• Users Online: 70
  • Home
  • Print this page
  • Email this page
Home Current issue Ahead of print Search About us Editorial board Archives Submit article Instructions Subscribe Contacts Login 


 
 Table of Contents  
REVIEW ARTICLE
Year : 2014  |  Volume : 9  |  Issue : 3  |  Page : 362-373

Diabetes and Retinal Vascular Dysfunction


1 Department of Ophthalmology and Visual Sciences, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA
2 Department of Pediatrics; McPherson Eye Research Institute, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA
3 Department of Ophthalmology and Visual Sciences; McPherson Eye Research Institute, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA

Date of Submission14-Dec-2013
Date of Acceptance19-Jan-2014
Date of Web Publication27-Oct-2014

Correspondence Address:
Nader Sheibani
Department of Ophthalmology and Visual Sciences, University of Wisconsin School of Medicine and Public Health, 1111 Highland Avenue, 9453 WIMR, Madison, WI 53705-2275
USA
Login to access the Email id

Source of Support: This work was supported by grants R01 EY016995, RC4 EY021357, R24 EY022883, R21 EY023024, P30 EY016665 and P30 CA014520 UW Paul P. Carbone Cancer Center Support Grant from the National Institutes of Health and an unrestricted departmental award from Research to Prevent Blindness. NS is a recipient of a Research Award from American Diabetes Association, 1-10-BS-160 and Retina Research Foundation. ESS is supported by a UW-Madison UW-Milwaukee Inter Campus grant and predoctoral fellowship from American Heart Association, 12PRE12030099., Conflict of Interest: None


DOI: 10.4103/2008-322X.143378

Rights and Permissions
  Abstract 

Diabetes predominantly affects the microvascular circulation of the retina resulting in a range of structural changes unique to this tissue. These changes ultimately lead to altered permeability, hyperproliferation of endothelial cells and edema, and abnormal vascularization of the retina with resulting loss of vision. Enhanced production of inflammatory mediators and oxidative stress are primary insults with significant contribution to the pathogenesis of diabetic retinopathy (DR). We have determined the identity of the retinal vascular cells affected by hyperglycemia, and have delineated the cell autonomous impact of high glucose on function of these cells. We discuss some of the high glucose specific changes in retinal vascular cells and their contribution to retinal vascular dysfunction. This knowledge provides novel insight into the molecular and cellular defects contributing to the development and progression of diabetic retinopathy, and will aid in the development of innovative, as well as target specific therapeutic approaches for prevention and treatment of DR.

Keywords: Diabetes; Retinal vasculature; Inflammation; Oxidative Stress; Thrombospondins


How to cite this article:
Shin ES, Sorenson CM, Sheibani N. Diabetes and Retinal Vascular Dysfunction. J Ophthalmic Vis Res 2014;9:362-73

How to cite this URL:
Shin ES, Sorenson CM, Sheibani N. Diabetes and Retinal Vascular Dysfunction. J Ophthalmic Vis Res [serial online] 2014 [cited 2017 Mar 28];9:362-73. Available from: http://www.jovr.org/text.asp?2014/9/3/362/143378


  Introduction Top


The retina is a specialized tissue for vision, which converts visible light into the neuronal signals perceived by the brain. The retina's unique vascular system provides nutrients and oxygen to the inner and outer retina, whose integrity is essential for sensing light. [1] Thus, the retinal vascular structure acquires blood-retinal barrier (BRB) characteristics, which are crucial for the integrity of retinal vascular structure and the regulation of the retinal microenvironment. In the retina, BRB consists of inner and outer components. Inner BRB consists of tight junctions between endothelial cells, and outer BRB is formed by tight junctions between retinal pigment epithelial cells. [2] In inner BRB, endothelial cells are surrounded by pericytes and the foot processes of astrocytes. [3] Astro-and peri-cytes are crucial for maintaining the structure of inner BRB. [2] Dysfunction of vascular cells in inner BRB during disease states causes breakdown of BRB leading to serious impairment of vision. In diabetic retinopathy (DR), the breakdown of BRB results in vascular leakage and subsequent macular edema, a major cause of vision impairment. [4] To better understand how the breakdown of BRB progresses, the cellular mechanisms responsible for the dysfunction of retinal vascular cells including endothelial cells, peri-and astro-cytes under pathological conditions need to be elucidated. The effects of high glucose conditions caused by diabetes on the function of retinal vascular cells, and the mechanisms for dysfunction of retinal vascular cells under high glucose conditions have been the subject of numerous studies. However, the detailed mechanisms involved, and the identity of the primary cellular target of diabetes have remained unknown. These gaps in our understanding of these mechanisms have hampered the development of effective therapies for DR. as one of the serious complications of diabetes and a leading cause of blindness among working-age people. [5] Delineating the identity of cellular and molecular mechanisms resulting in dysfunction of retinal vascular cells, and identification of protective mechanisms will provide the rationale for developing more specific and effective treatments.


  Diabetic retinopathy Top


Epidemiology of Diabetic Retinopathy

Diabetes mellitus is a serious worldwide health problem. In 2012, 371 million people were affected by diabetes and 4.7 million people died due to diabetes, including approximately 25.8 million patients in the United States. [6] Prevalence of diabetes increases the risk of serious diabetes complications. DR is one of the complications of diabetes and the main cause of blindness among working-age people. In the US, among patients with type 2 diabetes, estimated 40.3% of patients have DR and 8.2% of subjects have vision-threatening retinopathy. [7] For patients of type 1 diabetes, 86% and 42% of patients have retinopathy and vision-threatening stage of DR, respectively. [8] Studies based on 22,896 individuals with diabetes showed that 34.6% of patients have DR and the increasing risk was correlated with the duration of diabetes and inappropriate control of blood glucose and blood pressure. Vision-threatening stages of DR are proliferative DR and diabetic macular edema. Prevalence for proliferative DR was 6.96% and 6.81% for diabetic macular edema. [9] Impairment in vision due to DR remains a serious health issue worldwide. Tight blood glucose control and early detection and treatment of DR are effective to prevent vision loss and blindness caused by DR. [10]

Biochemical Changes in Diabetic Retinopathy

The pathogenesis of DR is very complex, because many factors contribute to the pathophysiology of DR Sustained hyperglycemia in retinal vasculature leads to accumulation of advanced glycation end-products (AGEs), inflammation, neuronal dysfunction and oxidative stress. [11] These biochemical changes under hyperglycemia are implicated in microvascular dysfunction leading to increased vascular permeability and capillary rarefaction. Consequently, these changes in retinal vasculature result in macular edema and neovascularization in the retina [Figure 1]. [12]

Excess glucose in the blood can have nonenzymatic chemical reactions with amino groups of proteins, lipids, and nucleic acids to produce AGEs. [13] This modification by AGEs causes vascular dysfunction in the diabetic retina. Accumulation of AGEs induces retinal pericyte apoptosis. In the early phase of apoptosis, caspase-10 mediates apoptosis of pericytes. [14] Angiotensin II also induces apoptosis of pericytes by AGEs through elevating. Receptor for AGEs (RAGE). [15] AGEs also increase vascular permeability in retinal endothelial cells. AGEs enhance translocation of protein kinase C-d (PKC-d) contributing to loss of tight junctions. [16] Thickening of basement membrane (BM) in capillaries is also observed in the diabetic retina. AGEs are also involved in the modification of BM. Accumulation of AGEs is associated with the expansion of BM and inhibition of the accumulation of AGE prevents BM thickening. [17] Furthermore, modified BM by AGEs prevents survival of retinal pericytes by regulating signaling pathway mediated by platelet-derived growth factor. [18] Thus, AGEs affect survival of pericytes by regulating signaling pathways and BM integrity of retinal blood vessels.
Figure 1: Pathophysiology of diabetic retinopathy. Hyperglycemia instigates biochemical changes leading to vascular dysfunction which increases vascular permeability followed by macular edema and retinal neovascularization.

