Review Article Diabetic Retinopathy: Animal Models ...downloads.hindawi.com/journals/jdr/2016/3789217.pdf3. Animal Models At present, most animal models of DR are rodents, mice, and

Review ArticleDiabetic Retinopathy Animal Models Therapiesand Perspectives

Xue Cai1 and James F McGinnis123

1Department of Ophthalmology Dean McGee Eye Institute Oklahoma University Health Sciences Center Oklahoma CityOK 73104 USA2Department of Cell Biology Oklahoma University Health Sciences Center Oklahoma City OK 73104 USA3Oklahoma Center for Neuroscience Oklahoma University Health Sciences Center Oklahoma City OK 73104 USA

Correspondence should be addressed to Xue Cai xue-caiouhscedu and James F McGinnis james-mcginnisouhscedu

Received 14 August 2015 Accepted 6 December 2015

Academic Editor Shuang-Xi Wang

Copyright copy 2016 X Cai and J F McGinnis This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Diabetic retinopathy (DR) is one of the major complications of diabetes Although great efforts have been made to uncover themechanisms underlying the pathology of DR the exact causes of DR remain largely unknown Because of multifactor involvementin DR etiology currently no effective therapeutic treatments for DR are available In this paper we review the pathology ofDR commonly used animal models and novel therapeutic approaches Perspectives and future directions for DR treatment arediscussed

1 Introduction

Diabetic retinopathy (DR) is one of the major complicationsof diabetes and is the leading cause of blindness amongworking people in developed countries The symptoms areelevated blood sugar levels blurred vision dark spots orflashing lights and sudden loss of vision The developmentof DR can be divided into nonproliferative DR (NPDRsubdivided into mild moderate and severe stages) withmicroaneurysms hard exudates hemorrhages and venousabnormalities [1 2] and proliferative DR (PDR advancedstage) with neovascularization preretinal or vitreous hem-orrhages and fibrovascular proliferation [1 2] Developmentof glaucoma retinal detachment and vision loss may alsohappen at this stage DR may cause macular edema whenblood and fluid leak into the retina caused by swelling ofthe central retina [3] DR is not easily diagnosed at earlystages but is more readily noticed with the advanced stagesor with edema Multiple techniques have been used fordetection diagnosis and evaluation of this disease includingfundoscopic photography fluorescence angiography B-scanultrasonography and optical coherence tomography (OCT)[4]

2 Pathology and Molecular Mechanism of DR

Initially DR was considered a microvascular complicationof endothelial dysfunction as it is characterized by capillarybasementmembrane (BM) thickening pericyte and endothe-lial cell loss blood-retinal barrier (BRB) breakdown andleakage acellular capillaries and neovascularization [5 6]However it is currently acknowledged that before the typicalfeatures of DR occur and can be clinically diagnosed cellularmolecular and functional changes are evidenced in the retina[7 8] where all types of retinal cells are affected includingganglion cells [5 6 9] Also thinning of the inner nuclearlayer (INL) reduction in synapse numbers and synaptic pro-teins changes in dendrite morphology and retinal pigmentepithelium (RPE) dysfunction occur in DR and result in thegradual loss of retinal function [9] In addition glia activationand innate immunitysterile inflammation [5 6] occur earlyin DRTherefore DR is not only a vascular disease but also aneurodegenerative disease

DR shares numerous similarities in its etiology andpathology with other neovascular diseases which have beendocumented to be associated with chronic inflammation

Hindawi Publishing CorporationJournal of Diabetes ResearchVolume 2016 Article ID 3789217 9 pageshttpdxdoiorg10115520163789217

2 Journal of Diabetes Research

including increased vascular permeability edema inflamma-tory cell infiltration tissue destruction neovascularizationproinflammatory cytokines and chemokines in the retina [310] Some of the potential risk factors leading to the pathologyof other neovascular diseases also contribute to the pathologyof DR

