcriptional epigenetic modification of RNA mainly, N6-methyladenosine (m6A), is an emerging concept in the scientific community. It has an indispensable effect in various physiological and pathological conditions. m6A mediates its effect through the various reader, writer, and eraser proteins. Recent studies have shown the impact of m6A RNA modification on various disease conditions, including diabetes, but its role in diabetic retinopathy is still unclear. However, change in m6A levels has been observed in various prime aggravators of DR pathogenesis, such as inflammation, oxidative stress, and angiogenesis. Further, various non-coding RNAs like microRNA, lncRNA, and circRNA are also associated with DR, and m6A has been shown to affect all these non-coding RNAs. This review is concerned with the possible mechanisms through which alteration in m6A modification of RNA can participate in the DR progression and pathogenesis and its expected role in metabolic memory phenomena.

Neurovascular unit in diabetic retinopathy: pathophysiological roles and potential therapeutical targets

Diabetic retinopathy (DR), one of the common complications of diabetes, is the leading cause of visual loss in working-age individuals in many industrialized countries. It has been traditionally regarded as a purely microvascular disease in the retina. However, an increasing number of studies have shown that DR is a complex neurovascular disorder that affects not only vascular structure but also neural tissue of the retina. Deterioration of neural retina could precede microvascular abnormalities in the DR, leading to microvascular changes. Furthermore, disruption of interactions among neurons, vascular cells, glia and local immune cells, which collectively form the neurovascular unit, is considered to be associated with the progression of DR early on in the disease. Therefore, it makes sense to develop new therapeutic strategies to prevent or reverse retinal neurodegeneration, neuroinflammation and impaired cell-cell interactions of the neurovascular unit in early stage DR. Here, we present current perspectives on the pathophysiology of DR as a neurovascular disease, especially at the early stage. Potential novel treatments for preventing or reversing neurovascular injuries in DR are discussed as well.

Evaluation of the neuronal and microvascular components of the macula in patients with diabetic retinopathy

PURPOSE

To investigate whether abnormal retinal microcirculation correlates with retinal neuronal changes in untreated diabetic eyes without macular edema.

METHODS

This study enrolled 29 diabetic patients without diabetic retinopathy (DR), 18 patients with mild non-proliferative diabetic retinopathy (NPDR), 15 patients with moderate NPDR, 14 patients with severe NPDR, 27 patients with proliferative diabetic retinopathy (PDR), and 25 healthy control subjects. Pattern electroretinography (PERG) and optical coherence tomography angiography (OCT-A) tests were performed.

RESULTS

Differences in the mean values for the area, acircularity index, and perimeter of foveal avascular zone were statistically significant between the healthy control group and the diabetic patients (P  < 0.05 for all). P50 and N95 amplitudes were statistically significantly lower in the PDR group compared to diabetic patients without DR, control, and moderate NPDR groups (P  < 0.05 for all). The whole retina vessel densities in superficial and deep capillary plexus were lower in the PDR group compared to the diabetic patients without DR and control group (P < 0.05 for all). There were statistically significant positive correlations between the amplitudes of the P50 and N95 waves with the vessel densities.

CONCLUSION

The existence of significant correlations between PERG and OCT-A parameters in diabetic patients has shown that vascular and neuronal changes in the macula affect each other in diabetic patients.

The lack of Irs2 induces changes in the immunocytochemical expression of aromatase in the mouse retina

