Signalling within the neurovascular unit in the mammalian retina

Neuroretinal Alterations in Schizophrenia and Bipolar Disorder: An Updated Meta-analysis

Schizophrenia (SZ) and bipolar disorder (BD) are characterized by major symptomatic, cognitive, and neuroanatomical changes. Recent studies have used optical coherence tomography (OCT) to investigate retinal changes in SZ and BD, but their unique and shared changes require further evaluation. Articles were identified using PubMed and Google Scholar. 39 studies met the inclusion criteria. Diagnostic groups were proband (SZ/BD combined), SZ, BD, and healthy control (HC) eyes. Meta-analyses utilized fixed and random effects models when appropriate, and publication bias was corrected using trim-and-fill analysis ("meta" package in R). Results are reported as standardized mean differences with 95% CIs. Data from 3145 patient eyes (1956 SZ, 1189 BD) and 3135 HC eyes were included. Studies identified thinning of the peripapillary retinal nerve fiber layer (pRNFL, overall and in 2 subregions), m-Retina (overall and all subregions), mGCL-IPL, mIPL, and mRPE in SZ patients. BD showed thinning of the pRNFL (overall and in each subregion), pGCC, and macular Retina (in 5 subregions), but no changes in thickness or volume for the total retina. Neither SZ nor BD patients demonstrated significant changes in the fovea, mRNFL, mGCL, mGCC, mINL, mOPL, mONL, or choroid thicknesses. Moderating effects of age, illness duration, and smoking on retinal structures were identified. This meta-analysis builds upon previous literature in this field by incorporating recent OCT studies and examining both peripapillary and macular retinal regions with respect to psychotic disorders. Overall, this meta-analysis demonstrated both peripapillary and macular structural retinal abnormalities in people with SZ or BD compared with HCs.

Diabetic retinopathy: New concepts of screening, monitoring, and interventions

The science of diabetes care has progressed to provide a better understanding of the oxidative and inflammatory lesions and pathophysiology of the neurovascular unit within the retina (and brain) that occur early in diabetes, even prediabetes. Screening for retinal structural abnormalities, has traditionally been performed by fundus examination or color fundus photography; however, these imaging techniques detect the disease only when there are sufficient lesions, predominantly hemorrhagic, that are recognized to occur late in the disease process after significant neuronal apoptosis and atrophy, as well as microvascular occlusion with alterations in vision. Thus, interventions have been primarily oriented toward the later-detected stages, and clinical trials, while demonstrating a slowing of the disease progression, demonstrate minimal visual improvement and modest reduction in the continued loss over prolonged periods. Similarly, vision measurement utilizing charts detects only problems of visual function late, as the process begins most often parafoveally with increasing number and progressive expansion, including into the fovea. While visual acuity has long been used to define endpoints of visual function for such trials, current methods reviewed herein are found to be imprecise. We review improved methods of testing visual function and newer imaging techniques with the recommendation that these must be utilized to discover and evaluate the injury earlier in the disease process, even in the prediabetic state. This would allow earlier therapy with ocular as well as systemic pharmacologic treatments that lower the and neuro-inflammatory processes within eye and brain. This also may include newer, micropulsed laser therapy that, if applied during the earlier cascade, should result in improved and often normalized retinal function without the adverse treatment effects of standard photocoagulation therapy.

Network pharmacology and biochemical experiments reveal the antiapoptotic mechanism of huperzine A for treating diabetic retinopathy

BACKGROUND/AIMS

Diabetic retinopathy is the most common eye disease that causes blindness in the working population. Neurodegeneration is the early sign of diabetic retinopathy, but no drug has been approved for delaying or reversing retinal neurodegeneration. Huperzine A, a natural alkaloid isolated from Huperzia serrata, displays neuroprotective and antiapoptotic effects in treating neurodegenerative disorders. Our study aims to investigate the effect of huperzine A in preventing retinal neurodegeneration of diabetic retinopathy and its possible mechanism.

METHODS

Diabetic retinopathy model was induced by streptozotocin. H&E staining, optical coherence tomography, immunofluorescence staining and angiogenic factors were used to determine the degree of retinal pathological injury. The possible molecular mechanism was unrevealed by network pharmacology analysis and further validated by biochemical experiments.

RESULTS

In our study, we demonstrated that huperzine A has a protective effect on the diabetes retina in a diabetic rat model. Based on the network pharmacology analysis and biochemical studies, huperzine A may treat diabetic retinopathy via key target HSP27 and apoptosis-related pathways. Huperzine A may modulate the phosphorylation of HSP27 and activate the antiapoptotic signalling pathway.

CONCLUSION

Our findings revealed that huperzine A might be a potential therapeutic drug to prevent diabetic retinopathy. It is the first-time combining network pharmacology analysis with biochemical studies to explore the mechanism of huperzine A in preventing diabetic retinopathy.

Role of myeloid cells in ischemic retinopathies: recent advances and unanswered questions

Myeloid cells including microglia and macrophages play crucial roles in retinal homeostasis by clearing cellular debris and regulating inflammation. These cells are activated in several blinding ischemic retinal diseases including diabetic retinopathy, where they may exert both beneficial and detrimental effects on neurovascular function and angiogenesis. Myeloid cells impact the progression of retinal pathologies and recent studies suggest that targeting myeloid cells is a promising therapeutic strategy to mitigate diabetic retinopathy and other ischemic retinal diseases. This review summarizes the recent advances in our understanding of the role of microglia and macrophages in retinal diseases and focuses on the effects of myeloid cells on neurovascular injury and angiogenesis in ischemic retinopathies. We highlight gaps in knowledge and advocate for a more detailed understanding of the role of myeloid cells in retinal ischemic injury to fully unlock the potential of targeting myeloid cells as a therapeutic strategy for retinal ischemia.

The retina-brain axis and diabetic retinopathy

Diabetic retinopathy (DR) is a major contributor to permanent vision loss and blindness. Changes in retinal neurons, glia, and microvasculature have been the focus of intensive study in the quest to better understand DR. However, the impact of diabetes on the rest of the visual system has received less attention. There are reports of associations of changes in the visual system with preclinical and clinical manifestations of diabetes. Simultaneous investigation of the retina and the brain may shed light on the mechanisms underlying neurodegeneration in diabetics. Additionally, investigating the links between DR and other neurodegenerative disorders of the brain including Alzheimer's and Parkinson's disease may reveal shared mechanisms for neurodegeneration and potential therapy options.

Hyperglycemia-regulated tRNA-derived fragment tRF-3001a propels neurovascular dysfunction in diabetic mice

Neurovascular dysfunction is a preclinical manifestation of diabetic complications, including diabetic retinopathy (DR). Herein, we report that a transfer RNA-derived RNA fragment, tRF-3001a, is significantly upregulated under diabetic conditions. tRF-3001a downregulation inhibits Müller cell activation, suppresses endothelial angiogenic effects, and protects against high-glucose-induced retinal ganglion cell injury in vitro. Furthermore, tRF-3001a downregulation alleviates retinal vascular dysfunction, inhibits retinal reactive gliosis, facilitates retinal ganglion cell survival, and preserves visual function and visually guided behaviors in STZ-induced diabetic mice and db/db diabetic mice. Mechanistically, tRF-3001a regulates neurovascular dysfunction in a microRNA-like mechanism by targeting GSK3B. Clinically, tRF-3001a is upregulated in aqueous humor (AH) samples of DR patients. tRF-3001a downregulation inhibits DR-induced human retinal vascular endothelial cell and Müller cell dysfunction in vitro and DR-induced retinal neurovascular dysfunction in C57BL/6J mice. Thus, targeting tRF-3001a-mediated signaling is a promising strategy for the concurrent treatment of vasculopathy and neuropathy in diabetes mellitus.