Diabetes is the number one risk factor for the develop-ment of DR Type 1 diabetes (juvenile diabetes in whichno insulin is made) is more likely to develop vision lossthan type 2 diabetes (adult onset diabetes with insufficientinsulin synthesis) In addition race (Hispanic and AfricanAmericans) smoking hyperglycemia (high blood sugar)hypertension (high blood pressure) and hyperlipidemia(high cholesterol) or dyslipidemia are also high risk factors[11 12] Vascular endothelial growth factor (VEGF) elevationinduces a decrease in the tight-junction proteins and break-down of the BRB [13] an increase of leukostasis within retinalvessels [14] inflammation [15 16] upregulation of ICAM-1(intercellular adhesionmolecule-1) expression an increase inall NOS (nitric oxide synthase) isoforms [17] and ametabolicimbalance in inorganic phosphate [18] all of which have beenreported to contribute to DR pathology Multiple intercon-necting biochemical pathways including an increased polyolpathway elevated hexosamine biosynthesis pathway (HBP)activation of protein kinase C (PKC) hemodynamic changesand advanced glycation end product (AGE) formation [5 614 19] have also been found to play key roles in developmentof DR RhoA is a small guanosine-51015840-triphosphate-bindingprotein and acts as aGTPaseTheRhoAmDia-1 (mammaliandiaphanous homolog-1)profiling-1 [20] or RhoAROCK1(Rho-associated coiled-coil-containing protein kinase 1) [21]pathways have been shown to be involved in the pathologyof DR via triggering microvascular endothelial dysfunctionActivation of these pathways leads to the increase of growthfactors such as VEGF and insulin-like growth factor-1 (IGF-1) activation of the renin-angiotensin-aldosterone system(RAAS) subclinical inflammation and capillary occlusion[14] Also increased endoplasmic reticulum (ER) stress andoxidative stress [22] resulting from deregulation of ER andmitochondrial quality control by autophagymitophagy RPEdysfunction genetic variants and epigenetic changes in chro-matin such as DNA methylation histone posttranslationalmodifications affecting gene transcription and regulationby noncoding RNAs [23ndash26] have also been shown to beassociated with DR Interestingly deletion of transforminggrowth factor-120573 (TGF-120573) signaling results in undifferentiatedpericytes that cause retinal changes in structure and functionwhich mimic those of DR [27] Loss of other gene functionssuch as BMP2 (bone morphogenetic protein 2) [28] and Toll-like receptor 4 [29] has been implicated in the pathogenesisof DR Activation of the P2X7 receptor a member of ligand-gated membrane ion channels resulted in the formationof large plasma membrane pores that exacerbate the devel-opment of DR through induction of inflammation [30]Recently a prooxidant and proapoptotic thioredoxin inter-acting protein (TXNIP) was shown to be highly upregulatedin DR and by high glucose (HG) in retinal cells in cultureTXNIP binds to thioredoxin (Trx) inhibiting its oxidantscavenging and thiol-reducing capacity Hence prolonged

overexpression of TXNIP causes ROSRNS stress mitochon-drial dysfunction inflammation and premature cell deathin DR [31] Collectively hyperglycemia-induced vasculardysfunction and subsequent tissue damage have been pro-posed to act through the following four main pathways [3233] (1) increased polyol pathway flux in which cytosolicredox imbalance occurs with an increased NADHNAD+ratio via the sorbitol pathway resulting in a decrease incytosolic NADPH and cellular functions (2) increased AGEformation in which nonenzymatic glycosylation of proteinsand production of AGEs alter gene expression and AGEs alsoinduce the synthesis of numerous inflammatory cytokines(3) activation of PKC via the formation of intracellulardiacylglycerol (DAG) and AGEs which contributes to thegeneration of ROS which induces VEGF and multiple othergrowth factors and transcription factors and (4) increasedhexosamine pathway flux in which fructose-6-phosphate isconverted to glucosamine-6-phosphate and finally to uridinediphosphate119873-acetyl glucosamineThis modification resultsin changes in gene expression and protein function Howevereach of the four major pathways is linked by overproductionof superoxide and increased generation of ROS [33] whichprovides a common target for potential treatment

3 Animal Models

At present most animal models of DR are rodents miceand rats Based on the experimental approaches to induceDR these models can be classified as chemically inducedspontaneous and genetically created However knowledgeof the molecular mechanisms underlying the initiation anddevelopment of DR is insufficient and largely unknownbecause there are no reliable and appropriate good animalmodels of spontaneous diabetes in which phenotypic char-acteristics exactly mimic the pathogenesis of clinical DRAlthough various traditionally used animal models of DRpresent a number of pathological changes similar to thoseof human DR several pathological characteristics of humanDR such as retinal neovascularization cannot yet be fullymimicked in any existing animal model of DR [34]

31 Chemically Induced Model The commonly used strepto-zotocin (STZ) or alloxan induced DR animal models (ratsor mice) exhibit rapid onset of hyperglycemia (3 days aftertreatment) and some of the symptoms of early DR (type Idiabetes) such as loss of retinal pericytes and capillariesthickening of the vascular basement membrane vascularocclusion and increased vascular permeability [3 34 35]However variability of pathological characteristics such asloss of retinal capillaries ganglion cell death and reductionof retinal function has been reported among different speciesand even within the same species [3 34 35]