Insulin receptor substrate (Irs) belongs to a family of proteins that mediate the intracellular signaling of insulin and IGF-1. Insulin receptor substrate 2 (Irs2) is necessary for retinal function, since its failure in Irs2-deficient mice in hyperglycemic situation promotes photoreceptor degeneration and visual dysfunction, like in diabetic retinopathy. The expression of P450 aromatase, which catalyzes androgen aromatization to form 17ß-estradiol, increases in some neurodegenerative diseases thus promoting the local synthesis of neuroestrogens that exert relevant neuroprotective functions. Aromatase is also expressed in neurons and glial cells of the central nervous system (CNS), including the retina. To further understand the role of Irs2 at the retinal level, we performed an immunocytochemical study in adult normoglycemic Irs2-deficient mice. For this aim, the retinal immunoexpression of neuromodulators, such as aromatase, glutamine synthetase (GS), and tyrosine hydroxylase (TH) was analyzed, joint to a morphometric and planimetric study of the retinal layers. Comparing with wild-type (WT) control mice, the Irs2-knockout (Irs2-KO) animals showed a significant increase in the immunopositivity to aromatase in almost all of the retinal layers. Besides, Irs2-KO mice exhibited a decreased immunopositive reaction for GS and TH, in Müller and amacrine cells, respectively; morphological variations were also found in these retinal cell types. Furthermore, the retina of Irs2-KO mice displayed alterations in the structural organization, and a generalized decrease in the retinal thickness was observed in each of the layers, except for the inner nuclear layer. Our findings suggest that the absence of Irs2 induces retinal neurodegenerative changes in Müller and amacrine cells that are unrelated to hyperglycemia. Accordingly, in the Irs2-KO mice, the increased retinal immunocytochemical reactivity of aromatase could be associated with an attempt to repair such neural retina injuries by promoting local neuroprotective mediators.

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.

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.

Flicker retinal vasodilation test using a combination of conventional electroretinogram flicker luminescence and blue wavelength autofluorescence

Currently, the only available objective measure of functioning of the retinal neuro-vascular unit (NVU) is the flicker retinal vasodilation test (FRVT). The FRVT measures change in retinal vessel diameter after exposure to flickers of light. Unfortunately, it is dependent on a technically adept machine, which is not readily available. In this hypothesis, we propose utilization of 2 commonly available retinal diagnostic tools, the electroretinogram (ERG) and the confocal scanning laser ophthalmoscopy-based fundus auto fluorescence (cSLO-FAF), for performing the FRVT. While the conventional machine based FRVT was performing the flicker luminescence and retinal imaging simultaneously, our hypothesized protocol is dependent on a rapidly performed 2 staged test, the effects of which need to be elicited in a pilot study. As neuroprotection is becoming a therapeutic reality for diseases like diabetic retinopathy, such adaptations may make research and therapy easier.

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.

Integrative Computational Approach Revealed Crucial Genes Associated With Different Stages of Diabetic Retinopathy

The increased incidence of diabetic retinopathy (DR) and the legacy effect associated with it has raised a great concern toward the need to find early diagnostic and treatment strategies. Identifying alterations in genes and microRNAs (miRNAs) is one of the most critical steps toward understanding the mechanisms by which a disease progresses, and this can be further used in finding potential diagnostic and prognostic biomarkers and treatment methods. We selected different datasets to identify altered genes and miRNAs. The integrative analysis was employed to find potential candidate genes (differentially expressed and aberrantly methylated genes that are also the target of altered miRNAs) and early genes (genes showing altered expression and methylation pattern during early stage of DR) for DR. We constructed a protein-protein interaction (PPI) network to find hub genes (potential candidate genes showing a greater number of interactions) and modules. Gene ontologies and pathways associated with the identified genes were analyzed to determine their role in DR progression. A total of 271 upregulated-hypomethylated genes, 84 downregulated-hypermethylated genes, 11 upregulated miRNA, and 30 downregulated miRNA specific to DR were identified. 40 potential candidate genes and 9 early genes were also identified. PPI network analysis revealed 7 hub genes (number of interactions >5) and 1 module (score = 5.67). Gene ontology and pathway analysis predicted enrichment of genes in oxidoreductase activity, binding to extracellular matrix, immune responses, leukocyte migration, cell adhesion, PI3K-Akt signaling pathway, ECM receptor interaction, etc., and thus their association with DR pathogenesis. In conclusion, we identified 7 hub genes and 9 early genes that could act as a potential prognostic, diagnostic, or therapeutic target for DR, and a few early genes could also play a role in metabolic memory phenomena.