Analysis of the morphology of retinal vascular cells in zebrafish (Danio rerio)

Background: Zebrafish (Danio rerio) have been established in recent years as a model organism to study Diabetic Retinopathy (DR). Loss of endothelial cells and pericytes is an early hallmark sign of developing DR in the mammalian retina. However, morphology, numbers, ratios, and distributions of different vascular cells in the retinal compartment in zebrafish have not yet been analyzed and compared with the mammalian retina. Methods: The retinal trypsin digest protocol was established on the zebrafish retina. Cell types were identified using the Tg(nflk:EGFP)-reporter line. Cells were quantified using quantitative morphometry. Results: Vascular cells in the zebrafish retina have distinct morphologies and locations. Nuclei of vascular mural cells appear as long and flat nuclei located near the vessel wall. Round nuclei within the vessel walls can be identified as endothelial cells. The vessel diameter decreases from central to peripheral parts of the retina. Additionally, the numbers of vascular cells decrease from central to peripheral parts of the retina. Discussion: The retinal trypsin digest protocol, which can be applied to the zebrafish retina, provides novel insights into the zebrafish retinal vascular architecture. Quantification of the different cell types shows that, in comparison to the mammalian retina, zebrafish have higher numbers of mural cells and an increased mural cell to endothelial cell ratio. This protocol enables to quantify mural cell and endothelial cell numbers, is easily adaptable to different transgenic and mutant zebrafish lines and will enable investigators to compare novel models on a single cell level.

Downregulation of miR-124-3p suppresses the development of the deep retinal blood vessels by enhancing the Stat1/Ripk1 pathway in mouse retinal microglia

The study aimed to investigate the role of microRNA (miR)-124-3p in retinal angiogenesis in a mouse model. An intravitreal injection of miR-124-3p antagomir was used to knockdown the expression of miR-124-3p in the mouse retina at postnatal day (P)3. Immunofluorescent staining of both retinal frozen sections and whole retina were used to observe retinal vascular development in the P6, P9 and P12 mice, as well as the changes in retinal ganglion cells, astrocytes, Müller cells and microglia. Whole retinal RNA extracted from P9 mice was used for transcriptome sequencing. Following gene set enrichment analysis, the enriched genes caused by miR-124-3p inhibition were analyzed by immunofluorescent staining and western blot. Results indicated that deep vascular development was significantly inhibited by the activation of M1 phenotype microglia. Moreover, there were no notable effects on superficial retinal vascular development, the retinal ganglion cells, astrocytes, and Müller cells. The expression of the Stat1/Irf9/Eif2ak2/Ripk1 axis in the miR-124-3p knockdown group was significantly increased. The microglia penetrated deep into the retina and the activation of Ripk1(+) microglia significantly increased, which was accompanied by an increased level of apoptosis to inhibit the deep vascular sprout. Downregulation of miR-124-3p during the early retinal development can suppress the development of the deep retinal blood vessels by enhancing the expression level of the Stat1/Irf9/Eif2ak2/Ripk1 axis and inducing the cell apoptosis of the activation of Ripk1(+) microglia.

Retinal Cell Damage in Diabetic Retinopathy

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

Serum glial fibrillary acidic protein and neurofilament light chain as biomarkers of retinal neurodysfunction in early diabetic retinopathy: results of the EUROCONDOR study

AIMS

Neurodegeneration and glial activation are primary events in the pathogenesis of diabetic retinopathy. Serum glial fibrillary acidic protein (GFAP) and neurofilament light chain (NfL) are biomarkers of underlying neuroinflammatory and neurodegenerative disease processes. The aim of the present study was to assess the usefulness of these serum biomarkers for the identification and monitoring of retinal neurodysfunction in subjects with type 2 diabetes.

METHODS

A case-control study was designed including 38 patients from the placebo arm of the EUROCONDOR clinical trial: 19 with and 19 without retinal neurodysfunction assessed by multifocal electroretinography. GFAP and NfL were measured by Simoa.

RESULTS

Serum levels of GFAP and NfL directly correlated with age (r = 0.37, p = 0.023 and r = 0.54, p < 0.001, respectively). In addition, a direct correlation between GFAP and NfL was observed (r = 0.495, p = 0.002). Serum levels of GFAP were significantly higher at baseline in those subjects in whom neurodysfunction progressed after the 2 years of follow-up (139.1 ± 52.5 pg/mL vs. 100.2 ± 54.6 pg/mL; p = 0.04).

CONCLUSIONS

GFAP could be a useful serum biomarker for retinal neurodysfunction. Monitoring retinal neurodysfunction using blood samples would be of benefit in clinical decision-making. However, further research is needed to validate this result as well as to establish the best cutoff values.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Thioredoxin 1 overexpression attenuated diabetes-induced endoplasmic reticulum stress in Müller cells via apoptosis signal-regulating kinase 1

As one of the common and serious chronic complications of diabetes mellitus (DM), the related mechanism of diabetic retinopathy (DR) has not been fully understood. Müller cell reactive gliosis is one of the early pathophysiological features of DR. Therefore, exploring the manner to reduce diabetes-induced Müller cell damage is essential to delay DR. Thioredoxin 1 (Trx1), one of the ubiquitous redox enzymes, plays a vital role in redox homeostasis via protein-protein interactions, including apoptosis signal-regulating kinase 1 (ASK1). Previous studies have shown that upregulation of Trx by some drugs can attenuate endoplasmic reticulum stress (ERS) in DR, but the related mechanism was unclear. In this study, we used DM mouse and high glucose (HG)-cultured human Müller cells as models to clarify the effect of Trx1 on ERS and the underlying mechanism. The data showed that the diabetes-induced Müller cell damage was increased significantly. Moreover, the expression of ERS and reactive gliosis was also upregulated in diabetes in vivo and in vitro. However, it was reversed after Trx1 overexpression. Besides, ERS-related protein expression, reactive gliosis, and apoptosis were decreased after transfection with ASK1 small-interfering RNA in stable Trx1 overexpression Müller cells after HG treatment. Taken together, Trx1 could protect Müller cells from diabetes-induced damage, and the underlying mechanism was related to inhibited ERS via ASK1.

Blockade of CB1 or Activation of CB2 Cannabinoid Receptors Is Differentially Efficacious in the Treatment of the Early Pathological Events in Streptozotocin-Induced Diabetic Rats

Oxidative stress, neurodegeneration, neuroinflammation, and vascular leakage are believed to play a key role in the early stage of diabetic retinopathy (ESDR). The aim of this study was to investigate the blockade of cannabinoid receptor 1 (CB1R) and activation of cannabinoid receptor 2 (CB2R) as putative therapeutics for the treatment of the early toxic events in DR. Diabetic rats [streptozotocin (STZ)-induced] were treated topically (20 μL, 10 mg/mL), once daily for fourteen days (early stage DR model), with SR141716 (CB1R antagonist), AM1710 (CB2R agonist), and the dual treatment SR141716/AM1710. Immunohistochemical-histological, ELISA, and Evans-Blue analyses were performed to assess the neuroprotective and vasculoprotective properties of the pharmacological treatments on diabetes-induced retinal toxicity. Activation of CB2R or blockade of CB1R, as well as the dual treatment, attenuated the nitrative stress induced by diabetes. Both single treatments protected neural elements (e.g., RGC axons) and reduced vascular leakage. AM1710 alone reversed all toxic insults. These findings provide new knowledge regarding the differential efficacies of the cannabinoids, when administered topically, in the treatment of ESDR. Cannabinoid neuroprotection of the diabetic retina in ESDR may prove therapeutic in delaying the development of the advanced stage of the disease.