32 Akita Mice The Akita (Ins2Akita+minus) mouse a sponta-neous diabetes model for early stage of DR (type I diabetes)is caused by a missense mutation in the diabetogenic Insulin2 gene (Ins2) and is characterized by a rapid onset ofhyperglycemia and hypoinsulinemia and marked reduction

Journal of Diabetes Research 3

of insulin secretion by 4 weeks of age [36] Significantincreases in vascular permeability were seen when measuredat 12 weeks after hyperglycemia The thickness of the innerplexiform layer (IPL) and INL in the peripheral region wasdecreased and the number of ganglion cells was significantlyreducedwhenmeasured at 22weeks after hyperglycemia [37]Recently Hombrebueno et al reported that the Akita miceexhibit progressive thinning of the retina and cone loss from 3months onwards severe impairment of synaptic connectivityat the outer plexiform layer (OPL) and significant reductionin the number of amacrine and ganglion cells [38 39] ERstress associated proteins were upregulated in this mousemodel [40] The transportation of proinsulin from the endo-plasmic reticulum (ER) to the Golgi apparatus is blockedand instead the mutant proinsulin is accumulated in theER forming complexes with BiP (binding immunoglobulinprotein) which are eventually degraded [41]

33 Kimba Mice The Kimba mice were generated bymicroinjection of human VEGF

165isoform driven by a

photoreceptor-specific promoter (rhodopsin) Pathologicalchanges in the retinal vasculature focal fluorescein leakagerelatively mild degree and slow onset of neovascularizationwere shown at 3-4 weeks of age and stable retinopathy per-sisted for 3months which resembles NPDR and early stage ofPDR [1] A thinner outer nuclear layer (ONL) and INL severeand extensive outer and inner retinal neovascularizationhemorrhage retinal detachment [1] microaneurysm leakycapillaries capillary dropout [42] leaky blood vessels andBRB loss [42 43] were presented in this mouse modelHowever the mice overexpressing photoreceptor-specifichVEGF are not on a hyperglycemic background and do notinduce choroidal neovascularization [1 42]

34 Akimba Mice The Akimba (Ins2AkitaVEGF+minus) mousegenerated from the Kimba (VEGF+minus) (trVEGF029) and theAkita (Ins2Akita) mice is a model for advanced DR [42]This model retains the parental retinal neovascularizationwith hyperglycemia and displays the majority of signs ofadvanced clinical DR includingmore diffuse vascular leakage(compared to the more focal leakage in Kimba mice) and theBRB disruption which was linked to decreased expressionof endothelial junction proteins pericyte dropout and vesselloss [42 43] With aging Akimba mice exhibit enhancedphotoreceptor loss thinning of the retina more severe andprogressive retinal vascular pathology capillary nonperfu-sion much higher prevalence and persistence of edemaand retinal detachment [42] Plasmalemma vesicle associatedprotein (PLVAP) is an endothelial cell specific protein whichis absent in intact BRB but is significantly increased inAkimba mice (and also in Kimba mice) Therefore PLVAPplays an important role in the regulation of BRB permeability[43]

35 dbdb Mice The dbdb (119897119890119901119903119889119887) mouse a spontaneousdiabetic model of type 2 diabetes [44 45] is caused by amutation in the leptin receptor gene It exhibits high glialactivation progressive loss of ganglion cells and significantreduction of neuroretinal thickness Significant abnormal

retinal function is pronounced at 16 weeks of age In additionsignificantly higher levels of glial fibrillary acidic protein(GFAP a marker for glial cells) expression increases inaccumulation of glutamate and downregulation of abundantneurotransmission genes were found at 8 weeks of age [44]Also breakdown of the BRB is a hallmark of the dbdb mice[46] and RPE dysfunction is concomitant with sustainedhyperglycemia [45] Proteomic analysis of 10-week-old reti-nas fromdbdb andwild typemice showed that 98membraneproteins out of a total of 844 were significantly differentiallyabundant in dbdb versus wild type mice in which 80 weredownregulated and 18 were upregulated in the dbdb retinas[47] The major proteins decreased are synaptic transmissionproteins especially the vesicular glutamate transporter 1(VGLUT1) [47] which is responsible for the loading ofglutamate into synaptic vesicles and is expressed at the ribbonsynapses in the photoreceptors and ldquoONrdquo bipolar cells [48]