Single-cell transcriptome analysis of the Akimba mouse retina reveals cell-type-specific insights into the pathobiology of diabetic retinopathy

AIMS/HYPOTHESIS

Diabetic retinopathy is a common complication of diabetes and a leading cause of visual impairment and blindness. Despite recent advances, our understanding of its pathophysiology remains incomplete. The aim of this study was to provide deeper insight into the complex network of molecular and cellular changes that underlie diabetic retinopathy by systematically mapping the transcriptional changes that occur in the different cellular compartments of the degenerating diabetic mouse retina.

METHODS

Single-cell RNA sequencing was performed on retinal tissue from 12-week-old wild-type and Akimba (Ins2^Akita×Vegfa^+/-) mice, which are known to replicate features of clinical diabetic retinopathy. This resulted in transcriptome data for 9474 retinal cells, which could be annotated to eight distinct retinal cell types. Using STRING analysis, we studied differentially expressed gene networks in neuronal, glial and immune cell compartments to create a comprehensive view on the pathological changes that occur in the Akimba retina. Using subclustering analysis, we further characterised macroglial and inflammatory cell subpopulations. Prominent findings were confirmed at the protein level using immunohistochemistry, western blotting and ELISA.

RESULTS

At 12 weeks, the Akimba retina was found to display degeneration of rod photoreceptors and presence of inflammatory cells, identified by subclustering analysis as monocyte, macrophage and microglial populations. Analysis of differentially expressed genes in the rod, cone, bipolar cell and macroglial compartments indicated changes in cell metabolism and ribosomal gene expression, gliosis, activation of immune system pathways and redox and metal ion dyshomeostasis. Experiments at the protein level supported a metabolic shift from glycolysis to oxidative phosphorylation (glyceraldehyde 3-phosphate dehydrogenase), activation of microglia/macrophages (isolectin-B4), metal ion and oxidative stress response (metallothionein and haem oxygenase-1) and reactive macroglia (glial fibrillary acidic protein and S100) in the Akimba retina, compared with wild-type mice. Our single-cell approach also indicates macroglial subpopulations with distinct fibrotic, inflammatory and gliotic profiles.

CONCLUSIONS/INTERPRETATION

Our study identifies molecular pathways underlying inflammatory, metabolic and oxidative stress-mediated changes in the Akimba mouse model of diabetic retinopathy and distinguishes distinct functional subtypes of inflammatory and macroglial cells.

DATA AVAILABILITY

RNA-seq data have been deposited in the ArrayExpress database at EMBL-EBI ( www.ebi.ac.uk/arrayexpress ) under accession number E-MTAB-9061. Graphical abstract.

Aflibercept ameliorates retinal pericyte loss and restores perfusion in streptozotocin-induced diabetic mice

INTRODUCTION

Anti-vascular endothelial growth factor (VEGF) agents are used worldwide for advanced-stage diabetic retinopathy (DR). In contrast, apart from blood glucose control, there are no specific treatments that can limit the progression of early-stage DR that starts with pericyte loss and the destruction of the blood-retinal barrier. Here, we examined the efficacy of aflibercept, a potent anti-VEGF agent, against early-DR pathologies in a murine model of streptozotocin (STZ)-induced DR.

RESEARCH DESIGN AND METHODS

STZ was intraperitoneally administered in 8-week-old C57BL/6N male mice. After 4 weeks, the mice were divided into aflibercept-treated and saline-treated groups. Eight weeks after the STZ injection, vascular permeability/leakage was measured with fluorescein angiography in live mice. At 4, 6, and 8 weeks after the STZ injection, the eyes were enucleated, flat-mounted, and stained for platelet-derived growth factor receptor-β to assess pericyte abundance, CD45 to assess leukocyte recruitment, and fluorescein isothiocyanate dextran to assess perfusion. VEGF levels were quantified in each group. The effects of aflibercept on pericyte number, perfusion status, and leukocyte recruitment/accumulation on mice with diabetes retina were evaluated.