Advanced glycation end products in diabetic retinopathy and phytochemical therapy

Advanced glycation end products (AGEs) are generated by the nonenzymatic glycation of proteins or lipids. Diabetic retinopathy (DR) is one common complication in patients with diabetes. The accumulation of AGEs in retinal cells is strongly associated with the development of DR. AGEs can induce the breakdown of redox balance and then cause oxidative stress in retinal cells, exerting cytopathic effects in the progression of DR. The interaction between AGEs and the receptor for AGE (RAGE) is involved in multiple cellular pathological alterations in the retina. This review is to elucidate the pathogenetic roles of AGEs in the progression of DR, including metabolic abnormalities, lipid peroxidation, structural and functional alterations, and neurodegeneration. In addition, disorders associated with AGEs can be used as potential therapeutic targets to explore effective and safe treatments for DR. In this review, we have also introduced antioxidant phytochemicals as potential therapeutic strategies for the treatment of DR.

Diabetes mellitus associated neurovascular lesions in the retina and brain: A review

Diabetes mellitus (DM) is now recognized as a system-wide, autoimmune, inflammatory, microvascular disorder, which, in the retina and brain results in severe multifocal injury now recognized as a leading cause, world-wide, of progressive vision loss and dementia. To address this problem, resulting primarily from variations in glycemia in the prediabetic and overt diabetic states, it must be realized that, although some of the injury processes associated with diabetes may be system wide, there are varying responses, effector, and repair mechanisms that differ from organ to organ or within varying cell structures. Specifically, within the retina, and similarly within the brain cortex, lesions occur of the "neurovascular unit", comprised of focal microvascular occlusions, inflammatory endothelial and pericyte injury, with small vessel leakage resulting in injury to astrocytes, Müller cells, and microglia, all of which occur with progressive neuronal apoptosis. Such lesions are now recognized to occur before the first microaneurysms are visible to imaging by fundus cameras or before they result in detectable symptoms or signs recognizable to the patient or clinician. Treatments, therefore, which currently are not initiated within the retina until edema develops or there is progression of vascular lesions that define the current staging of retinopathy, and in the brain only after severe signs of cognitive failure. Treatments, therefore are applied relatively late with some reduction in progressive cellular injury but with resultant minimal vision or cognitive improvement. This review article will summarize the multiple inflammatory and remediation processes currently understood to occur in patients with diabetes as well as pre-diabetes and summarize as well the current limitations of methods for assessing the structural and functional alterations within the retina and brain. The goal is to attempt to define future screening, monitoring, and treatment directions that hopefully will prevent progressive injury as well as enable improved repair and attendant function.

Neuroinflammation and neurodegeneration in diabetic retinopathy

Diabetic retinopathy (DR) is the most common complication of diabetes and has been historically regarded as a microangiopathic disease. Now, the paradigm is shifting toward a more comprehensive view of diabetic retinal disease (DRD) as a tissue-specific neurovascular complication, in which persistently high glycemia causes not only microvascular damage and ischemia but also intraretinal inflammation and neuronal degeneration. Despite the increasing knowledge on the pathogenic pathways involved in DR, currently approved treatments are focused only on its late-stage vasculopathic complications, and a single molecular target, vascular endothelial growth factor (VEGF), has been extensively studied, leading to drug development and approval. In this review, we discuss the state of the art of research on neuroinflammation and neurodegeneration in diabetes, with a focus on pathophysiological studies on human subjects, in vivo imaging biomarkers, and clinical trials on novel therapeutic options.

Investigation of heterocellular features of the mouse retinal neurovascular unit by 3D electron microscopy

The retina has a complex structure with a diverse collection of component cells that work together to facilitate vision. The retinal capillaries supplying the nutritional requirements to the inner retina have an intricate system of neural, glial and vascular elements that interconnect to form the neurovascular unit (NVU). The retina has no autonomic nervous system and so relies on the NVU as an interdependent, physical and functional unit to alter blood flow appropriately to changes in the physiological environment. The importance of this is demonstrated by alterations in NVU function being apparent in the blinding disease diabetic retinopathy and other diseases of the retina. It is, therefore, imperative to understand the anatomy of the components of the NVU that underlie its functioning and in particular the nanoscale arrangements of its heterocellular components. However, information on this in three spatial dimensions is limited. In the present study, we utilised the technique of serial block-face scanning electron microscopy (SBF-SEM), and computational image reconstruction, to enable the first three-dimensional ultrastructural analysis of the NVU in mouse retinal capillaries. Mouse isolated retina was prepared for SBF-SEM and up to 150 serial scanning electron microscopy images (covering z-axes distances of 12-8 mm) of individual capillaries in the superficial plexus and NVU cellular components digitally aligned. Examination of the data in the x-, y- and z-planes was performed with the use of semi-automated computational image analysis tools including segmentation, 3D image reconstruction and quantitation of cell proximities. A prominent feature of the capillary arrangements in 3D was the extensive sheath-like coverage by singular pericytes. They appeared in close register to the basement membrane with which they interwove in a complex mesh-like appearance. Breaks in the basement membrane appeared to facilitate pericyte interactions with other NVU cell types. There were frequent, close (<10 nm) pericyte-endothelial interactions with direct contact points and peg-and-socket-like morphology. Macroglia typically intervened between neurons and capillary structures; however, regions were identified where neurons came into closer contact with the basement membrane. A software-generated analysis to assess the morphology of the different cellular components of the NVU, including quantifications of convexity, sphericity and cell-to-cell closeness, has enabled preliminary semi-quantitative characterisation of cell arrangements with neighbouring structures. This study presents new data on the nanoscale spatial characteristics of components of the murine retinal NVU in 3D that has implications for our understanding of structural integrity (e.g. pericyte-endothelial cell anchoring) and function (e.g. possible paracrine communication between macroglia and pericytes). It also serves as a platform to inform future studies examining changes in NVU characteristics with different biological and disease circumstances. All raw and processed image data have been deposited for public viewing.

Effect of obstructive sleep apnoea on retinal microvascular function: a randomised controlled trial

PURPOSE

Retinal microvascular endothelial dysfunction is thought to be of importance in the development of ocular vascular diseases. Obstructive sleep apnoea (OSA) causes macrovascular endothelial dysfunction, but the effect of OSA on retinal microvascular endothelial function is not known. We aimed to determine the effect of OSA on retinal microvascular function.

METHODS

We conducted a multi-centre, double-blind, randomised, parallel, controlled trial in patients with known moderate-to-severe OSA, established on continuous positive airway pressure (CPAP). Participants were randomised to 14 nights of either continued CPAP or sham CPAP to generate a return of OSA. Retinal vascular responses to flickering light were measured using dynamic vessel analysis both at baseline and after 14 nights of intervention. The primary outcome was the change from baseline to follow-up in the area under the curve of the arteriolar response to flickering light, sham CPAP versus continued CPAP.

RESULTS

Nineteen patients were randomised to sham CPAP, and 18 patients were randomised to continued CPAP. There was no significant effect of CPAP withdrawal and return of OSA on retinal responses, with a change in the area under the curve of the arteriole response to flickering light of + 3.8 arbitrary units (95% CI - 10.6 to + 18.2, p = 0.59), sham CPAP versus continued CPAP.