36 New Animal Models In recent years two new animalmodels were reported One is a transgenic mouse overex-pressing insulin-like growth factor-1 (IGF-1) which developsthe most retinal alteration seen in human diabetic eyes on anonhyperglycemic background [49] and exhibits progressivedevelopment of vascular alteration (from NPDR to PDR)increased VEGF level BRB breakdown vascular permeabil-ity and glial alteration with age (3 months and older) [4950] Retinal neurodegeneration was seen at 6 months of agewith the number of bipolar and ganglion cells reduced anda 40 reduction of ONL and INL thickness was observedin 75-month-old mice Microarray analysis on 4-month-oldretinas with evidence of NPDR and gliosis [50] revealedupregulation of genes associated with retinal stress gliosisand angiogenesis Increased GFAP immunostaining was seenat 15months of age andwasmaintained throughout the entirelife Activation of ERK signalingwas detected at 3months andwas more pronounced at 75 months In addition expressionof oxidative stress markers was increased in particular astriking upregulation of all three subunits ofNADPHoxidaseimpaired glutamate recycling and significantly higher levelsof TNF-120572 and MCP-1 were seen at 75 months [51] The othermodel is the hyperhexosemic marmosets (Callithrix jacchus)which with a 30 galactose- (gal-) rich diet for two yearsdevelops significantly high blood glucose levels vascularpermeability macular edema increased number of acellularcapillaries pericyte loss vascular BM thickening increasedvessel tortuosity in the retinas and microaneurysms High-speed spectral domain OCT (SD-OCT) scan reveals sig-nificant thickening of the foveal and the juxtafoveal arearesulting from intraretinal fluid accumulation Also there arepotential break in the RPE and discontinuous photoreceptorlayers in the macular area starting at 15 months of galactosefeeding All these characteristics have striking similarities tothe human DR [52]

4 Current Therapies

During the nonproliferative stages treatment is usually notrecommended because normal visual function is not dis-turbed at these stages However at the advanced stages the


PDR treatment has to be undertaken Traditional approachesfor treatment of DR and associated microvasculature andneovascularization include laser treatment optimizing bloodglucose level and controlling blood pressure Currentlylaser treatment (photocoagulation) to stop the leakage andscattered laser burns to shrink abnormal blood vessels andprevent retinal detachment are effective and are widelyemployed and are the primary treatment strategy Surgicaltreatment to remove the vitreous (vitrectomy) is usually takenfor advanced PDR in type I diabetes if persistent vitreoushemorrhage or severe tractional retinal detachment occursIntravitreal injection of anti-VEGF (Avastin Lucentis andEylea) and corticosteroids to prevent abnormal blood vesselgrowth are effective and are also beneficial treatments forPDR [2 19 53]

Clinical trial (ClinicalTrialsgov number NCT01627249)phase III study (660 adults) with intravitreal injection ofAflibercept Bevacizumab or Ranibizumab for diabetic mac-ular edema (DME) showed that visual acuity was improvedandAflibercept is more effective when the initial visual acuityis worse [54] A five-year clinical trial study reported thatintravitreal injection of 05mg Ranibizumab with prompt(124 patients) or deferred (111 patients) focalgrid laser treat-ment for diabeticmacular edema resulted in themaintenanceof vision gains obtained by the first year through 5 yearsin most of the eyes [55] However another clinical trialstudy (322 of 582 eyes) showed that repeated intravitrealRanibizumab injections for DME may increase the risk ofsustained elevation of intraocular pressure or the need forocular hypotensive treatment [56] and a risk of stroke [2]Another clinical trial phase III study evaluating the safetyand bioactivity of intravitreal injection of a designed ankyrinrepeat protein (MP0112) for specific and high-affinity bindingto VEGF in patients with DME showed reduction of edemaand improvement of visual acuity although several patientsshowed inflammation [57] An ongoing clinical trial elim-inates the source of inflammation from a new preparation[57]

DR associated pathological factors molecular signalingpathways and other mechanisms underlying the pathologyof DR as well as the direct pathological defects (retinaldegeneration synaptic connection impairment and cell lossaccumulation of glutamate etc) provide a broad spec-trum of potential new therapeutic targets for the treat-ment of DR Therapeutic treatment strategies targetingthese molecules components or defects including variousfactors hyperglycemia- and glutamate-triggered pathwaysand microvascular impairment and angiogenesis have beenshown to produce an effective outcome [11 58 59] Chi-nese traditional medicine HF (He-Ying-Qing-Re formula)in which chlorogenic acid ferulic acid and arctin wereidentified as major components was shown to have anti-DR effects although hyperglycemia was not significantlyinhibited Its action on suppression of activation of AGEsand endothelial dysfunction occurs by inactivation of AGEsreceptor and their downstream Akt signaling pathway [60]Deletion of placental growth factor prevents DR by inactiva-tion of Akt and inhibition of the HIF1120572-VEGF pathway [1161] Recently angiopoietin-like 4 (ANGPTL 4) was identified

as a potential angiogenic factor which was upregulated in thePDR patients and was shown to be independent of VEGFlevels and localized in the area of retinal neovascularizationNeutralizing ANGPTL4 antibody can inhibit the angiogeniceffect in PDR patients with low VEGF levels or produce anadditive effect with anti-VEGF treatment for inhibition ofVEGF expression [62]