RESULTS

Our murine model successfully replicated the salient pathologies of DR such as pericytes loss, hyperpermeability, and perfusion blockage. Interestingly, numerous leukocytes and leukocyte clumps were found in diabetic retinal capillaries, especially in the non-perfused border area of the retina, suggesting a possible mechanism for non-perfusion and related pericyte damage. Treatment with aflibercept in mice with diabetes inhibited the upregulation of VEGF and the associated adhesion molecules while reducing the defects in perfusion. Aflibercept also attenuated pericyte loss in the diabetic retina.

CONCLUSION

VEGF inhibition through aflibercept treatment decreased leukocyte recruitment and aggregation, perfusion blockage, retinal hypoperfusion, and hyperpermeability in mice with diabetes and ultimately attenuated pericyte loss. Our findings suggest that anti-VEGF strategies may prove useful as possible therapies for limiting the progression of early-stage DR.

The effects of early diabetes on inner retinal neurons

Diabetic retinopathy is now well understood as a neurovascular disease. Significant deficits early in diabetes are found in the inner retina that consists of bipolar cells that receive inputs from rod and cone photoreceptors, ganglion cells that receive inputs from bipolar cells, and amacrine cells that modulate these connections. These functional deficits can be measured in vivo in diabetic humans and animal models using the electroretinogram (ERG) and behavioral visual testing. Early effects of diabetes on both the human and animal model ERGs are changes to the oscillatory potentials that suggest dysfunctional communication between amacrine cells and bipolar cells as well as ERG measures that suggest ganglion cell dysfunction. These are coupled with changes in contrast sensitivity that suggest inner retinal changes. Mechanistic in vitro neuronal studies have suggested that these inner retinal changes are due to decreased inhibition in the retina, potentially due to decreased gamma aminobutyric acid (GABA) release, increased glutamate release, and increased excitation of retinal ganglion cells. Inner retinal deficits in dopamine levels have also been observed that can be reversed to limit inner retinal damage. Inner retinal targets present a promising new avenue for therapies for early-stage diabetic eye disease.

Early diabetes impairs ON sustained ganglion cell light responses and adaptation without cell death or dopamine insensitivity

Retinal signaling under dark-adapted conditions is perturbed during early diabetes. Additionally, dopamine, the main neuromodulator of retinal light adaptation, is diminished in diabetic retinas. However, it is not known if this dopamine deficiency changes how the retina responds to increased light or dopamine. Here we determine whether light adaptation is impaired in the diabetic retina, and investigate potential mechanism(s) of impairment. Diabetes was induced in C57BL/6J male mice via 3 intraperitoneal injections of streptozotocin (75 mg/kg) and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell recordings of light-evoked and spontaneous inhibitory postsynaptic currents (IPSCs) or excitatory postsynaptic currents (EPSCs) were made from rod bipolar cells and ON sustained ganglion cells, respectively. Light responses were recorded before and after D1 receptor (D1R) activation (SKF-38393, 20 μM) or light adaptation (background of 950 photons·μm^-2 ·s^-1). Retinal whole mounts were stained for either tyrosine hydroxylase and activated caspase-3 or GAD65/67, GlyT1 and RBPMS and imaged. D1R activation and light adaptation both decreased inhibition, but the disinhibition was not different between control and diabetic rod bipolar cells. However, diabetic ganglion cell light-evoked EPSCs were increased in the dark and showed reduced light adaptation. No differences were found in light adaptation of spontaneous EPSC parameters, suggesting upstream changes. No changes in cell density were found for dopaminergic, glycinergic or GABAergic amacrine cells, or ganglion cells. Thus, in early diabetes, ON sustained ganglion cells receive excessive excitation under dark- and light-adapted conditions. Our results show that this is not attributable to loss in number or dopamine sensitivity of inhibitory amacrine cells or loss of dopaminergic amacrine cells.