CONCLUSIONS

CPAP withdrawal and a return of OSA had no significant effect on retinal microvascular responses. This contrasts with the effect of CPAP withdrawal on macrovascular endothelial function and suggests that OSA has different effects on macrovascular and microvascular endothelial function. ISRCTN 78082983, 23/10/2014, Prospectively registered.

Diabetic macular ischaemia- a new therapeutic target?

Diabetic macular ischaemia (DMI) is traditionally defined and graded based on the angiographic evidence of an enlarged and irregular foveal avascular zone. However, these anatomical changes are not surrogate markers for visual impairment. We postulate that there are vascular phenotypes of DMI based on the relative perfusion deficits of various retinal capillary plexuses and choriocapillaris. This review highlights several mechanistic pathways, including the role of hypoxia and the complex relation between neurons, glia, and microvasculature. The current animal models are reviewed, with shortcomings noted. Therefore, utilising the advancing technology of optical coherence tomography angiography (OCTA) to identify the reversible DMI phenotypes may be the key to successful therapeutic interventions for DMI. However, there is a need to standardise the nomenclature of OCTA perfusion status. Visual acuity is not an ideal endpoint for DMI clinical trials. New trial endpoints that represent disease progression need to be developed before irreversible vision loss in patients with DMI. Natural history studies are required to determine the course of each vascular and neuronal parameter to define the DMI phenotypes. These DMI phenotypes may also partly explain the development and recurrence of diabetic macular oedema. It is also currently unclear where and how DMI fits into the diabetic retinopathy severity scales, further highlighting the need to better define the progression of diabetic retinopathy and DMI based on both multimodal imaging and visual function. Finally, we discuss a complete set of proposed therapeutic pathways for DMI, including cell-based therapies that may provide restorative potential.

mTORC1 regulates high levels of protein synthesis in retinal ganglion cells of adult mice

Mechanistic target of rapamycin (mTOR) and mTOR complex 1 (mTORC1), linchpins of the nutrient sensing and protein synthesis pathways, are present at relatively high levels in the ganglion cell layer (GCL) and retinal ganglion cells (RGCs) of rodent and human retinas. However, the role of mTORCs in the control of protein synthesis in RGC is unknown. Here, we applied the SUrface SEnsing of Translation (SUnSET) method of nascent protein labeling to localize and quantify protein synthesis in the retinas of adult mice. We also used intravitreal injection of an adeno-associated virus 2 vector encoding Cre recombinase in the eyes of mtor- or rptor-floxed mice to conditionally knockout either both mTORCs or only mTORC1, respectively, in cells within the GCL. A novel vector encoding an inactive Cre mutant (CreΔC) served as control. We found that retinal protein synthesis was highest in the GCL, particularly in RGC. Negation of both complexes or only mTORC1 significantly reduced protein synthesis in RGC. In addition, loss of mTORC1 function caused a significant reduction in the pan-RGC marker, RNA-binding protein with multiple splicing, with little decrease of the total number of cells in the RGC layer, even at 25 weeks after adeno-associated virus-Cre injection. These findings reveal that mTORC1 signaling is necessary for maintaining the high rate of protein synthesis in RGCs of adult rodents, but it may not be essential to maintain RGC viability. These findings may also be relevant to understanding the pathophysiology of RGC disorders, including glaucoma, diabetic retinopathy, and optic neuropathies.

Piezo1 Is a Mechanosensor Channel in Central Nervous System Capillaries

Capillaries are equipped to sense neurovascular coupling agents released onto the outer wall of a capillary, translating these external signals into electrical/Ca2+ changes that play a crucial role in blood flow regulation and ensuring that neuronal demands are met. However, control mechanisms attributable to forces imposed onto the lumen are less clear. Here, we show that Piezo1 channels act as mechanosensors in central nervous system capillaries. Electrophysiological analyses confirmed expression and function of Piezo1 channels in brain cortical and retinal capillaries. Activation of Piezo1 channels evoked currents that were sensitive to endothelial cell-specific Piezo1 deletion. Using genetically encoded Ca2+ indicator mice and an ex vivo pressurized retina preparation, we found that activation of Piezo1 channels by mechanical forces triggered Ca2+ signals in capillary endothelial cells. Collectively, these findings indicate that Piezo1 channels are capillary mechanosensors that initiate crucial Ca2+ signals and could, therefore, have a profound impact on central nervous system blood flow control.

N-methyl-D-aspartic acid receptor-mediated vasodilation is attenuated in the retinas of diabetic rats

PURPOSE

Activation of N-methyl-d-aspartic acid (NMDA) receptors enhances nitric oxide (NO) production in retinal neuronal cells, and in turn, NO released from neuronal cells induces glial cell-mediated dilation of retinal arterioles in rats. The purpose of this study was to examine how neuronal cell-dependent, glial cell-mediated vasodilation is impacted in diabetic rat retinas.

MATERIALS AND METHODS

Diabetes was induced in 6-week-old male Wistar rats by combining streptozotocin injection and D-glucose feeding. Two weeks later, the dilator function of retinal arterioles was assessed.

RESULTS

Compared with non-diabetic rats, the dilator responses of retinal arterioles induced by intravitreal injection of NMDA and NOR3, an NO donor, were reduced in diabetic rats. Following the blockade of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with iberiotoxin, no significant difference in the retinal vasodilator response to NOR3 was observed between non-diabetic and diabetic rats. Intravitreal injection of 14,15-epoxyeicosatrienoic acid, a vasodilatory factor released from glial cells, dilated retinal arterioles, and the response was diminished by diabetes.

CONCLUSION

These findings suggest that the impaired BK_Ca channel function in vascular cells is responsible for the diminished neuronal cell-dependent, glial cell-mediated dilation of retinal arterioles during the early stage of diabetes.

Neuroprotective Effects of Tauroursodeoxicholic Acid Involves Vascular and Glial Changes in Retinitis Pigmentosa Model

Purpose

Retinitis pigmentosa is primarily characterized by a massive photoreceptor loss. But a global retinal remodeling occurs in later stages of the disease. At that phase, glial cells and retinal vasculature are also strongly affected. The main aim of the present work is to assess if the bile acid Tauroursodeoxicholic acid (TUDCA), which has a demonstrated neuroprotective effect in numerous neurodegenerative diseases, is able to prevent glial and vascular degeneration in the P23H rat retina.

Methods

Homozygous P23H (line 3) animals were injected weekly with a TUDCA (500 mg/kg, i.p.) or vehicle solution, from the postnatal day (P) 21 to P120. Sprague-Dawley rats (SD) were used as control. Retinal cross-sections and wholemounts were immunostained using different glial and vascular markers and visualized with confocal microscopy. Retinal blood vessels were stained with nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry and retinal vascular networks were drawn by hand using a camera lucida.

Results

At P120, the photoreceptor degeneration observed in P23H rats was accompanied by a reduction in the vascular network density and complexity at the deep capillary plexus. In addition, astrocytes showed gliotic features and the outer processes of Müller cells displayed an aberrant distribution in ring-shaped structures. When treated with TUDCA, P23H rats displayed better-preserved vessels and capillary loops in the deep capillary plexus which are associated with the partial preservation of photoreceptors. TUDCA treatment also increased the number of astrocytes and reduced the presence of Müller cell process clusters in the outer retina.

Conclusion

This work suggests that, besides its neuroprotective effect on photoreceptor cells, TUDCA treatment also protects from vascular and glial degeneration, a fact that encourages the use of TUDCA as a powerful therapy for neurodegenerative diseases.