Preclinical therapies targeting other factors have beenreported A single intravitreal injection of a vector express-ing insulin-like growth factor binding protein-3 (IGFBP-3)into diabetic rat retina after 2 months of diabetes restoresnormal insulin signal transduction via regulation of theinsulin receptorTNF-120572 (tumor necrosis factor-alpha) path-way and leads to the reduction of proapoptotic markersor increases of antiapoptotic markers and the restorationof retinal function [63] Blockage of TNF-120572 by intravitrealand intraperitoneal delivery of anti-TNF-120572 antibody in STZ-induced mice and Akita mice resulted in a dose-dependentprevention of increased retinal leukostasis acellular capil-lary BRB breakdown and cell death [64] Intraperitonealinjection of anti-VEGFR1 antibody (MF1) prevents vascularleakage and inhibits inflammation associated gene expressionand abnormal distribution of tight-junction proteins in STZ-induced mice and Akita mice [65]

Fenofibrate is a peroxisome proliferator-activatedreceptor-120572 (PPAR-120572) agonist and is known for clinical treat-ment for dyslipidemia Recently it was shown to signif-icantly ameliorate retinal vascular leakage and leukostasisin DR of STZ-induced diabetic rats and Akita mice throughdownregulation of ICAM-1 MCP-1 (monocyte chemoat-tractant protein-1) and NF-120581B (nuclear factor-kappa B)signaling [66] Clinical studies demonstrated that Fenofibratehas protective effects on progression of proliferative DR intype 2 diabetic patients [67 68] Now the use of thismedication for DR is approved [69]

Omega-3 polyunsaturated fatty acid (120596-3PUFA) has beenshown to be decreased in STZ-induced diabetic rat retina[70] 120596-3PUFA rich diets enhanced glucose homeostasis andpreserved retinal function in dbdbmice but the effect isindependent of preservation of retinal vasculature integrityinflammatory modulation and retinal neuroprotection [71]

5 Novel Potential Therapeutic Targets

Because of the complicated etiology of DR drugs such asinhibitors for signaling pathways and growth factors havebeen shown to be effective for the treatment of DR buthave limitations Currently intravitreal injection of anti-VEGF and corticosteroids are popular therapeutics but ahigh proportion of patients (sim40) do not respond to thesetherapies [58 72]This implies that other factors or pathwaysindependent of VEGF are involved in the development ofmicrovasculature and neovascularization Therefore there isan urgent need for finding potential target candidates andfor the development of new treatment strategies for DRtherapy

Epigenetic chromatin modifications (DNA methylationhistone posttranslational modifications and regulation bynoncoding RNAs) acting on both cis- and trans-chromatin


structural elements can be regulated by TXNIP [25] Aber-rant epigenetic modifications have been identified in DR andimplicated in the progression of DR [25 26] MicroRNAs(miRNAs) are a group of noncoding RNA sequences whichare short and highly conservative and can posttranscription-ally control gene expression by degradation or repression oftarget mRNAs They are implicated in a variety of biologicalactivities including modulation of glucose angiogenesisand inflammatory responses as well as pathogenesis ofdiabetes and related complications such as DR [10] Howeverconflicting data were seen with different miRNAs It hasbeen shown that retinal miRNA expression was altered inearly DR rats induced by STZ in which miRNAs weredifferentially regulated compared to the controls withoutDR [73] Downregulation of miR-200b has been shown toincrease VEGF expression and polycomb repressive com-plex 2 (PRC2) (histone methyltransferase complex) repressesmiR-200b through its histone H3 lysine-27 trimethylationThus inhibition of PRC2 through histone methylation ofmiR-200b increases miR-200b and reduces VEGF in STZ-induced diabetic rats [74] The 31015840-untranslated region (31015840-UTR) ofmRNA sequence contains regulatory regions includ-ing binding sites for miRNAs to repress translation anddegrade mRNA transcripts In DR rats miRNA-195 wassignificantly upregulated after one month of diabetes andthe antioxidant enzyme MnSOD level was reduced In situhybridization indicated that miR-195 was overexpressed inthe cells of INL and ONL and ganglion cell layers but sirtuin1 (SIRT1) was downregulated SIRT1 is involved in manybiological processes including cell survival and metabolismand miR-195 binds to the 31015840-UTR of SIRT1 to regulate itsexpression Intravitreal injection of miR-195 antagomir leadsto downregulation of SIRT1 thus preventing DR damagecaused by SIRT1-mediated downregulation of MnSOD [75]Collectively increasing amounts of data demonstrate theactive involvement and critical role of miRNAs in devel-opment of DR although the exact mechanisms by whichmiRNA or miRNAs act are not known Increased knowledgeof how miRNAs function as therapeutic agents will lead totheir effective use in the treatment of DR