Pattern-Reversal Visual Evoked Potentials Tests in Persons with Type 2 Diabetes Mellitus with and without Diabetic Retinopathy

Background

Currently, diabetic retinopathy (DR) has a wide recognition as a neurovascular rather than a microvascular diabetic complication with an increasing need for enhanced detection approaches. Pattern-reversal visual evoked potentials (PRVEPs) test, as an objective electrophysiological measure of the optic nerve and retinal function, can be of great value in the detection of diabetic retinal changes.

Objectives

The use of two sizes of checkerboard PRVEPs testing to detect any neurological changes in persons with type 2 diabetes mellitus (T2DM) with and without a clinically detected DR. Also, to compare the results according to the candidate age, duration, and glycemic status of T2DM.

Methods

This study included 50 candidates as group A with T2DM and did not have a clinically detected DR and 50 candidates as group B with T2DM and had a clinically detected early DR and 50 candidates as controls who were neither diabetic nor had any other medical or ophthalmic condition that might affect PRVEPs test results. The PRVEPs were recorded in the consultant unit of ophthalmology in Almawani Teaching Hospital. Monocular PRVEPs testing of both eyes was done by using large (60 min) and small (15 min) checks to measure N75 latency and P100 latency and amplitude.

Results

There was a statistically significant P100 latency delay and P100 amplitude reduction in both groups A and B in comparison with the controls. The difference between groups A and B was also significant. In both test results of groups A and B, the proportions of abnormal P100 latency were higher than those of P100 amplitude with a higher abnormal proportions in 15 min test.

Conclusions

The PRVEP test detected neurological changes, mainly as conductive alterations affecting mostly the foveal region prior to any overt DR clinical changes, and these alterations were heightened by the presence of DR clinical changes.

Astaxanthin Protects Retinal Photoreceptor Cells against High Glucose-Induced Oxidative Stress by Induction of Antioxidant Enzymes via the PI3K/Akt/Nrf2 Pathway

Diabetic retinopathy (DR) is a major microvascular complication that can lead to severe visual impairment in patients with diabetes. The elevated oxidative stress and increased reactive oxygen species (ROS) production induced by hyperglycemia have been reported to play an important role in the complex pathogenesis of DR. Astaxanthin (AST), a natural carotenoid derivative, has been recently recognized as a strong free radical scavenger and might, therefore, be beneficial in different diseases, including DR. In this study, we evaluated the potential role of AST as an antioxidative and antiapoptotic agent in protecting retinal cells and also investigated the involvement of the PI3K/Akt/Nrf2 pathway in AST-mediated effects. We treated high glucose-cultured mouse photoreceptor cells (661W) with different concentrations of AST and analyzed ROS production and cell apoptosis in the different regimens. Moreover, we also analyzed the expression of PI3K, Akt, Nrf2, and Phase II enzymes after AST treatment. Our results showed that AST dose-dependently reduced ROS production and attenuated 661W cell apoptosis in a high glucose environment. Importantly, its protective effect was abolished by treatment with PI3K or Nrf2 inhibitors, indicating the involvement of the PI3K/Akt/Nrf2 pathway. These results suggest AST as a nutritional supplement that could benefit patients with DR.

The potential role of lncRNAs in diabetes and diabetic microvascular complications

Long noncoding RNAs (lncRNAs) are a group of noncoding RNAs that are longer than 200 nucleotides without protein-coding potential. Becasuse of which these RNAs have no significant protein-coding potential, they were initially considered as "junk-products" of transcription without biological meaning. Nevertheless, recent research advancements have shown that lncRNAs are involved in many physiological processes such as cell cycle regulation, cell apoptosis and survival, cancer migration and metabolism. This review described the function of lncRNAs and the potential underlying mechanism involved in diabetes and diabetic microvascular complications. The roles of lncRNAs in the pathogenesis of type 2 diabetes mellitus have only recently been recognized, involving hepatic glucose production and insulin resistance. We further investigated the mechanisms of lncRNAs in diabetic nephropathy (DN), including the roles of lncRNAs in mesangial cells (MCs) proliferation and fibrosis, inflammatory processes, extracellular matrix accumulation in the glomeruli and tubular injury. We also discussed the potential mechanism of lncRNAs in diabetic retinopathy (DR), including aberrant neovascularization and neuronal dysfunction. This review summarized the current knowledge of the functions and underlying mechanisms of lncRNAs in type 2 diabetes mellitus and related renal and retinal complications. Accumulating evidence suggests the potential of lncRNAs as therapeutic targets for clinical applications in the management of diabetes.