Click here to view


Reactive oxygen species (ROS) is generated from mitochondrial electron transport chain, cytochrome p450s, the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase nitric oxide synthases (NOSs). ROS produced by cells is required to maintain normal cellular functions. However, excess production of ROS result in pathological conditions and oxidative stress. [19] Diabetes causes oxidative stress, and augmented level of oxidative stress contributes to the pathogenesis of diabetic vascular complications, including DR. [20] In the diabetic retina, ROS levels are elevated and related with vascular dysfunction, including loss of pericytes, formation of acellular capillaries, and vascular leakage and thickening of BM. [21],[22] Oxidative stress in the diabetic retina has causal links with metabolic dysfunction such as vascular inflammation, RAGE activation, activation of PKCs, and activation of nuclear factor kappaB (NF-kB). [23],[24],[25],[26] Photoreceptor cells have been recently shown to be an early source of oxidative stress and local inflammation in the diabetic retina. Diabetes increased generation of ROS by photoreceptor cells followed by induction of pro-inflammatory mediators, including inducible nitric oxide synthase (iNOS) and intercellular adhesion molecule 1 (ICAM-1). [27]

Inflammation is one of the major factors induced during diabetes leading to macular edema, ischemia and neovascularization. [10] Physiological changes in the diabetic retina are associated with inflammation, and anti-inflammatory therapy ameliorates development of DR in various animal models. [28] Several pro-inflammatory mediators are involved in the pathogenesis of DR. iNOS is an enzyme that catalyzes the reaction for nitric oxide (NO) generation. iNOS is up-regulated in the diabetic retina and plays a crucial role in the pathogenesis of vascular lesion during the early stage of DR. [29],[30] Inhibition of iNOS by aminoguanidine inhibited the progression of retinal dysfunction caused by DR in the rodent model of diabetes. [31]

Eicosanoids, mediators of inflammation which are also affected by diabetic conditions have two major families; prostaglandins and leukotriens. Cyclooxygenase-2 (cox-2) is an enzyme that catalyzes the synthesis of prostaglandins. Cox-2 expression is up-regulated in the diabetic retina and treatment with cox-2 inhibitor prevents damage in the diabetic retina. [32],[33] Production of leukotrienes by lipoxygenases (LOs), especially 5LO, by leukocytes promotes the retinal capillary degeneration in diabetic mice [34] and contributes to increased levels of pro-inflammatory cytokines. Tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) are increased in the retinas of diabetic animals. [35],[36] Vascular endothelial growth factor (VEGF) also has a pro-inflammatory role and is elevated in the diabetic retina. The increase in VEGF level promotes neovascularization and damage in the retina, [37] and its inhibition ameliorates vascular leakage in the diabetic retina. [38]

Antiinflammatory molecules are also affected by diabetic conditions and further contribute to the severity of DR. Pigment epithelium-derived factor (PEDF) is a member of the super family of serine protease inhibitors. It has neuroprotective, antiangiogenic, and antiinflammatory features. [39] PEDF levels are attenuated in the diabetic retina and associated with up-regulation of VEGF levels. [40] PEDF and VEGF act reciprocally in regulating retinal inflammation under diabetic conditions. Although a role for PEDF in modulation of Wnt signaling has been demonstrated, the detailed mechanisms involved need further elucidation. [41]

Thrombospondin-1 (TSP-1) was one of the first endogenous inhibitors of angiogenesis identified, which is expressed at significantly high levels in vitreous and aqueous humor of various species, including humans. [42] We also showed that diabetes resulted in a significant down-regulation of TSP-1 in vitreous and aqueous humor samples prepared from rats which were made diabetic by streptozotocin (STZ). We later showed this is the case in vitreous samples prepared from humans with diabetes. [43] However, the vitreous level of PEDF was not significantly affected in these patients. What was interesting is that we observed two isoforms of PEDF in vitreous samples prepared from humans with or without diabetes. The presence of higher molecular weight isoform was always associated with reduced levels of TSP-1, especially in diabetic patients. Furthermore, we recently showed that lack of TSP-1 and/or its selective down-regulation in the endothelium exacerbates the development and pathogenesis of DR. [44] These results, along with other published works from our laboratory and others regarding the antiangiogenic and antiinflammatory activities of TSP-1, support an important role for TSP-1 in the pathogenesis of DR.

The neuronal dysfunction in the retina is also an important effect of diabetes. Neuronal functional abnormalities of diabetic retina may precede the onset of vascular structural change in the retina. [45] Diabetes affects cells involved in neurosensory function of the retina through altered metabolisms and enhanced apoptosis. [5] Apoptosis in the neural retina was detected earlier than apoptosis in retinal vascular cells, and the rate of apoptosis in the neural retina was not affected by the duration of diabetes. [46] Neural apoptosis is associated with the breakdown of BRB and it is mediated by inflammation and oxidative stress. [47] Furthermore, extracellular glutamate is another factor that induces the apoptosis of neuronal cells. In the retina of rat, glutamate level was elevated after 2 months of diabetes. [48] At the early stages of DR, the ability of the Mόller cells to remove glutamate from the extracellular space is attenuated, and glutamate homeostasis is altered. The increased level of glutamate in diabetic retina has toxic effects on retinal neurons by inducing apoptosis. [49] Oxidative stress caused by diabetic conditions also increases glutamate level in the diabetic retina. [48] Reducing glutamate levels in the diabetic retina prevents neuronal cell death and degeneration. Neuronal cell death is closely related with retinal vascular cell death. The vascular defects appear in the same region as neuronal dysfunction in DR. [50] These results imply that there are intimate interactions between neuronal cells and vascular cells in the retina in order to maintain the integrity of BRB and proper tissue perfusion. These interactions need further elucidation, and retinal glial cells may be a key player here.

Besides nonenzymatic protein modifications producing AGEs, diabetic conditions affect posttranslational modifications of proteins which may play a pivotal role in the pathogenesis of DR. Uridine diphosphate (UDP)-N-acetylglucosamine (GlcNAc) is an end product of the hexosamine biosynthetic pathway. This UDP-GlcNAc is a donor of O-linked GlcNAc (O-GlcNAc) for protein modification. [51] High glucose conditions elevate the levels of O-GlcNAcylated proteins in retinal vascular cells, more specifically in pericytes. [52]

Diabetic conditions also increase histone acetylation related with production of pro-inflammatory proteins including iNOS, ICAM-1 and VEGF in the retina. [53] Moreover, high glucose conditions in the retina affect glycosylation of proteins. Synaptophysin is a presynaptic protein in synaptic vesicles and mediates release of neurotransmitters. Diabetic conditions increase mannose rich glycosylation of synaptophysin and enhances its degradation in the retina. [54] Thus, high glucose conditions modulate various posttranslational modifications of proteins leading to the pathogenesis of DR. However, the impact of diabetic conditions on posttranslational modifications of target proteins, and their identity in the retina needs further investigation.

Vascular Changes in Diabetic Retinopathy

Alterations in vascular structure under diabetic conditions results in the breakdown of BRB and diabetic edema. [12] The vascular unit of the retina is composed of endothelial cells, peri- and astro-cytes. Diabetes affects the integrity of BRB by altering the structure of neurovascular unit of the retina. In the diabetic retina, VEGF level is increased contributing to vascular leakage. [55] VEGF activates PKC-β which phosphorylates the tight junction protein, occludin. The phosphorylated occludin is then ubiquitinated and targeted for degradation leading to increased vascular permeability. [56]

Pericytes have a crucial role in maintaining vascular stability and early depletion of pericytes is a hallmark of DR causing the formation of pericyte ghosts. The pericyte ghosts mark the space having pocket shape in BMs and is formed after pericytes. [57] While degeneration of pericytes leads to pericyte ghosts, the number of endothelial cells in retinal vessels is not affected. Pericyte loss also increases proliferation of endothelial cells contributing to microaneurysm formation in retinal vessels. The loss of pericytes in diabetic retinal vasculature can be detected by the formation of acellular capillaries. Acellular capillaries are BM tubes without cell nuclei that have at least one-fourth of the normal capillary diameter. [58] The density of pericytes inversely correlates with vascular abnormalities of the retina. Reduction of pericyte numbers causes vascular regression leading to retinopathy. [59] Loss and degeneration of pericytes affect the integrity and maintenance of BRB. Consequently, pericyte dysfunction leads to capillary dilation, microaneurysms and increased vascular permeability resulting in vascular leakage and macular edema. [12]

In the diabetic retina, nonperfused and obliterated microvessels are observed in the early stages of retinopathy. Vessel closure promotes the proliferative retinal neovascularization and can be histologically observed as the prevalence of acellular capillaries increases. [60] Adhesion of leukocyte to the retinal blood vessel induces endothelial cell death, pericyte loss and vascular closure leading to ischemia [12] which is the primary stimulus for neovascularization. [61] Leukocyte adhesion to retinal vessels is mediated by ICAM-1 and vascular cellular adhesion molecule-1. [62] The activation of neovascularization is mediated by hypoxia-inducible factor 1-alpha (HIF-1α) which is a transcription factor and regulates the expression of various pro-angiogenic genes including VEGF, VEGF receptor-1, and angiopoietin-1 (Ang-1). [63] Aberrant activation of these pro-angiogenic factors under diabetic conditions results in pathological neovascularization. Thus, vascular changes in the diabetic retina promote the breakdown of BRB, macular edema, and proliferative neovascularization leading to retinal detachment and impairment of vision [Figure 2].
Figure 2: Vascular changes in diabetic retina. Pericytes control vessel stability and proliferation of endothelial cells. Pericyte loss contributes to breakdown of blood-retinal barrier (BRB) and damage to the endothelium, and formation of acellular capillaries. These vascular changes caused by pericyte loss lead to ischemia, proliferative vascularization of retina, and retinal detachment and loss of vision.