Reactive oxygen species (ROS) the primary causativefactor for a variety of diseases have been shown to play animportant role in promoting DR [12 58 76] As a treatmenttarget evidence from preclinical and clinical studies indi-cates that antioxidant therapies which directly target ROS-producing enzymes are beneficial although the outcomeof large clinical trials has been less promising [76] How-ever nuclear factor erythroid 2-related factor 2 (Nrf2) theregulator of phase II enzymes system and the network ofcytoprotective genes [77 78] is still attractive Its activatorshave been proven effective in prevention of the developmentand progression of DR [79] Here we specifically point outthat nanomedicine attracts more attention in the past severalyears because it has been beneficial in a variety of medicalapplications including its promising effects ondisease therapy[80 81] We have been using cerium oxide nanoparticles(nanoceria) to treat several animal models for ocular diseasesand demonstrated their nontoxic and long-lasting effective-ness in delaying retinal degeneration in tubby mice [82] and

inhibiting retinal and choroidal neovascularization [83] Dueto their unique physicochemical features nanoceria them-selves exhibit superoxide dismutase and catalase activitiesunder redox conditions and can upregulate phase II enzymes[84] and regulate the common antioxidant gene networkdownstreamof Trx [85]Nanoceria have an atom-comparablesize which enables them to freely cross the cellular andnuclear membrane barriers In addition they do not needrepeat dosing as is required by other antioxidants Thus onesingle dose produces sustained protective effects [82ndash84]which suggests their great potential to be excellent agents forthe treatment of DR

Stem cells emerged as a regenerative therapeutic strategyfor treatment of a variety of diseases because they areundifferentiated and retain their stem cell characteristicsand possess the potential to differentiate into many differentcell types under certain biological conditions [86 87] Stemcells have been obtained from multiple sources and havebeen shown to have a great potential for tissue repair andocular disease treatment [87 88] Human embryonic stemcells (hESCs) can differentiate into more than 99 pureRPE cells and integrate into the host RPE layer and becomematured Phase III clinical trials for assessing the tolerabilityand safety of subretinal transplantation of hESC-derivedRPE cells in patients with Stargardtrsquos macular dystrophy(ClinicalTrialsgov number NCT01345006) and advanceddry AMD (ClinicalTrialsgov number NCT01344993) haveshown that hESCs improve visual acuity [89] Assessment oftheir medium- and long-term safety graft and survival inpatients is ongoing [90] Mesenchymal stromal cells (MSCs)have been shown to have multiple effects including tissuerepair secretion of neuroprotective growth factors suppres-sion of host immune response and lowering glucose levels[91 92] Bone marrow derived MSCs have been reported tobe differentiated into retinal cells and rescue retinal degen-eration in several animal models [91] Clinical trial phase Iassessing their effects on visual acuity in patientswith retinitispigmentosa (RP) (ClinicalTrialsgov number NCT01068561)has been completed and phase III in patients with AMD andStargardt (ClinicalTrialsgov number NCT01518127) will becompleted in December 2015 (also see review [92]) Howeverno clinical study of therapeutic effects of MSCs in DR hasbeen reported Progress has also been made in using severalclasses of stem cells (EPCs endothelial progenitor cellsASCs adipose stromal cells PSCs pluripotent stem cells) tostimulate both neuroregeneration and vascular regenerationin the diabetic retina [92] EPCs are circulating cells and canbe recruited to the sites of vessel damage and tissue ischemiaand promote vascular healing and reperfusion [93] Clinicalstudies have shown that altered numbers of EPCs were foundin patients of type I and type II diabetes withNPDR and PDRsuggesting that EPCs are potential biomarkers for DME andPDR and may be used as therapeutic modalities to treat DR[72] Preclinical study of STZ-induced diabetic rats receivinga single intravitreal injection of human derived ASCs attwo months after diabetes onset showed significant decreasesin vascular leakage and apoptotic cells and downregulationof inflammatory gene expression and improved rod b-waveamplitude within one week after injection [94] Furthermore


mouse ASCs (mASCs) were intravitreally injected into 5-week-old Akimba mice and the mASCs integrated andassociated with retinal microvasculature Injection of TGF-1205731-preconditionedmASCs into P9 Akimba pups resulted in agreat decrease in capillary dropout areas and avascular areas[95] These results suggest that regenerative medicine couldbe a permanent solution for fighting diabetes and associatedcomplications