Diabetic Retinopathy: Role of Neurodegeneration and Therapeutic Perspectives

ABSTRACT

Retinal neurodegeneration plays a significant role in the pathogenesis of diabetic retinopathy, the leading cause of preventable blindness. The hallmarks of diabetes-induced neurodegeneration are neural cell apoptosis and glial activation, which seem even before vascular lesions can be detected by ophthalmoscopic examination. The molecular mediators of retinal neurodegeneration include proinflamma- tory cytokines, oxidative stress, mitochondrial dysfunction, and the molecular pathways closely related to chronic hyperglycemia. In this article, an overview of the main components of neurodegeneration, its key underlying mechanisms, and the more useful experimental models for investigative purposes will be given. In addition, the results of most relevant treatments based on neuroprotection, and the research gaps that should be filled will be critically reviewed.

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.

Neurovascular Impairment and Therapeutic Strategies in Diabetic Retinopathy

Diabetic retinopathy has recently been defined as a highly specific neurovascular complication of diabetes. The chronic progression of the impairment of the interdependence of neurovascular units (NVUs) is associated with the pathogenesis of diabetic retinopathy. The NVUs consist of neurons, glial cells, and vascular cells, and the interdependent relationships between these cells are disturbed under diabetic conditions. Clinicians should understand and update the current knowledge of the neurovascular impairments in diabetic retinopathy. Above all, neuronal cell death is an irreversible change, and it is directly related to vision loss in patients with diabetic retinopathy. Thus, neuroprotective and vasoprotective therapies for diabetic retinopathy must be established. Understanding the physiological and pathological interdependence of the NVUs is helpful in establishing neuroprotective and vasoprotective therapies for diabetic retinopathy. This review focuses on the pathogenesis of the neurovascular impairments and introduces possible neurovascular protective therapies for diabetic retinopathy.

Fractalkine-induced microglial vasoregulation occurs within the retina and is altered early in diabetic retinopathy

This work identifies a role for microglia, the innate immune cells of the CNS, in the local control of the retinal vasculature and identifies deficits early in diabetes. Microglia contact neurons and vasculature and express several vasoactive agents. Activation of microglial fractalkine-Cx3cr1 signaling leads to capillary constriction and blocking the renin-angiotensin system (RAS) with candesartan abolishes microglial-mediated vasoconstriction in the retina. In early diabetes, reduced retinal blood flow is coincident with capillary constriction, increased microglial–vessel association, loss of microglial–capillary regulation, and altered microglial expression of the RAS pathway. While candesartan restores retinal capillary diameter early in diabetes, targeting of microglial–vascular regulation is required to prevent coincident dilation of large retinal vessels and reduced retinal blood flow.

Local blood flow control within the central nervous system (CNS) is critical to proper function and is dependent on coordination between neurons, glia, and blood vessels. Macroglia, such as astrocytes and Müller cells, contribute to this neurovascular unit within the brain and retina, respectively. This study explored the role of microglia, the innate immune cell of the CNS, in retinal vasoregulation, and highlights changes during early diabetes. Structurally, microglia were found to contact retinal capillaries and neuronal synapses. In the brain and retinal explants, the addition of fractalkine, the sole ligand for monocyte receptor Cx3cr1, resulted in capillary constriction at regions of microglial contact. This vascular regulation was dependent on microglial Cx3cr1 involvement, since genetic and pharmacological inhibition of Cx3cr1 abolished fractalkine-induced constriction. Analysis of the microglial transcriptome identified several vasoactive genes, including angiotensinogen, a constituent of the renin-angiotensin system (RAS). Subsequent functional analysis showed that RAS blockade via candesartan abolished microglial-induced capillary constriction. Microglial regulation was explored in a rat streptozotocin (STZ) model of diabetic retinopathy. Retinal blood flow was reduced after 4 wk due to reduced capillary diameter and this was coincident with increased microglial association. Functional assessment showed loss of microglial–capillary response in STZ-treated animals and transcriptome analysis showed evidence of RAS pathway dysregulation in microglia. While candesartan treatment reversed capillary constriction in STZ-treated animals, blood flow remained decreased likely due to dilation of larger vessels. This work shows microglia actively participate in the neurovascular unit, with aberrant microglial–vascular function possibly contributing to the early vascular compromise during diabetic retinopathy.

The role of inflammation and neurodegeneration in diabetic macular edema

The pathogenesis of diabetic macular edema (DME) is complex. Persistently high blood glucose activates multiple cellular pathways and induces inflammation, oxidation stress, and vascular dysfunction. Retinal ganglion cells, macroglial and microglial cells, endothelial cells, pericytes, and retinal pigment epithelium cells are involved. Neurodegeneration, characterized by dysfunction or apoptotic loss of retinal neurons, occurs early and independently from the vascular alterations. Despite the increasing knowledge on the pathways involved in DME, only limited therapeutic strategies are available. Besides antiangiogenic drugs and intravitreal corticosteroids, alternative therapeutic options tackling inflammation, oxidative stress, and neurodegeneration have been considered, but none of them has been currently approved.

Obstructive Sleep Apnoea, Other Sleep Parameters and Diabetic Retinopathy

PURPOSE OF REVIEW

To summarise the association between obstructive sleep apnoea and diabetic retinopathy and diabetic macular oedema. To examine the effects of other sleep parameters on diabetic retinopathy and diabetic macular oedema. To discuss the pathophysiology of diabetic eye changes and how it is related to obstructive sleep apnoea.

RECENT FINDINGS

Conflicting data exists in terms of the association of diabetic eye changes with sleep apnoea and/or other sleep parameters. Various cross-sectional studies show PDR to be associated with the prevalence of OSA. Patients who underwent continuous positive airway pressure (CPAP) treatment were significantly less likely to develop pre/proliferative DR. Secondary sleep parameters generally are not associated with DR except for long duration of sleep. Differences in reporting could be due to the different thresholds set to define OSA/ODI and severity of DR/DME, in addition to factors used in multivariate analysis. There is a need for further studies with long-term follow-up and to assess the impact of CPAP on the development and progression of diabetic eye change(s).

Magnetic Resonance Imaging-Visible Perivascular Spaces in the Basal Ganglia Are Associated With the Diabetic Retinopathy Stage and Cognitive Decline in Patients With Type 2 Diabetes

Purpose: The aim of this study was to evaluate whether perivascular space (PVS) severity and retinal ganglion cell layer (GCL) thickness differed based on the stage of diabetic retinopathy (DR) and the cognitive status in patients with DR.

Methods: A total of 81 patients with DR (51 in the non-proliferative group and 30 in the proliferative group) were included in this retrospective, cross-sectional study. PVS severity was assessed in the basal ganglia (BG) and centrum semiovale using MRI. The total cerebral small vessel disease (SVD) score was determined based on the numbers of lacunes and microbleeds and the severity of white matter hyperintensity. Optical coherence tomography was used to measure foveal and perifoveal GCL thicknesses. Cerebral SVD markers and cognitive function were compared between the groups, and correlations between the BG-PVS severity and the Mini-Mental Status Examination (MMSE) scores and GCL parameters were evaluated.

Results: Patients with proliferative DR had higher BG-PVS severity (P = 0.012), higher total cerebral SVD scores (P = 0.035), reduced GCL thicknesses in the inferior (P = 0.027), superior (P = 0.046), and temporal (P = 0.038) subfields compared to patients with non-proliferative DR. In addition, the BG-PVS severity was negatively correlated with the MMSE score (P = 0.007), and the GCL thickness was negatively correlated with the BG-PVS severity (P-values < 0.05 for inferior, superior, and temporal subfields).