Click here to view


Epigenetic Changes in Diabetic Retinopathy

Epigenetic modifications have important roles in regulation of gene expression and contribute to the pathogenesis of various diseases. The epigenetic changes associated with the pathological complications of diabetes including DR have been investigated. [64] In human umbilical vein endothelial cell, high glucose conditions elevate the level of p300, a transcriptional activator with histone acetyltransferase activity. The increased p300 protein promotes its binding to the promoter of genes whose altered expression occurs during diabetes including endothelin-1, VEGF and fibronectin. Furthermore, histone acetylation and phosphorylation of histone H2AX are up-regulated under high glucose conditions. [65] In the retina of diabetic rats, expression level and activity of histone deacetylases are augmented, while the activities of histone acetyltransferases are decreased, contributing to the attenuation of acetylated histone H3 level. These changes in histone acetylation are sustained even after termination of hyperglycemia and contribute to the "metabolic memory." [66] Moreover, diabetic conditions are shown to regulate the expression of matrix metalloproteinase-9 (MMP-9) through epigenetic modifications. During diabetes, attenuated histone H3 dimethyl lysine (H3K9me2) and increased acetyl H3K9 (Ac-H3K9) are observed in the retina, which result in MMP-9 activation and mitochondrial damage. [67]

Diabetes also disrupts the expression pattern of microRNAs (miRNAs) contributing to the pathogenesis of DR. [68] In the retina of STZ-induced diabetic rats, expression of several miRNAs is altered. The expression of miR-200b was attenuated under high glucose conditions and was accompanied by increased VEGF levels. In addition, injection of miR-200b mimic prevents VEGF-mediated permeability and angiogenesis. [69] Furthermore, the level of miR-200b is up-regulated in retinas of Akita/+ mice, which spontaneously develop diabetes. miRNA-200b down-regulates oxidation resistance 1 gene expression leading to oxidative stress in human Mόller cell. [70] When the expression profile of miRNAs was investigated in the retina of normal and diabetic rats, it was found that NF-kB, VEGF and p53-responsive miRNAs were up-regulated. [71] HIF-1α and VEGF are regulated by common miRNAs. Silencing of HIF-1α transcript decreases expression of VEGF and vice versa. Over expression of common miRNAs attenuated VEGF and HIF-1α expression and inhibit vascular permeability in the retina of STZ-induced diabetic mice. [72] Thus, aberrant expression of miRNAs in diabetes has a significant impact on pathogenesis of DR and deserves further investigation.


  Blood-Retinal Barrier Top


The Cellular Organization of Retinal Vasculature

To understand the function of BRB in maintaining the integrity of retinal structure, elucidation of the retinal vascular cell structure and organization is required. The retina is a tissue for perception of light, and it converts light into electrochemical signals, which are transmitted to brain through photoreceptor and retinal neuronal circuits for vision. [5] The retina is located in the posterior part of the eye and neural tissue lined between the vitreous body and the choroid for systemic circulation. The retina has four major cellular components which are affected by diseases such as diabetes. Blood vessels are the first cellular component consisting of endothelial cells and pericytes. Pericytes are smooth muscle cell-like cells enveloping capillaries. Endothelial cells line the interior surface of blood vessels for BRB. Glial cells, the second cellular component of the retina consist of Mόller cells and astrocytes. Mόller cells support neuronal cells and modulate function of neurons. Mόller cells elongate from inner limiting membrane to the pigment epithelium. Astrocytes are associated with retinal blood vessels by wrapping them with their processes. Glial cells are located at the interface between the neurons and retinal blood vessels, and incorporate vascular and neuronal activity of the retina. They may be the key mediators of neurovascular dysfunction associated with various pathological conditions of the central nervous system including DR. The third cellular component of the retina is the neurons. Neurons are involved in photo-transduction and convey electrochemical impulses to the brain for sensory functions. Retinal neurons consist of five primary cell types: Photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells. Microglia is the fourth cellular component of the retina whose. The microglial cells are involved in modulation of immune function in retina to maintain retinal homeostasis. They respond to stress and injury by release of cytokines, and phagocytosis and clearance of the dead cells. [5],[73]

The inner BRB is formed by retinal endothelial cells associating with astrocytes and pericytes. Anatomically, endothelial cells are surrounded by basal lamina, which astrocytes and pericytes rest on. Pericytes envelop the outer side of the capillary wall and interact with endothelial cells through peg-and-socket contacts, which contain cell junction proteins. The end-foot of astrocytes encircles capillaries and basal lamina is located between the astrocytes and pericytes. Pericytes and astrocytes provide vascular integrity by interacting with endothelial cells. [74] Dysfunction in interactions among vascular cells can cause the breakdown of BRB resulting in retinal disease. An area of significant interest in our laboratory has been how hyperglycemia impacts the function of various cellular components of the retinal neurovascular system.

Role of Endothelial Cells in Retinal Vasculature

In the inner BRB, retinal endothelial cells are the main components, which form the physical barriers between vascular lumen and the retina. [75] Transport of metabolites and nutrients between blood and the retina is selectively regulated by the physical barrier formed by endothelial cells to maintain homeostasis in the retina. [76] This physical barrier also regulates transport of ions and fluids providing an optimal condition, which is crucial for visual function. There are two routes for the selective transport in the retina: The paracellular aqueous pathway regulated by junctions between endothelial cells, and the transcellular pathway mediated by specialized vesicles including caveolae or transport proteins. [4] The junction between endothelial cells for the paracellular pathway consists of tight junctions, adherens junctions and gap junctions. These junctions at attachment sites between endothelial cells also regulate signaling for maintaining cell position, inhibit the growth by contact inhibition, and protect cells from apoptosis. [77]

Endothelial cells express cell-type specific transmembrane proteins for adhesion including vascular endothelial cadherin (VE-cadherin) at adherens junctions and claudins at tight junctions. [78],[79] Transmembrane proteins at tight junctions include occludin, claudins and junctional adhesion molecules. [80] Occludin associates with zonula occludens (ZO)-1, ZO-2 and ZO-3, which directly bind to filamentous actin (F-actin). [81] Tight junctions act as a gate for paracellular transport, and a fence for maintaining cell polarity. [82] At adherens junctions, VE-cadherin is a main component associating with intracellular proteins including β-catenin, p120, and plakoglobin. Regulation of VE-cadherin expression and/or phosphorylation can affect the overall endothelial cell junctions and vascular stability through modulation of intracellular signaling pathways. [83]

At gap junctions, a hemi-channel on each of two neighboring endothelial cells makes contact. The hemi-channels consist of six connexins. Small molecules (<1000 dalton [Da]) pass freely through gap junctions. Gap junctions are involved in electrical or chemical communications. [4] Gap junctions are also involved in the regulation of barrier function by modulating expression and localization of tight junction proteins such as occludin, claudin-5 and ZO-1. [84] How hyperglycemia impacts the function of these proteins in endothelial cells are under intense investigation.

Role of Pericytes in Retinal Vasculature

Pericytes play an essential role in maintaining BRB. Pericytes or mural cells envelope capillary walls and share basal lamina with endothelial cells. Pericytes have an elongated stellate shape and extend finger-like processes to cover the capillary wall formed by endothelial cells. [85] They directly interact with endothelial cells through N-cadherin and connexin-43 hemi-channels. [86],[87] The ratio of pericyte-to-endothelial in the retina which is approximately 1:1, is higher than in any other organ (1:3 in the brain and 1:10 in lung). [88] This higher coverage by pericyte in retinal microvessels may imply the crucial role of pericytes in the retina in maintaining BRB structure and vascular integrity. Determining the pivotal role of pericytes in the regulation of retinal vascular functions has been the topic of numerous studies.

In pericyte-deficient mice, permeability of blood barrier was increased by up-regulation of endothelial transcytosis and abnormal polarization of astrocyte end-feet surrounding blood vessels. [75] Smooth muscle α-actin (SMA) is expressed in smooth muscle cells and pericytes when activated, and related with contractile properties of these cells. In SMA null mice generated by global gene targeting, retinal structure, vascular pattern and covering of vessels by mural cells were normal. However, permeability in retinal vessels was significantly increased in SMA null mice. [89] These results suggest that pericytes regulate the maintenance of blood barrier and structural integrity of blood vessels in the retina. Furthermore, pericytes regulate vessel stability by affecting survival of endothelial cells. VEGF produced by pericytes regulates the survival of endothelial cells. [90] Pericytes also promote the apoptosis of endothelial cells in selective vessel patterning by expressing endosialin, a type I transmembrane glycoprotein. [91] Transforming growth factor-β1 (TGF-β1) signaling between endothelial cells and pericytes are also involved in the apoptosis of retinal endothelial cells. Systemic inhibition of TGF-β signaling results in abnormalities in retinal vascular structure including impaired perfusion of the superficial vascular plexus and vascular leakage through enhanced apoptosis of endothelial cells. [92] Pericytes also have a significant role in the pathogenesis of diabetes complications including DR. In the diabetic retina, pericyte loss is one of the early hallmarks of DR and contributes to BRB disruption. Apoptosis of pericytes during diabetes leads to depletion of pericytes from retinal vasculature, formation of microaneurysms and acellular capillaries. [93] Thus, mechanisms which protect pericytes from cytotoxic effects of high glucose may be beneficial and important for development of new therapy.