Nevertheless as we previously mentioned DR has a com-plicated etiology and involves many factors Among thesecausative factors genetic background seems to contributemost heavily and current approaches for the treatment of DRcan only delay the disease progression and do not providea complete treatment or cure for DR Correction of thedefective gene(s) appears to be potentially the most effectiveway for DR treatment (see below) In the clinic the challengefaced is the lack of detection methods for as yet unknownearly clinical symptoms which would enable immediate andproper treatment for inhibition of the progression of NPDRto PDR

6 Perspective and Future Direction forDR Treatment

With wide exploration of the etiology of the diseases usingmodern molecular techniques one finds that almost allthe diseases are linked with mutation(s) of a specific geneor multiple genes Current effective gene therapy methodsinvolve gene replacement therapy in which the defective copyof the gene is replaced by the wild type allele to complimentthe defect or knockdown of the defective gene by RNAinterference (RNAi) silences the effects of the mutated geneor introduces a gene to produce a product causing cellapoptosis (httpwwwghrnlmnihgovhandbook) None ofthe above mentioned strategies can completely eliminatethe products or effects of the defective genes indicatingthat the diseases cannot be completely cured CRISPRCas9-mediated genome editing which emerged as a new ther-apeutic strategy for defective gene repairing has attractedsignificant attention in recent years Indeed at the 2015annual ARVO (the association for research in vision andophthalmology) meeting several laboratories reported theirprogress in using this approach to correct (or repair) mutantgene sequences from patient-derived induced pluripotentstem cells (iPSCs) for treatment of inherited ocular dis-eases such as retinitis pigmentosa AMD and other retinaldiseases [96ndash99] Considering the similarity in the patho-genesis of AMD and DR CRISPRCas9-mediated selectiveengineering of genes associated with DR or angiogenesisis expected to produce positive and effective treatment ofDR

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was supported in part by NIH NEI P30EY021725R01EY018724 and R01EY022111 and unrestricted funds fromPHF and RPB

References

[1] C-M Lai S A Dunlop L A May et al ldquoGeneration of trans-genic mice with mild and severe retinal neovascularisationrdquoBritish Journal ofOphthalmology vol 89 no 7 pp 911ndash916 2005

[2] N Cheung P Mitchell and T Y Wong ldquoDiabetic retinopathyrdquoThe Lancet vol 376 no 9735 pp 124ndash136 2010

[3] R Robinson V A Barathi S S Chaurasia T YWong and T SKern ldquoUpdate on animal models of diabetic retinopathy frommolecular approaches to mice and higher mammalsrdquo DiseaseModels and Mechanisms vol 5 no 4 pp 444ndash456 2012

[4] D A Salz and A J Witkin ldquoImaging in diabetic retinopathyrdquoMiddle East African Journal of Ophthalmology vol 22 no 2 pp145ndash150 2015

[5] A M Abu El-Asrar L Dralands L Missotten I A Al-Jadaanand K Geboes ldquoExpression of apoptosis markers in the retinasof human subjects with diabetesrdquo Investigative Ophthalmologyand Visual Science vol 45 no 8 pp 2760ndash2766 2004

[6] A J Barber E Lieth S A Khin D A Antonetti A GBuchanan and T W Gardner ldquoNeural apoptosis in the retinaduring experimental and human diabetes Early onset and effectof insulinrdquo The Journal of Clinical Investigation vol 102 no 4pp 783ndash791 1998

[7] M S Ola and A S Alhomida ldquoNeurodegeneration in diabeticretina and its potential drug targetsrdquo Current Neuropharmacol-ogy vol 12 no 4 pp 380ndash386 2014

[8] J S Ng M A Bearse Jr M E Schneck S Barez and A JAdams ldquoLocal diabetic retinopathy prediction by multifocalERG delays over 3 yearsrdquo Investigative Ophthalmology andVisual Science vol 49 no 4 pp 1622ndash1628 2008

[9] A J Barber ldquoDiabetic retinopathy recent advances towardsunderstanding neurodegeneration and vision lossrdquo ScienceChina Life Sciences vol 58 no 6 pp 541ndash549 2015

[10] R Mastropasqua L Toto F Cipollone D Santovito PCarpineto and L Mastropasqua ldquoRole of microRNAs in themodulation of diabetic retinopathyrdquo Progress in Retinal and EyeResearch vol 43 pp 92ndash107 2014

[11] A Das P G McGuire and S Rangasamy ldquoDiabetic macularedema pathophysiology and novel therapeutic targetsrdquo Oph-thalmology vol 122 no 7 pp 1375ndash1394 2015

[12] H-P Hammes Y Feng F Pfister and M Brownlee ldquoDiabeticretinopathy targeting vasoregressionrdquo Diabetes vol 60 no 1pp 9ndash16 2011