Conclusion: BG-PVS severity and retinal GCL thickness may represent novel imaging biomarkers reflecting the stage of DR and cognitive decline in diabetic patients. Furthermore, these results suggest a possible link between cerebral and retinal neurodegeneration at the clinical level.

The effect of AAV-mediated downregulation of Claudin-3 on the development of mouse retinal vasculature

Retinal vascular development is a very tightly regulated and organized process of vessel formation and regression to generate the mature vasculature system. Claudin-3 has been found to be required for the normal development of the neural retina and its vessels in zebrafish in our recent study. In this study, we investigated whether Claudin-3 played a role in the development of mouse retinal vasculature. Immunofluorescent staining was performed to detect the expression and localization of Claudin-3 in the mouse retina. Intravitreal injection of a recombinant adeno-associated virus (AAV) expressing a short hairpin RNA targeting Claudin-3 mRNA was performed to down-regulate Claudin-3 expression in retina in neonatal (Postnatal Day 3, P3) C57BL/6J mice. Retinal vessels were examined by isolectin B4 immunofluorescent staining on the whole-mount retinas and frozen retinal sections at P10. The apoptotic retinal ganglion cells (RGCs) were measured by TdT-mediated dUTP nick-end labelling (TUNEL) staining. Vascular endothelial growth factor A (VEGF-A) expression was detected by immunofluorescent staining. The protein levels of Claudin-3, VEGF-A and B cell lymphoma 2 (Bcl-2) were evaluated by Western blot at P7, P10 and P14. We found that Claudin-3 mainly expressed in the RGCs and progressively increased during the retinal development. The AAV-mediated downregulation of Claudin-3 at P3 impeded the development of retinal deep vascularization of P10 mouse, but without effect on the development of the retinal superficial plexus. Claudin-3 knockdown increased RGC apoptosis and reduced the expression of VEGF-A and Bcl-2 in the retinas. These results suggested that the downregulation of Claudin-3 induced RGC apoptosis and impeded the mouse retinal vascular development by downregulating the levels of VEGF-A and Bcl-2.

Advances in the Relationship Between Pyroptosis and Diabetic Neuropathy

Pyroptosis is a novel programmed cell death process that promotes the release of interleukin-1β (IL-1β) and interleukin-18 (IL-18) by activating inflammasomes and gasdermin D (GSDMD), leading to cell swelling and rupture. Pyroptosis is involved in the regulation of the occurrence and development of cardiovascular and cerebrovascular diseases, tumors, and nerve injury. Diabetes is a metabolic disorder characterized by long-term hyperglycemia, insulin resistance, and chronic inflammation. The people have paid more and more attention to the relationship between pyroptosis, diabetes, and its complications, especially its important regulatory significance in diabetic neurological diseases, such as diabetic encephalopathy (DE) and diabetic peripheral neuropathy (DPN). This article will give an in-depth overview of the relationship between pyroptosis, diabetes, and its related neuropathy, and discuss the regulatory pathway and significance of pyroptosis in diabetes-associated neuropathy.

The "Neuro-Glial-Vascular" Unit: The Role of Glia in Neurovascular Unit Formation and Dysfunction

The neurovascular unit (NVU) is a complex multi-cellular structure consisting of endothelial cells (ECs), neurons, glia, smooth muscle cells (SMCs), and pericytes. Each component is closely linked to each other, establishing a structural and functional unit, regulating central nervous system (CNS) blood flow and energy metabolism as well as forming the blood-brain barrier (BBB) and inner blood-retina barrier (BRB). As the name suggests, the “neuro” and “vascular” components of the NVU are well recognized and neurovascular coupling is the key function of the NVU. However, the NVU consists of multiple cell types and its functionality goes beyond the resulting neurovascular coupling, with cross-component links of signaling, metabolism, and homeostasis. Within the NVU, glia cells have gained increased attention and it is increasingly clear that they fulfill various multi-level functions in the NVU. Glial dysfunctions were shown to precede neuronal and vascular pathologies suggesting central roles for glia in NVU functionality and pathogenesis of disease. In this review, we take a “glio-centric” view on NVU development and function in the retina and brain, how these change in disease, and how advancing experimental techniques will help us address unanswered questions.

Macular Microvascular Modifications in Progressive Lamellar Macular Holes

Lamellar macular holes (LMHs) may show morphological and functional deterioration over time, yet no definite prognostic factor for progression has been identified. Since neurovascular retinal unit impairment may take part in neurodegeneration, we compare progressive LMHs to stable ones in optical coherence tomography (OCT) angiography parameters.

METHODS

OCT B scans of eyes with LMH were analyzed to detect the presence of tissue loss (TL) over time, allowing us to identify a TL group and a stable (ST) group (14 patients each). The best corrected visual acuity (BCVA) at each considered imaging time point was collected. Lastly, patients underwent macular OCT angiography.

RESULTS

BCVA at last follow up was significantly reduced in the TL group compared to both the ST group and TL group baseline assessment. SCP foveal vessel density (VD), SCP and deep capillary plexus (DCP) perfusion density (PD) and parafoveal PD were lower in the TL group. Linear correlations between quantitative TL over time and parafoveal PD in SCP and between the speed of TL and BCVA variation during follow up were also detected.

CONCLUSIONS

TL in LMHs is associated with both OCT angiography modifications and BCVA deterioration over time. We suggest these findings to be a manifestation of foveal Muller cell impairment in progressive LMHs.

Neurovascular Unit: A New Target for Treating Early Stages of Diabetic Retinopathy

The concept of diabetic retinopathy as a microvascular disease has evolved and is now considered a more complex diabetic complication in which neurovascular unit impairment plays an essential role and, therefore, can be considered as a main therapeutic target in the early stages of the disease. However, neurodegeneration is not always the apparent primary event in the natural story of diabetic retinopathy, and a phenotyping characterization is recommendable to identify those patients in whom neuroprotective treatment might be of benefit. In recent years, a myriad of treatments based on neuroprotection have been tested in experimental models, but more interestingly, there are drugs with a dual activity (neuroprotective and vasculotropic). In this review, the recent evidence concerning the therapeutic approaches targeting neurovascular unit impairment will be presented, along with a critical review of the scientific gaps and problems which remain to be overcome before our knowledge can be transferred to clinical practice.

The Pathogenesis and Therapeutic Approaches of Diabetic Neuropathy in the Retina

Diabetic retinopathy is a major retinal disease and a leading cause of blindness in the world. Diabetic retinopathy is a neurovascular disease that is associated with disturbances of the interdependent relationship of cells composed of the neurovascular units, i.e., neurons, glial cells, and vascular cells. An impairment of these neurovascular units causes both neuronal and vascular abnormalities in diabetic retinopathy. More specifically, neuronal abnormalities including neuronal cell death and axon degeneration are irreversible changes that are directly related to the vision reduction in diabetic patients. Thus, establishment of neuroprotective and regenerative therapies for diabetic neuropathy in the retina is an emergent task for preventing the blindness of patients with diabetic retinopathy. This review focuses on the pathogenesis of the neuronal abnormalities in diabetic retina including glial abnormalities, neuronal cell death, and axon degeneration. The possible molecular cell death pathways and intrinsic survival and regenerative pathways are also described. In addition, therapeutic approaches for diabetic neuropathy in the retina both in vitro and in vivo are presented. This review should be helpful for providing clues to overcome the barriers for establishing neuroprotection and regeneration of diabetic neuropathy in the retina.