Role of Astrocytes in Retinal Vasculature

Astrocytes originate from the optic nerve head and migrate into the developing retina laying down the scaffolding for retinal vascularization. [94] Astrocytes are closely associated with the developing retinal vasculature with significant impact on retinal vessels function. [95] Pathological conditions such as diabetes and ischemia cause degeneration of astrocytes contributing to dysfunction of retinal vasculature. [96] Astrocytes are known for their interactions with blood vessels of central nervous system and help to establish and maintain barrier properties of the vasculature. [97] Retinal astrocytes help to enhance barrier function by releasing soluble proteins. Retinal astrocytes promote the integrity of blood barrier by increasing tight junction protein expression in retinal endothelial cells. [98] A-kinase anchor protein 12 (AKAP12) is derived by astrocytes. AKAP reduces VEGF level and increases TSP-1 levels leading to enhanced blood barrier through the inhibition of PKC-ζ phosphorylation and Rho kinase activity. [99]

Astrocytes also regulate the expression of glial-derived neurotrophic factor and VEGF in a reciprocal manner through stimulation of retinoic acid receptor α to antagonize loss of tight junction for maintaining vascular integrity under hyperglycemic conditions. [100] In addition, astrocytes secrete sonic hedgehog to activate Hedgehog signaling pathway in endothelial cells, consequently decreasing permeability and pro-inflammatory mediators in endothelial cells. [101] Not only secretion of proteins by astrocytes, but also physical interactions between astrocytes and vascular cells in the retina is important for the integrity of barrier structure. Astrocytes provide a scaffolding for endothelial cells organization into capillaries, and enhance the tight junction between endothelial cells. [102] Dysfunctions of retinal astrocytes in communication with endothelial cell and physical association with blood vessels contribute to BRB breakdown leading to pathological states. [74]


  Physiological changes in retinal vascular cells under hyperglycemia Top


0Physiological Changes in Retinal Endothelial Cells

In hyperglycemic conditions, endothelial cells of retinal blood vessels are directly exposed to a microenvironment with high concentration of glucose. Therefore, effects of high glucose conditions on the physiology of retinal endothelial cells have been intensively studied. High glucose conditions cause loss of retinal endothelial cells or structural damage. [103] The cellular components affected by high glucose conditions are mitochondria. High concentration of glucose increased membrane potential and level of ROS in mitochondria of bovine retinal endothelial cells. Increase in ROS level and membrane potential was correlated with the rate of cell death. [104] In rat retinal endothelial cells, high glucose conditions increased mitochondrial fragmentation with concomitant increases in cytochrome c release leading to apoptosis. [105] Type 2A protein phosphatase (PP2A) is also involved in the apoptosis of retinal endothelial cells induced by hyperglycemic conditions which increase PP2A activity through up-regulation of methylation followed by the activation of apoptotic proteins, superoxide, and NF-kB. [106] However, apoptosis of retinal endothelial cells under high glucose is independent of caspase activity. [107]

High glucose conditions also affect junctional properties of retinal endothelial cells. Connexin 43 is one of the protein components of hemi-channels which allow neighboring cells to contact. Hyperglycemia reduces the expression of connexin 43 and decreases cell-cell communication triggering apoptosis. [108] Moreover, in human retinal endothelial cells, high glucose conditions disrupted tight junctions by decreasing occludin expression through the activation of VEGF and insulin-like growth factor-I receptors. [109] Hyperglycemic conditions also affect tight junctions by decreasing ZO-1 levels in the retina of diabetic mice. [110] Consequently, such conditions disturb junctional integrity leading to increased retinal endothelial cell permeability. [111] However, the majority of in vitro studies indicate the lack of apoptosis in microvascular endothelial cells exposed to high glucose, especially those from the retina. [112] However, this may not be the case in vivo. The loss of endothelial cells in vivo may be the result of loss of pericytes leading to vascular dysfunction and formation of acellular capillaries.

Cell adhesive and migratory activities of retinal endothelial cells are critical for angiogenesis. High glucose conditions enhanced the migration of mouse and human retinal endothelial cells which this enhanced migration was mediated through activation of Src, PI3K/Akt1/eNOS and ERKs pathways [113],[114] as well as through the up-regulation of heparanase. The activation of Akt and ERK phosphorylation are also involved in the up-regulation of heparanase. [115] Increased migration under high glucose conditions contributes to increased angiogenic activity of retinal endothelial cells and is consistent with down-regulation of TSP-1 in microvascular endothelial cells under hyperglycemia and enhanced migration of TSP-1-deficient retinal endothelial cells. [42],[112],[116]

Physiological Changes in Retinal Pericytes

Depletion of pericytes is a hallmark of DR and the underlying mechanisms involved remain the subject of numerous investigations. Various mechanisms have been attributed to the progress of pericyte apoptosis under diabetic conditions. NF-kB is activated under high concentration of glucose leading to expression of proapoptotic proteins in retinal pericytes. [117] Under high glucose conditions, aldose reductase catalyzes synthesis of sugar polyol which accumulates resulting in the apoptosis of retinal pericytes. [118] PKC-d and Src homology-2 domain-containing phosphatase-1 (SHP-1) are also activated by high glucose conditions leading to pericyte apoptosis in an NF-kB independent signaling pathway. [119] Furthermore, hyperglycemic conditions cause endoplasmic reticulum (ER) stress and oxidative stress contributing to apoptosis of pericytes. [120]

Up-regulation of oxidative stress in pericytes under hyperglycemia is regulated by thioredoxin interacting protein. [121] Apoptosis of pericytes is also mediated by pericytes-reactive antibodies. High glucose conditions render pericytes more susceptible to antibody-mediated attack leading to damage in pericytes and the damaged pericytes exhibit reduced activity in T-cell inhibition. [122] In addition, morphology and function of mitochondria in pericytes are affected by high glucose conditions which disrupted mitochondrial network and enhanced its fragmentation. Membrane potential is elevated, and oxygen consumption is attenuated in mitochondria under high glucose conditions. Changes in mitochondria morphology and metabolism causes apoptosis of retinal pericytes. [123]

High glucose conditions induce not only apoptosis, but also dysfunction of retinal pericytes. In bovine retinal pericytes, actin fibers were disassembled and lost under high glucose conditions. [124] In diabetic mice with high blood glucose levels, pericytes in retinal vessels exhibit reduced adherence to endothelium. Ang-2 is involved in this reduced adherence of pericytes to endothelium. [125] Pericytes also protect retinal endothelial cells from inflammation-induced apoptosis by inhibiting proliferation of activated T-cell. Which this inhibition is attenuated under high glucose conditions. [126] Thus, hyperglycemia impair survival and function of retinal pericytes leading to diabetic complications in the retina.

Our recent work shows that increased production of inflammatory mediators including IL-1β and TNF-α results in sustained activity of signal transducer and activator of transcription 1 (STAT1) and increased expression of proapoptotic Bcl-2 family member, Bim. In addition, Bim expression was responsible for increased oxidative stress and loss of pericytes, since retinal pericytes prepared from Bim deficient mice were resistant to high glucose mediated oxidative stress and apoptosis. In addition, although the use of antioxidant N-acetylcysteine was protective, it did not affect Bim expression, thus suggesting that increased Bim expression by high glucose is up-stream of oxidative stress. [127] We showed that high glucose results in increased production of inflammatory mediators and activation of STAT1 transcription factor which drives Bim expression and death of pericytes.

Physiological Changes in Retinal Astrocytes

Astrocytes have a crucial role in the structural integrity of BRB and high glucose conditions affect both BRB structures and astrocytes in the retinal vasculature. Diabetes contribute to the reduction of astrocytes in peripapillary and far peripheral retina and inhibit astrocytes from associating with superficial large vessels. [58],[128] In addition to morphological changes in astrocytes, high concentration of glucose also affect cellular physiology of astrocytes. In the retina of diabetic mice, expression of glial fibrillary acidic protein, a marker of glial activation, is increased transiently in astrocytes, but not in Mόller cells. [129] Aquaporins (AQP) are water channel proteins that regulate the water-electrolyte balance in the retina. High glucose conditions impact the distribution of AQP1 and AQP4 in retinal astrocytes. In normal retina, AQP4 is mainly expressed around retinal blood vessels, whereas under diabetic conditions, the mainly expressed protein is AQP1. [130]

Diabetes also affects gap junctions in astrocytes. Connexins are important proteins in the formation of gap junctions and cell-cell communication. Connexin-26 and -43 protein levels were attenuated in the astrocytes of diabetic rats. Decrease in connexin expression was followed by significant loss of astrocytes. [131] Furthermore, high glucose conditions activate defense mechanisms against ER stress in astrocytes. Cultured astrocytes under high glucose and retinal astrocytes from diabetic rat are resistant to ER stress under hyperglycemic conditions. [132]

Our recent studies show that high level of glucose do not induce the apoptosis of astrocytes, but increase their proliferation and adhesion due to alteration in intracellular signaling pathways involved in cell survival, migration, and proliferation. High glucose conditions also resulted in production of inflammatory mediators in astrocytes. Under high glucose conditions, astrocytes migration was attenuated, which corresponded with their inability to undergo morphogenesis in Matrigel. In addition, the conditioned medium from astrocytes cultured under hyperglycemia was sufficient to block capillary morphogenesis of retinal endothelial cells. These activities were associated with increased oxidative stress in astrocytes culture under high glucose conditions and were reversed in the presence of the antioxidant N-acetylcysteine. The increased oxidative stress in astrocytes was concomitant with increased nuclear localization of NRF2, the oxidative stress activated transcription factor, and up-regulation of oxidative protective genes heme oxygenase-1 and periredoxin-2. These results are consistent with a protective role recently demonstrated for NRF2, mainly activated in the glial cells, in protection from DR. [133]