Site-specific abnormalities in the visual system of a mouse model of CDKL5 deficiency disorder

CDKL5 deficiency disorder (CDD) is a neurodevelopmental disorder characterized by a severe global developmental delay and early-onset seizures. Notably, patients show distinctive visual abnormalities often clinically diagnosed as cortical visual impairment. However, the involvement of cerebral cortical dysfunctions in the origin of the symptoms is poorly understood. CDD mouse models also display visual deficits, and cortical visual responses can be used as a robust biomarker in CDKL5 mutant mice. A deeper understanding of the circuits underlying the described visual deficits is essential for directing preclinical research and translational approaches. Here, we addressed this question in two ways: first, we performed an in-depth morphological analysis of the visual pathway, from the retina to the primary visual cortex (V1), of CDKL5 null mice. We found that the lack of CDKL5 produced no alteration in the organization of retinal circuits. Conversely, CDKL5 mutants showed reduced density and altered morphology of spines and decreased excitatory synapse marker PSD95 in the dorsal lateral geniculate nucleus and in V1. An increase in the inhibitory marker VGAT was selectively present in V1. Second, using a conditional CDKL5 knockout model, we showed that selective cortical deletion of CDKL5 from excitatory cells is sufficient to produce abnormalities of visual cortical responses, demonstrating that the normal function of cortical circuits is dependent on CDKL5. Intriguingly, these deficits were associated with morphological alterations of V1 excitatory and inhibitory synaptic contacts. In summary, this work proposes cortical circuit structure and function as a critically important target for studying CDD.

Contrast sensitivity is associated with outer-retina thickness in early-stage diabetic retinopathy

PURPOSE

To determine the relationship between contrast sensitivity (CS) and outer-retina thickness (ORT) in diabetics who have minimal or no diabetic retinopathy (DR).

METHODS

Twenty non-diabetic control subjects and 40 type-2 diabetic subjects participated (20 had no clinically apparent DR [NDR] and 20 had mild non-proliferative DR [NPDR]). No subject had a history of treatment for macular oedema. Letter CS, microperimetry (MP) sensitivity and visual acuity (VA) were measured. Letter CS and MP measurements were performed over the central 6° of the visual field. Spectral domain optical coherence tomography (SD-OCT) images were obtained at corresponding locations, outer-retina thickness was quantified, and structure-function relationships were evaluated.

RESULTS

Analysis of variance indicated significant letter CS differences among the groups (p < 0.001). Letter CS was reduced significantly for the mild NPDR group (p < 0.001; 33% reduction), but not the NDR group (p = 0.08). There were no significant differences in MP sensitivity or ORT among the groups (both p > 0.10). Nevertheless, Hoeffding's D tests indicated significant associations between ORT and letter CS (p < 0.001) and between ORT and MP sensitivity for the mild NPDR group (p = 0.01). VA was not significantly associated with ORT for either diabetic group (both p > 0.49).

CONCLUSIONS

Outer-retina thickness is associated with letter CS and MP sensitivity, but not VA, in mild NPDR. This finding highlights the usefulness of simple letter CS measures and suggests neural dysfunction can occur in the absence of marked structural abnormalities in early-stage DR.