Imaging retinal microvascular manifestations of carotid artery disease in older adults: from diagnosis of ocular complications to understanding microvascular contributions to cognitive impairment

Carotid artery stenosis (CAS) is a consequence of systemic atherosclerotic disease affecting the aging populations of the Western world. CAS is frequently associated with cognitive impairment. However, the mechanisms contributing to the development of vascular cognitive impairment (VCI) associated with CAS are multifaceted and not fully understood. In addition to embolization and decreased blood flow due to the atherosclerotic lesion in the carotid artery, microcirculatory dysfunction in the cerebral circulation also plays a critical role in CAS-related VCI. To better understand the microvascular contributions to cognitive decline associated with CAS and evaluate microvascular protective effects of therapeutic interventions, it is essential to examine the structural and functional changes of the microvessels in the central nervous system (CNS). However, there are some limitations of in vivo brain vascular imaging modalities. The retinal microvasculature provides a unique opportunity to study pathogenesis of cerebral small vessel disease and VCI, because the cerebral circulation and the retinal circulation share similar anatomy, physiology and embryology. Similar microvascular pathologies may manifest in the brain and the retina, thus ocular examination can be used as a noninvasive screening tool to investigate pathological changes in the CNS associated with CAS. In this review, ocular signs of CAS and the retinal manifestations of CAS-associated microvascular dysfunction are discussed. The advantages and limitation of methods that are capable of imaging the ocular circulation (including funduscopy, fluorescein angiography, Doppler sonography, optical coherence tomography [OCT] and optical coherence tomography angiography [OCTA]) are discussed. The potential use of dynamic retinal vessel analysis (DVA), which allows for direct visualization of neurovascular coupling responses in the CNS, for understanding microvascular contributions to cognitive decline in CAS patients is also considered.

The role of peroxisome proliferator-activated receptors in healthy and diseased eyes

Peroxisome Proliferator-Activated Receptors (PPARs) are a family of nuclear receptors that play essential roles in modulating cell differentiation, inflammation, and metabolism. Three subtypes of PPARs are known: PPAR-alpha (PPARα), PPAR-gamma (PPARγ), and PPAR-beta/delta (PPARβ/δ). PPARα activation reduces lipid levels and regulates energy homeostasis, activation of PPARγ results in regulation of adipogenesis, and PPARβ/δ activation increases fatty acid metabolism and lipolysis. PPARs are linked to various diseases, including but not limited to diabetes, non-alcoholic fatty liver disease, glaucoma and atherosclerosis. In the past decade, numerous studies have assessed the functional properties of PPARs in the eye and key PPAR mechanisms have been discovered, particularly regarding the retina and cornea. PPARγ and PPARα are well established in their functions in ocular homeostasis regarding neuroprotection, neovascularization, and inflammation, whereas PPARβ/δ isoform function remains understudied. Naturally, studies on PPAR agonists and antagonists, associated with ocular pathology, have also gained traction with the development of PPAR synthetic ligands. Studies on PPARs has significantly influenced novel therapeutics for diabetic eye disease, ocular neuropathy, dry eye, and age-related macular degeneration (AMD). In this review, therapeutic potentials and implications will be highlighted, as well as reported adverse effects. Further investigations are necessary before any of the PPARs ligands can be utilized, in the clinics, to treat eye diseases. Future research on the prominent role of PPARs will help unravel the complex mechanisms involved in order to prevent and treat ocular diseases.

Advancing Diabetic Retinopathy Research: Analysis of the Neurovascular Unit in Zebrafish

Diabetic retinopathy is one of the most important microvascular complications associated with diabetes mellitus, and a leading cause of vision loss or blindness worldwide. Hyperglycaemic conditions disrupt microvascular integrity at the level of the neurovascular unit. In recent years, zebrafish (Danio rerio) have come into focus as a model organism for various metabolic diseases such as diabetes. In both mammals and vertebrates, the anatomy and the function of the retina and the neurovascular unit have been highly conserved. In this review, we focus on the advances that have been made through studying pathologies associated with retinopathy in zebrafish models of diabetes. We discuss the different cell types that form the neurovascular unit, their role in diabetic retinopathy and how to study them in zebrafish. We then present new insights gained through zebrafish studies. The advantages of using zebrafish for diabetic retinopathy are summarised, including the fact that the zebrafish has, so far, provided the only animal model in which hyperglycaemia-induced retinal angiogenesis can be observed. Based on currently available data, we propose potential investigations that could advance the field further.

The Contribution of Microglia to the Development and Maturation of the Visual System

Microglia, the resident immune cells of the central nervous system (CNS), were once considered quiescent cells that sat in readiness for reacting to disease and injury. Over the last decade, however, it has become clear that microglia play essential roles in maintaining the normal nervous system. The retina is an easily accessible part of the central nervous system and therefore much has been learned about the function of microglia from studies in the retina and visual system. Anatomically, microglia have processes that contact all synapses within the retina, as well as blood vessels in the major vascular plexuses. Microglia contribute to development of the visual system by contributing to neurogenesis, maturation of cone photoreceptors, as well as refining synaptic contacts. They can respond to neural signals and in turn release a range of cytokines and neurotrophic factors that have downstream consequences on neural function. Moreover, in light of their extensive contact with blood vessels, they are also essential for regulation of vascular development and integrity. This review article summarizes what we have learned about the role of microglia in maintaining the normal visual system and how this has helped in understanding their role in the central nervous system more broadly.

The Impacts of Unfolded Protein Response in the Retinal Cells During Diabetes: Possible Implications on Diabetic Retinopathy Development

Diabetic retinopathy (DR) is a vision-threatening, chronic, and challenging eye disease in the diabetic population. Despite recent advancements in the clinical management of diabetes, DR remains the major cause of blindness in working-age adults. A better understanding of the molecular and cellular basis of DR development will aid in identifying therapeutic targets. Emerging pieces of evidence from recent research in the field of ER stress have demonstrated a close association between unfolded protein response (UPR)-associated cellular activities and DR development. In this minireview article, we shall provide an emerging understating of how UPR influences DR pathogenesis at the cellular level.

Protecting the aging eye with hydrogen sulfide

Research demonstrates that senescence is associated with tissue and organ dysfunction, and the eye is no exception. Sequelae arising from aging have been well defined as distinct clinical entities and vision impairment has significant psychosocial consequences. Retina and adjacent tissues like retinal pigmented epithelium and choroid are the key structures that are required for visual perception. Any structural and functional changes in retinal layers and blood retinal barrier could lead to age-related macular degeneration, diabetic retinopathy, and glaucoma. Further, there are significant oxygen gradients in the eye that can lead to excessive reactive oxygen species, resulting in endoplasmic reticulum and mitochondrial stress response. These radicals are source of functional and morphological impairment in retinal pigmented epithelium and retinal ganglion cells. Therefore, ocular diseases could be summarized as disturbance in the redox homeostasis. Hyperhomocysteinemia is a risk factor and causes vascular occlusive disease of the retina. Interestingly, hydrogen sulfide (H2S) has been proven to be an effective antioxidant agent, and it can help treat diseases by alleviating stress and inflammation. Concurrent glutamate excitotoxicity, endoplasmic reticulum stress, and microglia activation are also linked to stress; thus, H2S may offer additional interventional strategy. A refined understanding of the aging eye along with H2S biology and pharmacology may help guide newer therapies for the eye.

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.