  Concluding remarks and future perspective Top


High glucose conditions affects morphology and physiology of retinal vascular cells including endothelial cells, pericytes, and astrocytes leading to dysfunction of retinal vasculature. The key factors in this dysfunction are inflammation and increased oxidative stress. However, these alterations have specific impacts on various retinal vascular cells. Although high glucose induces apoptosis of retinal pericytes, it had minimal impact on the apoptosis of retinal astrocytes or endothelial cells. Additionally, the studies on pericytes response to high glucose conditions indicated that inflammation is the primary cause of their demise since up-regulation of Bim by inflammatory mediators is responsible for increased oxidative stress and loss of pericytes. We have also observed that although pericytes are the major source of inflammatory mediators, retinal astrocytes and endothelial cells may respond to high glucose with increased production of inflammatory mediators, as well. However, the reason for selective up-regulation of Bim in pericytes and their loss in response to high glucose still needs further investigation. Our recent studies of proteasome activity and O-GlcNAC modification in these cells [134],[135] suggest that inefficient proteasome activity and/or enhanced accumulation of O-GlcNAC modified proteins may be responsible for selective sensitivity of pericytes to increased concentrations of glucose. These possibilities are the subject of current investigation and will provide new insight into the mechanisms leading to selective sensitivity of pericytes to hyperglycemia. Furthermore, our studies support the critical role for astroglial cells, as an intermediary between neuroretina and retinal circulation. We believe astroglial cells are the essential regulator of retinal neurovascular system and their role in the pathogenesis of central nervous system diseases including DR deserves further attention and careful evaluation.


  Acknowledgments Top


This work was supported by grants R01 EY016995, RC4 EY021357, R24 EY022883, R21 EY023024, P30 EY016665 and P30 CA014520 UW Paul P. Carbone Cancer Center Support Grant from the National Institutes of Health and an unrestricted departmental award from Research to Prevent Blindness. NS is a recipient of a Research Award from American Diabetes Association, 1-10-BS-160 and Retina Research Foundation. ESS is supported by a UW-Madison UW-Milwaukee Inter Campus grant and predoctoral fellowship from American Heart Association, 12PRE12030099.

 
  References Top

1.Runkle EA, Antonetti DA. The blood-retinal barrier: Structure and functional significance. Methods Mol Biol 2011;686:133-148.  Back to cited text no. 1
    
2.Kaur C, Foulds WS, Ling EA. Blood-retinal barrier in hypoxic ischaemic conditions: Basic concepts, clinical features and management. Prog Retin Eye Res 2008;27:622-647.  Back to cited text no. 2
    
3.Hosoya K, Tachikawa M. Inner blood-retinal barrier transporters: Role of retinal drug delivery. Pharm Res 2009;26:2055-2065.  Back to cited text no. 3
    
4.Klaassen I, Van Noorden CJ, Schlingemann RO. Molecular basis of the inner blood-retinal barrier and its breakdown in diabetic macular edema and other pathological conditions. Prog Retin Eye Res 2013;34:19-48.  Back to cited text no. 4
    
5.Antonetti DA, Barber AJ, Bronson SK, Freeman WM, Gardner TW, Jefferson LS, et al. Diabetic retinopathy: Seeing beyond glucose-induced microvascular disease. Diabetes 2006;55:2401-2411.  Back to cited text no. 5
    
6.Guariguata L. By the numbers: new estimates from the IDF Diabetes Atlas Update for 2012. Diabetes Res Clin Pract. 2012;98:524-525.  Back to cited text no. 6
[PUBMED]    
7.Kempen JH, O'Colmain BJ, Leske MC, Haffner SM, Klein R, Moss SE, et al. The prevalence of diabetic retinopathy among adults in the United States. Arch Ophthalmol 2004;122:552-563.  Back to cited text no. 7
    
8.Roy MS, Klein R, O'Colmain BJ, Klein BE, Moss SE, Kempen JH. The prevalence of diabetic retinopathy among adult type 1 diabetic persons in the United States. Arch Ophthalmol 2004;122:546-551.  Back to cited text no. 8
    
9.Yau JW, Rogers SL, Kawasaki R, Lamoureux EL, Kowalski JW, Bek T, et al. Global prevalence and major risk factors of diabetic retinopathy. Diabetes Care 2012;35:556-564.  Back to cited text no. 9
[PUBMED]    
10.Bandello F, Lattanzio R, Zucchiatti I, Del Turco C. Pathophysiology and treatment of diabetic retinopathy. Acta Diabetol 2013;50:1-20.  Back to cited text no. 10
    
11.Bhagat N, Grigorian RA, Tutela A, Zarbin MA. Diabetic macular edema: pathogenesis and treatment. Surv Ophthalmol 2009;54:1-32.  Back to cited text no. 11
    
12.Bandello F, Lattanzio R, Zucchiatti I, Del Turco C. Pathophysiology and treatment of diabetic retinopathy. Acta Diabetol 2013;50:1-20.  Back to cited text no. 12
    
13.Singh R, Barden A, Mori T, Beilin L. Advanced glycation end-products: A review. Diabetologia 2001;44:129-146.  Back to cited text no. 13
    
14.Lecomte M, Denis U, Ruggiero D, Lagarde M, Wiernsperger N. Involvement of caspase-10 in advanced glycation end-product-induced apoptosis of bovine retinal pericytes in culture. Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease. 2004;1689(3):202-11.  Back to cited text no. 14
    
15.Yamagishi S, Takeuchi M, Matsui T, Nakamura K, Imaizumi T, Inoue H. Angiotensin II augments advanced glycation end product-induced pericyte apoptosis through RAGE overexpression. FEBS Lett 2005;579:4265-4270.  Back to cited text no. 15
    
16.Kim JH, Kim JH, Jun HO, Yu YS, Kim KW. Inhibition of protein kinase C delta attenuates blood-retinal barrier breakdown in diabetic retinopathy. Am J Pathol 2010;176:1517-1524.  Back to cited text no. 16
[PUBMED]    
17.Gardiner TA, Anderson HR, Stitt AW. Inhibition of advanced glycation end-products protects against retinal capillary basement membrane expansion during long-term diabetes. J Pathol 2003;201:328-333.  Back to cited text no. 17
    
18.Stitt AW, Hughes SJ, Canning P, Lynch O, Cox O, Frizzell N, et al. Substrates modified by advanced glycation end-products cause dysfunction and death in retinal pericytes by reducing survival signals mediated by platelet-derived growth factor. Diabetologia 2004;47:1735-1746.  Back to cited text no. 18
    
19.Kowluru RA, Chan PS. Oxidative stress and diabetic retinopathy. Exp Diabetes Res 2007;2007:43603.  Back to cited text no. 19
    
20.Baynes JW. Role of oxidative stress in development of complications in diabetes. Diabetes 1991;40:405-412.  Back to cited text no. 20
[PUBMED]    
21.Li J, Wang JJ, Yu Q, Chen K, Mahadev K, Zhang SX. Inhibition of reactive oxygen species by Lovastatin downregulates vascular endothelial growth factor expression and ameliorates blood-retinal barrier breakdown in db/db mice: Role of NADPH oxidase 4. Diabetes 2010;59:1528-1538.  Back to cited text no. 21
    
22.Zheng Z, Chen H, Ke G, Fan Y, Zou H, Sun X, et al. Protective effect of perindopril on diabetic retinopathy is associated with decreased vascular endothelial growth factor-to-pigment epithelium-derived factor ratio: involvement of a mitochondria-reactive oxygen species pathway. Diabetes 2009;58:954-964.  Back to cited text no. 22
    
23.Al-Shabrawey M, Rojas M, Sanders T, Behzadian A, El-Remessy A, Bartoli M, et al. Role of NADPH oxidase in retinal vascular inflammation. Invest Ophthalmol Vis Sci 2008;49:3239-3244.  Back to cited text no. 23
    
24.Warboys CM, Toh HB, Fraser PA. Role of NADPH oxidase in retinal microvascular permeability increase by RAGE activation. Invest Ophthalmol Vis Sci 2009;50:1319-1328.  Back to cited text no. 24
    
25.Pricci F, Leto G, Amadio L, Iacobini C, Cordone S, Catalano S, et al. Oxidative stress in diabetes-induced endothelial dysfunction involvement of nitric oxide and protein kinase C. Free Radic Biol Med 2003;35:683-694.  Back to cited text no. 25
    
26.Zheng Z, Chen H, Li J, Li T, Zheng B, Zheng Y, et al. Sirtuin 1-mediated cellular metabolic memory of high glucose via the LKB1/AMPK/ROS pathway and therapeutic effects of metformin. Diabetes 2012;61:217-228.  Back to cited text no. 26
    
27.Du Y, Veenstra A, Palczewski K, Kern TS. Photoreceptor cells are major contributors to diabetes-induced oxidative stress and local inflammation in the retina. Proc Natl Acad Sci USA 2013;110:16586-16591.  Back to cited text no. 27
    
28.Tang J, Kern TS. Inflammation in diabetic retinopathy. Prog Retin Eye Res 2011;30:343-358.  Back to cited text no. 28
    
29.Zheng L, Du Y, Miller C, Gubitosi-Klug RA, Ball S, Berkowitz BA, et al. Critical role of inducible nitric oxide synthase in degeneration of retinal capillaries in mice with streptozotocin-induced diabetes. Diabetologia 2007;50:1987-1996.  Back to cited text no. 29
    