Recent Developments in Diabetic Retinal Neurodegeneration: A Literature Review

Neurodegeneration plays a significant role in the complex pathology of diabetic retinopathy. Evidence suggests the onset of neurodegeneration occurs early on in the disease, and so a greater understanding of the process is essential for prompt detection and targeted therapies. Neurodegeneration is a common pathway of assorted processes, including activation of inflammatory pathways, reduction of neuroprotective factors, DNA damage, and apoptosis. Oxidative stress and formation of advanced glycation end products amplify these processes and are elevated in the setting of hyperglycemia, hyperlipidemia, and glucose variability. These key pathophysiologic mechanisms are discussed, as well as diagnostic modalities and novel therapeutic avenues, with an emphasis on recent discoveries. The aim of this article is to highlight the crucial role of neurodegeneration in diabetic retinopathy and to review the molecular basis for this neuronal dysfunction, its diagnostic features, and the progress currently made in relevant therapeutic interventions.

The Therapeutic Role of Carotenoids in Diabetic Retinopathy: A Systematic Review

BACKGROUND

Carotenoids are a large group of natural pigments that occur in many foods, fruits, and vegetables. Several studies have shown a number of biological properties of carotenoids, particularly beneficial impacts on cancer, metabolic, neurodegenerative, and cardiovascular diseases. However, recent evidence has shown that these compounds could prevent, delay, and ameliorate diabetic retinopathy (DR). The aim of current study was to review the therapeutic effects of carotenoids in the treatment of DR and discuss the molecular mechanisms that are behind these pharmacological activities.

METHODS

Six online databases (Medline/PubMed, Scopus, Web of Knowledge, Embase, ScienceDirect, and ProQuest) were searched until September 2019. The systematic review was carried out using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist.

RESULTS

A total of 25 studies were included after the final retrieval. A relationship was observed between carotenoids and management of DR. Findings also demonstrated that the underlying mechanism of beneficial effects of these compounds was antioxidant, anti-inflammatory, anti-angiogenic, and neuroprotective properties.

CONCLUSION

Carotenoids potentially delay the initiation and prevent the progression of DR; however, ample preclinical studies are required to confirm their effect, and adequate clinical trials are needed to really understand how well these compounds influence DR among humans.

Evaluation of Early Retinal Nerve Injury in Type 2 Diabetes Patients Without Diabetic Retinopathy

Objectives: To investigate the damage to the retinal nerve fiber layer (RNFL) and ganglion cell complex layer (GCL+) in diabetic patients without retinal microangioma and to determine the kind of nerve damage more likely to indicate early injury. Subjects and Methods: We included 360 patients (360 eyes) with type 2 diabetes mellitus and 168 healthy volunteers (168 eyes). Patients with retinal microangioma were excluded by fundus fluorescein angiography (FFA). The parameters around the optic disc and macular area were measured by optical coherence tomography (OCT). Results: The peripapillary RNFL thickness was thinner in the temporal (72.98 ± 13.76 μm, P < 0.0001) and inferior (120.71 ± 21.43 μm, P = 0.0103) sectors in patients with no diabetic retinopathy (NDR) compared to healthy controls. The reduction of retinal thickness in the macular region was prominent in the inferior sector in patients (34.74 ± 4.92 μm, P < 0.0001) compared to normal controls. Thinning of GCL+ in the second region of the macular area was significant in patients with NDR compared to normal controls (P < 0.05). However, no difference in the GCL+ and retinal thicknesses of the central macular region was observed between the patients with NDR and healthy controls. Using the 5th percentile (P5) of normal controls as the reference value, we found that the parameters with the highest indices in patients with NDR were the inferior and temporal peripapillary RNFL thickness (13.0%), the inferior RNFL thickness in the macular area (20%), the inferior retinal thickness in the outer ring of the macular area (10.8%), and the inferior GCL+ thickness in the macular area (10.6%). The GCL+ and RNFL thicknesses in the central macular area accounted for the smallest proportion in P5 of normal controls (3%). Conclusions: Retinal nerve injury can occur in patients without retinal microangioma. The inferior RNFL in the macular area and the inferior and temporal peripapillary RNFL were most sensitive to glucose damage. These areas might be associated with early detection of diabetic retinopathy (DR) as they are more likely to indicate early damage.