Metabolic Dysregulation and Neurovascular Dysfunction in Diabetic Retinopathy

Diabetic retinopathy is a major cause of ocular complications in patients with type 1 and type 2 diabetes in developed countries. Due to the continued increase in the number of people with obesity and diabetes in the United States of America and globally, the incidence of diabetic retinopathy is expected to increase significantly in the coming years. Diabetic retinopathy is widely accepted as a combination of neurodegenerative and microvascular changes; however, which change occurs first is not yet understood. Although the pathogenesis of diabetic retinopathy is very complex, regulated by numerous signaling pathways and cellular processes, maintaining glucose homeostasis is still an essential component for normal physiological functioning of retinal cells. The maintenance of glucose homeostasis is finely regulated by coordinated interplay between glycolysis, Krebs cycle, and oxidative phosphorylation. Glycolysis is the most conserved metabolic pathway in biology and is tightly regulated to maintain a steady-state concentration of glycolytic intermediates; this regulation is called scheduled or regulated glycolysis. However, an abnormal increase in glycolytic flux generates large amounts of intermediate metabolites that can be shunted into different damaging pathways including the polyol pathway, hexosamine pathway, diacylglycerol-dependent activation of the protein kinase C pathway, and Amadori/advanced glycation end products (AGEs) pathway. In addition, disrupting the balance between glycolysis and oxidative phosphorylation leads to other biochemical and molecular changes observed in diabetic retinopathy including endoplasmic reticulum-mitochondria miscommunication and mitophagy dysregulation. This review will focus on how dysregulation of glycolysis contributes to diabetic retinopathy.

Retinal biomarkers for Alzheimer's disease and vascular cognitive impairment and dementia (VCID): implication for early diagnosis and prognosis

Cognitive impairment and dementia are major medical, social, and economic public health issues worldwide with significant implications for life quality in older adults. The leading causes are Alzheimer's disease (AD) and vascular cognitive impairment/dementia (VCID). In both conditions, pathological alterations of the cerebral microcirculation play a critical pathogenic role. Currently, the main pathological biomarkers of AD-β-amyloid peptide and hyperphosphorylated tau proteins-are detected either through cerebrospinal fluid (CSF) or PET examination. Nevertheless, given that they are invasive and expensive procedures, their availability is limited. Being part of the central nervous system, the retina offers a unique and easy method to study both neurodegenerative disorders and cerebral small vessel diseases in vivo. Over the past few decades, a number of novel approaches in retinal imaging have been developed that may allow physicians and researchers to gain insights into the genesis and progression of cerebromicrovascular pathologies. Optical coherence tomography (OCT), OCT angiography, fundus photography, and dynamic vessel analyzer (DVA) are new imaging methods providing quantitative assessment of retinal structural and vascular indicators-such as thickness of the inner retinal layers, retinal vessel density, foveal avascular zone area, tortuosity and fractal dimension of retinal vessels, and microvascular dysfunction-for cognitive impairment and dementia. Should further studies need to be conducted, these retinal alterations may prove to be useful biomarkers for screening and monitoring dementia progression in clinical routine. In this review, we seek to highlight recent findings and current knowledge regarding the application of retinal biomarkers in dementia assessment.

Oxidative stress and diabetic retinopathy: Molecular mechanisms, pathogenetic role and therapeutic implications

Oxidative stress, a cytopathic outcome of excessive generation of ROS and the repression of antioxidant defense system for ROS elimination, is involved in the pathogenesis of multiple diseases, including diabetes and its complications. Retinopathy, a microvascular complication of diabetes, is the primary cause of acquired blindness in diabetic patients. Oxidative stress has been verified as one critical contributor to the pathogenesis of diabetic retinopathy. Oxidative stress can both contribute to and result from the metabolic abnormalities induced by hyperglycemia, mainly including the increased flux of the polyol pathway and hexosamine pathway, the hyper-activation of protein kinase C (PKC) isoforms, and the accumulation of advanced glycation end products (AGEs). Moreover, the repression of the antioxidant defense system by hyperglycemia-mediated epigenetic modification also leads to the imbalance between the scavenging and production of ROS. Excessive accumulation of ROS induces mitochondrial damage, cellular apoptosis, inflammation, lipid peroxidation, and structural and functional alterations in retina. Therefore, it is important to understand and elucidate the oxidative stress-related mechanisms underlying the progress of diabetic retinopathy. In addition, the abnormalities correlated with oxidative stress provide multiple potential therapeutic targets to develop safe and effective treatments for diabetic retinopathy. Here, we also summarized the main antioxidant therapeutic strategies to control this disease.

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Oxidative stress can both contribute to and result from hyperglycemia-induced metabolic abnormalities in retina.

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Genes important in regulation of ROS are epigenetically modified, increasing ROS accumulation in retina.

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Oxidative stress is closely associated with the pathological changes in the progress of diabetic retinopathy.

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Antioxidants ameliorate retinopathy through targeting multiple steps of oxidative stress.

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.

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

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

Oxidative Stress and Vascular Dysfunction in the Retina: Therapeutic Strategies

Many retinal diseases, such as diabetic retinopathy, glaucoma, and age-related macular (AMD) degeneration, are associated with elevated reactive oxygen species (ROS) levels. ROS are important intracellular signaling molecules that regulate numerous physiological actions, including vascular reactivity and neuron function. However, excessive ROS formation has been linked to vascular endothelial dysfunction, neuron degeneration, and inflammation in the retina. ROS can directly modify cellular molecules and impair their function. Moreover, ROS can stimulate the production of inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) causing inflammation and cell death. However, there are various compounds with direct or indirect antioxidant activity that have been used to reduce ROS accumulation in animal models and humans. In this review, we report on the physiological and pathophysiological role of ROS in the retina with a special focus on the vascular system. Moreover, we present therapeutic approaches for individual retinal diseases targeting retinal signaling pathways involving ROS.

Chrysin Inhibits High Glucose-Induced Migration on Chorioretinal Endothelial Cells via VEGF and VEGFR Down-Regulation

Background: Diabetes mellitus (DM) is a chronic inflammatory disease, which causes multiple complications. Diabetic retinopathy (DR) is among these complications and is a dominant cause of vision loss for diabetic patients. Numerous studies have shown that chrysin, a flavonoid, has many biological activities such as anti-oxidation and anti-inflammation. However, it is rarely used in ocular diseases. In this study, we examined the inhibitory effects of flavonoid on high glucose induced migration of chorioretinal endothelial cells (RF/6A cells) and its mechanism. Materials and methods: The viability of RF/6A cells treated with chrysin was examined with a 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. The migration of RF/6A cells was assessed by the transwell migration and scratch wound assays. The expression of AKT, ERK, vascular endothelial growth factor (VEGF), HIF−1α and MMP-2 were determined by western blotting. To observe the mRNA expression of VEGF receptor (VEGFR), qRT-PCR, was utilized. Results: The results showed that chrysin can dose-dependently inhibit the RF/6A cell migration in vitro transwell and the scratch wound assays which are induced by high glucose. After pretreatment of RF/6A cells with different concentrations of chrysin, they did not produce any cytotoxicity in MTT assay. Moreover, chrysin down-regulated both phosphorylated AKT and ERK, as well as attenuated the expression levels of MMP-2. It also decreased the expression of the VEGF transcription factor and VEGF. Furthermore, it was shown that chrysin could suppress the protein and mRNA expression levels of VEGFR. Conclusion: The results indicate that chrysin could down-regulate the phosphorylation of AKT, ERK and MMP-2 and reduce the effects of VEGF and VEGFR in a high glucose environment. It further inhibits the high glucose-induced migration of RE/6A cells. Therefore, chrysin may have the potential for visual protection.