30.Mishra A, Newman EA. Inhibition of inducible nitric oxide synthase reverses the loss of functional hyperemia in diabetic retinopathy. Glia 2010;58:1996-2004.  Back to cited text no. 30
    
31.Mishra A, Newman EA. Aminoguanidine reverses the loss of functional hyperemia in a rat model of diabetic retinopathy. Front Neuroenergetics 2011;3:10.  Back to cited text no. 31
    
32.Perrone L, Devi TS, Hosoya KI, Terasaki T, Singh LP. Inhibition of TXNIP expression in vivo blocks early pathologies of diabetic retinopathy. Cell Death Dis 2010;1:e65.  Back to cited text no. 32
    
33.Ayalasomayajula SP, Kompella UB. Subconjunctivally administered celecoxib-PLGA microparticles sustain retinal drug levels and alleviate diabetes-induced oxidative stress in a rat model. Eur J Pharmacol 2005;511:191-198.  Back to cited text no. 33
    
34.Talahalli R, Zarini S, Tang J, Li G, Murphy R, Kern TS, et al. Leukocytes regulate retinal capillary degeneration in the diabetic mouse via generation of leukotrienes. J Leukoc Biol 2013;93:135-143.  Back to cited text no. 34
    
35.Krady JK, Basu A, Allen CM, Xu Y, LaNoue KF, Gardner TW, et al. Minocycline reduces proinflammatory cytokine expression, microglial activation, and caspase-3 activation in a rodent model of diabetic retinopathy. Diabetes 2005;54:1559-1565.  Back to cited text no. 35
    
36.Li J, Wang JJ, Yu Q, Wang M, Zhang SX. Endoplasmic reticulum stress is implicated in retinal inflammation and diabetic retinopathy. FEBS Lett 2009;583:1521-1527.  Back to cited text no. 36
    
37.Yang W, Yu X, Zhang Q, Lu Q, Wang J, Cui W, et al. Attenuation of streptozotocin-induced diabetic retinopathy with low molecular weight fucoidan via inhibition of vascular endothelial growth factor. Exp Eye Res 2013;115:96-105.  Back to cited text no. 37
    
38.Hu Y, Chen Y, Ding L, He X, Takahashi Y, Gao Y, et al. Pathogenic role of diabetes-induced PPAR-a down-regulation in microvascular dysfunction. Proc Natl Acad Sci USA 2013;110:15401-15406.  Back to cited text no. 38
    
39.Liu X, Chen HH, Zhang LW. Potential therapeutic effects of pigment epithelium-derived factor for treatment of diabetic retinopathy. Int J Ophthalmol 2013;6:221-227.  Back to cited text no. 39
    
40.Yoshida Y, Yamagishi S, Matsui T, Jinnouchi Y, Fukami K, Imaizumi T, et al. Protective role of pigment epithelium-derived factor (PEDF) in early phase of experimental diabetic retinopathy. Diabetes Metab Res Rev 2009;25:678-686.  Back to cited text no. 40
    
41.Park K, Lee K, Zhang B, Zhou T, He X, Gao G, et al. Identification of a Novel Inhibitor of the Canonical Wnt Pathway. Mol Cell Biol. 2011 July 15, 2011;31(14):3038-51.  Back to cited text no. 41
    
42.Sheibani N, Sorenson CM, Cornelius LA, Frazier WA. Thrombospondin-1, a natural inhibitor of angiogenesis, is present in vitreous and aqueous humor and is modulated by hyperglycemia. Biochem Biophys Res Commun 2000;267:257-261.  Back to cited text no. 42
    
43.Wang S, Gottlieb JL, Sorenson CM, Sheibani N. Modulation of thrombospondin 1 and pigment epithelium-derived factor levels in vitreous fluid of patients with diabetes. Arch Ophthalmol 2009;127:507-513.  Back to cited text no. 43
    
44.Sorenson CM, Wang S, Gendron R, Paradis H, Sheibani N. Thrombospondin-1 deficiency exacerbates the pathogenesis of diabetic retinopathy. J Diabetes Metab 2013;Suppl 12:005. doi:10.4172/2155-6156.S12-005  Back to cited text no. 44
    
45.Han Y, Bearse MA Jr, Schneck ME, Barez S, Jacobsen CH, Adams AJ. Multifocal electroretinogram delays predict sites of subsequent diabetic retinopathy. Invest Ophthalmol Vis Sci 2004;45:948-954.  Back to cited text no. 45
    
46.Barber AJ, Gardner TW, Abcouwer SF. The significance of vascular and neural apoptosis to the pathology of diabetic retinopathy. Invest Ophthalmol Vis Sci 2011;52:1156-1163.  Back to cited text no. 46
    
47.El-Remessy AB, Al-Shabrawey M, Khalifa Y, Tsai NT, Caldwell RB, Liou GI. Neuroprotective and blood-retinal barrier-preserving effects of cannabidiol in experimental diabetes. Am J Pathol 2006;168:235-244.  Back to cited text no. 47
    
48.Kowluru RA, Engerman RL, Case GL, Kern TS. Retinal glutamate in diabetes and effect of antioxidants. Neurochem Int 2001;38:385-390.  Back to cited text no. 48
    
49.Davalli AM, Perego C, Folli FB. The potential role of glutamate in the current diabetes epidemic. Acta Diabetol 2012;49:167-183.  Back to cited text no. 49
    
50.Ng JS, Bearse MA Jr, Schneck ME, Barez S, Adams AJ. Local diabetic retinopathy prediction by multifocal ERG delays over 3 years. Invest Ophthalmol Vis Sci 2008;49:1622-1628.  Back to cited text no. 50
    
51.Ngoh GA, Facundo HT, Zafir A, Jones SP. O-GlcNAc signaling in the cardiovascular system. Circ Res 2010;107:171-185.  Back to cited text no. 51
    
52.Gurel Z, Sieg KM, Shallow KD, Sorenson CM, Sheibani N. Retinal O-linked N-acetylglucosamine protein modifications: Implications for postnatal retinal vascularization and the pathogenesis of diabetic retinopathy. Mol Vis 2013;19:1047-1059.  Back to cited text no. 52
    
53.Kadiyala CS, Zheng L, Du Y, Yohannes E, Kao HY, Miyagi M, et al. Acetylation of retinal histones in diabetes increases inflammatory proteins: Effects of minocycline and manipulation of histone acetyltransferase (HAT) and histone deacetylase (HDAC). J Biol Chem 2012;287:25869-25880.  Back to cited text no. 53
    
54.D'Cruz TS, Weibley BN, Kimball SR, Barber AJ. Post-translational processing of synaptophysin in the rat retina is disrupted by diabetes. PLoS One 2012;7:e44711.  Back to cited text no. 54
    
55.Kim YH, Kim YS, Roh GS, Choi WS, Cho GJ. Resveratrol blocks diabetes-induced early vascular lesions and vascular endothelial growth factor induction in mouse retinas. Acta Ophthalmol 2012;90:e31-e37.  Back to cited text no. 55
    
56.Murakami T, Frey T, Lin C, Antonetti DA. Protein kinase cß phosphorylates occludin regulating tight junction trafficking in vascular endothelial growth factor-induced permeability in vivo. Diabetes 2012;61:1573-1583.  Back to cited text no. 56
    
57.Mizutani M, Kern TS, Lorenzi M. Accelerated death of retinal microvascular cells in human and experimental diabetic retinopathy. J Clin Invest 1996;97:2883-2890.  Back to cited text no. 57
    
58.Barber AJ, Antonetti DA, Kern TS, Reiter CE, Soans RS, Krady JK, et al. The Ins2Akita mouse as a model of early retinal complications in diabetes. Invest Ophthalmol Vis Sci 2005;46:2210-2218.  Back to cited text no. 58
    
59.Enge M, Bjarnegård M, Gerhardt H, Gustafsson E, Kalén M, Asker N, et al. Endothelium-specific platelet-derived growth factor-B ablation mimics diabetic retinopathy. EMBO J 2002;21:4307-4316.  Back to cited text no. 59
    
60.Engerman RL. Pathogenesis of diabetic retinopathy. Diabetes 1989;38:1203-1206.  Back to cited text no. 60
[PUBMED]    
61.Miyamoto K, Khosrof S, Bursell SE, Rohan R, Murata T, Clermont AC, et al. Prevention of leukostasis and vascular leakage in streptozotocin-induced diabetic retinopathy via intercellular adhesion molecule-1 inhibition. Proc Natl Acad Sci USA 1999;96:10836-10841.  Back to cited text no. 61
    
62.Bai N, Tang S, Ma J, Luo Y, Lin S. Increased expression of intercellular adhesion molecule-1, vascular cellular adhesion molecule-1 and leukocyte common antigen in diabetic rat retina. Yan Ke Xue Bao 2003;19:176-183.  Back to cited text no. 62
    
63.Costa PZ, Soares R. Neovascularization in diabetes and its complications. Unraveling the angiogenic paradox. Life Sci 2013;92:1037-1045.  Back to cited text no. 63
    
64.Villeneuve LM, Natarajan R. The role of epigenetics in the pathology of diabetic complications. Am J Physiol Renal Physiol 2010;299:F14-F25.  Back to cited text no. 64
    
65.Chen S, Feng B, George B, Chakrabarti R, Chen M, Chakrabarti S. Transcriptional coactivator p300 regulates glucose-induced gene expression in endothelial cells. Am J Physiol Endocrinol Metab 2010;298:E127-E137.  Back to cited text no. 65
    
66.Zhong Q, Kowluru RA. Role of histone acetylation in the development of diabetic retinopathy and the metabolic memory phenomenon. J Cell Biochem 2010;110:1306-1313.  Back to cited text no. 66
    
67.Zhong Q, Kowluru RA. Regulation of matrix metalloproteinase-9 by epigenetic modifications and the development of diabetic retinopathy. Diabetes 2013;62:2559-2568.  Back to cited text no. 67
    
68.He S, Li X, Chan N, Hinton DR. Review: Epigenetic mechanisms in ocular disease. Mol Vis 2013;19:665-674.  Back to cited text no. 68
    
69.McArthur K, Feng B, Wu Y, Chen S, Chakrabarti S. MicroRNA-200b regulates vascular endothelial growth factor-mediated alterations in diabetic retinopathy. Diabetes 2011;60:1314-1323.  Back to cited text no. 69
    
70.Murray AR, Chen Q, Takahashi Y, Zhou KK, Park K, Ma JX. MicroRNA-200b downregulates oxidation resistance 1 (Oxr1) expression in the retina of type 1 diabetes model. Invest Ophthalmol Vis Sci 2013;54:1689-1697.  Back to cited text no. 70
    
71.Kovacs B, Lumayag S, Cowan C, Xu S. MicroRNAs in early diabetic retinopathy in streptozotocin-induced diabetic rats. Invest Ophthalmol Vis Sci 2011;52:4402-4409.  Back to cited text no. 71
    
72.Ling S, Birnbaum Y, Nanhwan MK, Thomas B, Bajaj M, Ye Y. MicroRNA-dependent cross-talk between VEGF and HIF1a in the diabetic retina. Cell Signal 2013;25:2840-2847.  Back to cited text no. 72
    
73.Gardner TW, Antonetti DA, Barber AJ, LaNoue KF, Levison SW. Diabetic retinopathy: More than meets the eye. Surv Ophthalmol 2002;47 Suppl 2:S253-S262.  Back to cited text no. 73
    
74.Abbott NJ, Rönnbäck L, Hansson E. Astrocyte-endothelial interactions at the blood-brain barrier. Nat Rev Neurosci 2006;7:41-53.  Back to cited text no. 74
    
75.Armulik A, Genové G, Mäe M, Nisancioglu MH, Wallgard E, Niaudet C, et al. Pericytes regulate the blood-brain barrier. Nature 2010;468:557-561.  Back to cited text no. 75
    
76.Al Ahmad A, Gassmann M, Ogunshola OO. Maintaining blood-brain barrier integrity: Pericytes perform better than astrocytes during prolonged oxygen deprivation. J Cell Physiol 2009;218:612-622.  Back to cited text no. 76
    
77.Dejana E. Endothelial cell-cell junctions: Happy together. Nat Rev Mol Cell Biol 2004;5:261-270.  Back to cited text no. 77
[PUBMED]    
78.Vestweber D. VE-cadherin: the major endothelial adhesion molecule controlling cellular junctions and blood vessel formation. Arterioscler Thromb Vasc Biol 2008;28:223-232.  Back to cited text no. 78
    
79.Taddei A, Giampietro C, Conti A, Orsenigo F, Breviario F, Pirazzoli V, et al. Endothelial adherens junctions control tight junctions by VE-cadherin-mediated upregulation of claudin-5. Nat Cell Biol 2008;10:923-934.  Back to cited text no. 79
    
80.González-Mariscal L, Betanzos A, Nava P, Jaramillo BE. Tight junction proteins. Prog Biophys Mol Biol 2003;81:1-44.  Back to cited text no. 80
    
81.Wolburg H, Noell S, Mack A, Wolburg-Buchholz K, Fallier-Becker P. Brain endothelial cells and the glio-vascular complex. Cell Tissue Res 2009;335:75-96.  Back to cited text no. 81
    
82.Sawada N, Murata M, Kikuchi K, Osanai M, Tobioka H, Kojima T, et al. Tight junctions and human diseases. Med Electron Microsc 2003;36:147-156.  Back to cited text no. 82
    
83.Dejana E, Giampietro C. Vascular endothelial-cadherin and vascular stability. Curr Opin Hematol 2012;19:218-223.  Back to cited text no. 83
    
84.Nagasawa K, Chiba H, Fujita H, Kojima T, Saito T, Endo T, et al. Possible involvement of gap junctions in the barrier function of tight junctions of brain and lung endothelial cells. J Cell Physiol 2006;208:123-132.  Back to cited text no. 84
    
85.Quaegebeur A, Segura I, Carmeliet P. Pericytes: Blood-brain barrier safeguards against neurodegeneration? Neuron 2010;68:321-323.  Back to cited text no. 85
    
86.Armulik A, Abramsson A, Betsholtz C. Endothelial/pericyte interactions. Circ Res 2005;97:512-523.  Back to cited text no. 86
    
87.Bobbie MW, Roy S, Trudeau K, Munger SJ, Simon AM, Roy S. Reduced connexin 43 expression and its effect on the development of vascular lesions in retinas of diabetic mice. Invest Ophthalmol Vis Sci 2010;51:3758-3763.  Back to cited text no. 87
    
88.Shepro D, Morel NM. Pericyte physiology. FASEB J 1993;7:1031-1038.  Back to cited text no. 88
    
89.Tomasek JJ, Haaksma CJ, Schwartz RJ, Vuong DT, Zhang SX, Ash JD, et al. Deletion of smooth muscle alpha-actin alters blood-retina barrier permeability and retinal function. Invest Ophthalmol Vis Sci 2006;47:2693-2700.  Back to cited text no. 89
    
90.Darland DC, Massingham LJ, Smith SR, Piek E, Saint-Geniez M, D'Amore PA. Pericyte production of cell-associated VEGF is differentiation-dependent and is associated with endothelial survival. Dev Biol 2003;264:275-288.  Back to cited text no. 90
    
91.Simonavicius N, Ashenden M, van Weverwijk A, Lax S, Huso DL, Buckley CD, et al. Pericytes promote selective vessel regression to regulate vascular patterning. Blood 2012;120:1516-1527.  Back to cited text no. 91
    
92.Walshe TE, Saint-Geniez M, Maharaj AS, Sekiyama E, Maldonado AE, D'Amore PA. TGF-beta is required for vascular barrier function, endothelial survival and homeostasis of the adult microvasculature. PLoS One 2009;4:e5149.  Back to cited text no. 92
    
93.Ejaz S, Chekarova I, Ejaz A, Sohail A, Lim CW. Importance of pericytes and mechanisms of pericyte loss during diabetes retinopathy. Diabetes Obes Metab 2008;10:53-56.  Back to cited text no. 93
    
94.Watanabe T, Raff MC. Retinal astrocytes are immigrants from the optic nerve. Nature 1988;332:834-837.  Back to cited text no. 94
    
95.Schnitzer J. Retinal astrocytes: Their restriction to vascularized parts of the mammalian retina. Neurosci Lett 1987;78:29-34.  Back to cited text no. 95
[PUBMED]    
96.Chan-Ling T, Stone J. Degeneration of astrocytes in feline retinopathy of prematurity causes failure of the blood-retinal barrier. Invest Ophthalmol Vis Sci 1992;33:2148-2159.  Back to cited text no. 96
    
97.Choi YK, Kim KW. Blood-neural barrier: Its diversity and coordinated cell-to-cell communication. BMB Rep 2008;41:345-352.  Back to cited text no. 97
    
98.Gardner TW, Lieth E, Khin SA, Barber AJ, Bonsall DJ, Lesher T, et al. Astrocytes increase barrier properties and ZO-1 expression in retinal vascular endothelial cells. Invest Ophthalmol Vis Sci 1997;38:2423-2427.  Back to cited text no. 98
    
99.Choi YK, Kim KW. AKAP12 in astrocytes induces barrier functions in human endothelial cells through protein kinase Czeta. FEBS J 2008;275:2338-2353.  Back to cited text no. 99
    
100.Nishikiori N, Osanai M, Chiba H, Kojima T, Mitamura Y, Ohguro H, et al. Glial cell-derived cytokines attenuate the breakdown of vascular integrity in diabetic retinopathy. Diabetes 2007;56:1333-1340.  Back to cited text no. 100
    
101.Alvarez JI, Dodelet-Devillers A, Kebir H, Ifergan I, Fabre PJ, Terouz S, et al. The Hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence. Science 2011;334:1727-1731.  Back to cited text no. 101
    
102.Kim JH, Kim JH, Yu YS, Kim DH, Kim KW. Recruitment of pericytes and astrocytes is closely related to the formation of tight junction in developing retinal vessels. J Neurosci Res 2009;87:653-659.  Back to cited text no. 102
[PUBMED]    
103.Joussen AM, Smyth N, Niessen C. Pathophysiology of diabetic macular edema. Dev Ophthalmol 2007;39:1-12.  Back to cited text no. 103
    
104.Cui Y, Xu X, Bi H, Zhu Q, Wu J, Xia X, et al. Expression modification of uncoupling proteins and MnSOD in retinal endothelial cells and pericytes induced by high glucose: The role of reactive oxygen species in diabetic retinopathy. Exp Eye Res 2006;83:807-816.  Back to cited text no. 104
    
105.Trudeau K, Molina AJ, Guo W, Roy S. High glucose disrupts mitochondrial morphology in retinal endothelial cells: implications for diabetic retinopathy. Am J Pathol 2010;177:447-455.  Back to cited text no. 105
    
106.Du Y, Kowluru A, Kern TS. PP2A contributes to endothelial death in high glucose: Inhibition by benfotiamine. Am J Physiol Regul Integr Comp Physiol 2010;299:R1610-7.  Back to cited text no. 106
    
107.Leal EC, Aveleira CA, Castilho AF, Serra AM, Baptista FI, Hosoya K, et al. High glucose and oxidative/nitrosative stress conditions induce apoptosis in retinal endothelial cells by a caspase-independent pathway. Exp Eye Res 2009;88:983-991.  Back to cited text no. 107
    
108.Li AF, Roy S. High glucose-induced downregulation of connexin 43 expression promotes apoptosis in microvascular endothelial cells. Invest Ophthalmol Vis Sci 2009;50:1400-1407.  Back to cited text no. 108
    
109.Spoerri PE, Afzal A, Li Calzi S, Shaw LC, Cai J, Pan H, et al. Effects of VEGFR-1, VEGFR-2, and IGF-IR hammerhead ribozymes on glucose-mediated tight junction expression in cultured human retinal endothelial cells. Mol Vis 2006;12:32-34.  Back to cited text no. 109
    
110.Bhattacharjee PS, Huq TS, Potter V, Young A, Davenport IR, Graves R, et al. High-glucose-induced endothelial cell injury is inhibited by a Peptide derived from human apolipoprotein E. PLoS One 2012;7:e52152.  Back to cited text no. 110
    
111.Tien T, Barrette KF, Chronopoulos A, Roy S. Effects of high glucose-induced CX43 downregulation on occludin and ZO-1 expression and tight junction barrier function in retinal endothelial cells. Invest Ophthalmol Vis Sci 2013;54:6518-6525.  Back to cited text no. 111
    
112.Huang Q, Sheibani N. High glucose promotes retinal endothelial cell migration through activation of Src, PI3K/Akt1/eNOS, and ERKs. Am J Physiol Cell Physiol 2008;295:C1647-C1657.  Back to cited text no. 112
    
113.Chen X, Li J, Li M, Zeng M, Li T, Xiao W, et al. KH902 suppresses high glucose-induced migration and sprouting of human retinal endothelial cells by blocking VEGF and PIGF. Diabetes Obes Metab 2013;15:224-233.  Back to cited text no. 113
    
114.Huang Q, Sheibani N. High glucose promotes retinal endothelial cell migration through activation of Src, PI3K/Akt1/eNOS, and ERKs. Am J Physiol Cell Physiol 2008;295:C1647-C1657.  Back to cited text no. 114
    
115.Yuan L, Hu J, Luo Y, Liu Q, Li T, Parish CR, et al. Upregulation of heparanase in high-glucose-treated endothelial cells promotes endothelial cell migration and proliferation and correlates with Akt and extracellular-signal-regulated kinase phosphorylation. Mol Vis 2012;18:1684-1695.  Back to cited text no. 115
    
116.Su X, Sorenson CM, Sheibani N. Isolation and characterization of murine retinal endothelial cells. Mol Vis 2003;9:171-178.  Back to cited text no. 116
    
117.Romeo G, Liu WH, Asnaghi V, Kern TS, Lorenzi M. Activation of nuclear factor-kappaB induced by diabetes and high glucose regulates a proapoptotic program in retinal pericytes. Diabetes 2002;51:2241-2248.  Back to cited text no. 117
    
118.Takamura Y, Tomomatsu T, Kubo E, Tsuzuki S, Akagi Y. Role of the polyol pathway in high glucose-induced apoptosis of retinal pericytes and proliferation of endothelial cells. Invest Ophthalmol Vis Sci 2008;49:3216-3223.  Back to cited text no. 118
    
119.Geraldes P, Hiraoka-Yamamoto J, Matsumoto M, Clermont A, Leitges M, Marette A, et al. Activation of PKC-delta and SHP-1 by hyperglycemia causes vascular cell apoptosis and diabetic retinopathy. Nat Med 2009;15:1298-1306.  Back to cited text no. 119
    
120.Fu D, Wu M, Zhang J, Du M, Yang S, Hammad SM, et al. Mechanisms of modified LDL-induced pericyte loss and retinal injury in diabetic retinopathy. Diabetologia 2012;55:3128-3140.  Back to cited text no. 120
    
121.Devi TS, Hosoya K, Terasaki T, Singh LP. Critical role of TXNIP in oxidative stress, DNA damage and retinal pericyte apoptosis under high glucose: Implications for diabetic retinopathy. Exp Cell Res 2013;319:1001-1012.  Back to cited text no. 121
    
122.Li Y, Smith D, Li Q, Sheibani N, Huang S, Kern T, et al. Antibody-mediated retinal pericyte injury: Implications for diabetic retinopathy. Invest Ophthalmol Vis Sci 2012;53:5520-5526.  Back to cited text no. 122
    
123.Trudeau K, Molina AJ, Roy S. High glucose induces mitochondrial morphology and metabolic changes in retinal pericytes. Invest Ophthalmol Vis Sci 2011;52:8657-8664.  Back to cited text no. 123
    
124.Beltramo E, Berrone E, Giunti S, Gruden G, Perin PC, Porta M. Effects of mechanical stress and high glucose on pericyte proliferation, apoptosis and contractile phenotype. Exp Eye Res 2006;83:989-994.  Back to cited text no. 124
    
125.Pfister F, Feng Y, vom Hagen F, Hoffmann S, Molema G, Hillebrands JL, et al. Pericyte migration: A novel mechanism of pericyte loss in experimental diabetic retinopathy. Diabetes 2008;57:2495-2502.  Back to cited text no. 125
    
126.Tu Z, Li Y, Smith DS, Sheibani N, Huang S, Kern T, et al. Retinal pericytes inhibit activated T cell proliferation. Invest Ophthalmol Vis Sci 2011;52:9005-9010.  Back to cited text no. 126
    
127.Shin ES, Huang Q, Gurel Z, Palenski TL, Zaitoun I, Sorenson CM, et al. STAT1-mediated Bim expression promotes the apoptosis of retinal pericytes under high glucose conditions. Cell Death Dis 2014;5:e986.  Back to cited text no. 127
    
128.Rungger-Brändle E, Dosso AA, Leuenberger PM. Glial reactivity, an early feature of diabetic retinopathy. Invest Ophthalmol Vis Sci 2000;41:1971-1980.  Back to cited text no. 128
    
129.Feit-Leichman RA, Kinouchi R, Takeda M, Fan Z, Mohr S, Kern TS, et al. Vascular damage in a mouse model of diabetic retinopathy: Relation to neuronal and glial changes. Invest Ophthalmol Vis Sci 2005;46:4281-4287.  Back to cited text no. 129
    
130.Iandiev I, Pannicke T, Reichenbach A, Wiedemann P, Bringmann A. Diabetes alters the localization of glial aquaporins in rat retina. Neurosci Lett 2007;421:132-136.  Back to cited text no. 130
    
131.Ly A, Yee P, Vessey KA, Phipps JA, Jobling AI, Fletcher EL. Early inner retinal astrocyte dysfunction during diabetes and development of hypoxia, retinal stress, and neuronal functional loss. Invest Ophthalmol Vis Sci 2011;52:9316-9326.  Back to cited text no. 131
    
132.Lind KR, Ball KK, Cruz NF, Dienel GA. The unfolded protein response to endoplasmic reticulum stress in cultured astrocytes and rat brain during experimental diabetes. Neurochem Int 2013;62:784-795.  Back to cited text no. 132
    
133.Xu Z, Wei Y, Gong J, Cho H, Park JK, Sung ER, et al. NRF2 plays a protective role in diabetic retinopathy in mice. Diabetologia 2014;57:204-213.  Back to cited text no. 133
    
134.Aghdam SY, Gurel Z, Ghaffarieh A, Sorenson CM, Sheibani N. High glucose and diabetes modulate cellular proteasome function: Implications in the pathogenesis of diabetes complications. Biochem Biophys Res Commun 2013;432:339-344.  Back to cited text no. 134
    
135.Gurel Z, Sieg KM, Shallow KD, Sorenson CM, Sheibani N. Retinal O-linked N-acetylglucosamine protein modifications: Implications for postnatal retinal vascularization and the pathogenesis of diabetic retinopathy. Mol Vis 2013;19:1047-1059.  Back to cited text no. 135
    


    Figures

  [Figure 1], [Figure 2]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

 
  In this article
Abstract
Introduction
Diabetic retinopathy
Physiological ch...
Concluding remar...
Acknowledgments
Blood-Retinal Ba...
References
Article Figures

 Article Access Statistics
    Viewed1446    
    Printed23    
    Emailed0    
    PDF Downloaded205    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]