[13] D A Antonetti A J Barber S Khin E Lieth J M Tarbelland TW Gardner ldquoVascular permeability in experimental dia-betes is associated with reduced endothelial occludin contentvascular endothelial growth factor decreases occludin in retinalendothelial cells Penn State Retina Research Grouprdquo Diabetesvol 47 no 12 pp 1953ndash1959 1998

[14] J M Tarr K Kaul M Chopra E M Kohner and R ChibberldquoPathophysiology of diabetic retinopathyrdquo ISRN Ophthalmol-ogy vol 2013 Article ID 343560 13 pages 2013

[15] A M Joussen V Poulaki M L Le et al ldquoA central role forinflammation in the pathogenesis of diabetic retinopathyrdquo TheFASEB Journal vol 18 no 12 pp 1450ndash1452 2004


[16] F Semeraro A Cancarini R dellrsquoOmo S Rezzola M RRomano and C Costagliola ldquoDiabetic retinopathy vascularand inflammatory diseaserdquo Journal of Diabetes Research vol2015 Article ID 582060 16 pages 2015

[17] E C Leal A Manivannan K-I Hosoya et al ldquoInduciblenitric oxide synthase isoform is a key mediator of leukostasisand blood-retinal barrier breakdown in diabetic retinopathyrdquoInvestigative Ophthalmology and Visual Science vol 48 no 11pp 5257ndash5265 2007

[18] H Vorum and J Ditzel ldquoDisturbance of inorganic phosphatemetabolism in diabetes mellitus its relevance to the pathogene-sis of diabetic retinopathyrdquo Journal of Ophthalmology vol 2014Article ID 135287 8 pages 2014

[19] ADas S StroudAMehta and S Rangasamy ldquoNew treatmentsfor diabetic retinopathyrdquoDiabetes Obesity andMetabolism vol17 no 3 pp 219ndash230 2015

[20] Q Lu L Lu W Chen H Chen X Xu and Z ZhengldquoRhoAmDia-1profilin-1 signaling targets microvascularendothelial dysfunction in diabetic retinopathyrdquo GraefersquosArchive for Clinical and Experimental Ophthalmology vol 253no 5 pp 669ndash680 2015

[21] Q Y Lu W Chen L Lu et al ldquoInvolvement of RhoAROCK1signaling pathway in hyperglycemia-induced microvascularendothelial dysfunction in diabetic retinopathyrdquo InternationalJournal of Clinical and Experimental Pathology vol 7 no 10 pp7268ndash7277 2014

[22] T Oshitari N Hata and S Yamamoto ldquoEndoplasmic reticulumstress and diabetic retinopathyrdquo Vascular Health and RiskManagement vol 4 no 1 pp 115ndash122 2008

[23] ZH Tang LWang F Zeng andK Zhang ldquoHuman genetics ofdiabetic retinopathyrdquo Journal of Endocrinological Investigationvol 37 no 12 pp 1165ndash1174 2014

[24] M A Reddy E Zhang and R Natarajan ldquoEpigenetic mech-anisms in diabetic complications and metabolic memoryrdquoDiabetologia vol 58 no 3 pp 443ndash455 2015

[25] L Perrone C Matrone and L P Singh ldquoEpigenetic modifica-tions and potential new treatment targets in diabetic retinopa-thyrdquo Journal of Ophthalmology vol 2014 Article ID 789120 10pages 2014

[26] F A A Kwa and T R Thrimawithana ldquoEpigenetic modifi-cations as potential therapeutic targets in age-related maculardegeneration and diabetic retinopathyrdquo Drug Discovery Todayvol 19 no 9 pp 1387ndash1393 2014

[27] B M Braunger S V Leimbeck A Schlecht C Volz H Jagleand E R Tamm ldquoDeletion of ocular transforming growthfactor 120573 signaling mimics essential characteristics of diabeticretinopathyrdquoThe American Journal of Pathology vol 185 no 6pp 1749ndash1768 2015

[28] K A Hussein K Choksi S Akeel et al ldquoBone morphogeneticprotein 2 a potential new player in the pathogenesis of diabeticretinopathyrdquo Experimental Eye Research vol 125 pp 79ndash882014

[29] H Wang H Shi J Zhang et al ldquoToll-like receptor 4 in bonemarrow-derived cells contributes to the progression of diabeticretinopathyrdquo Mediators of Inflammation vol 2014 Article ID858763 7 pages 2014

[30] T Sugiyama ldquoRole of P2X7 receptors in the development ofdiabetic retinopathyrdquoWorld Journal of Diabetes vol 5 no 2 pp141ndash145 2014

[31] L P Singh ldquoThioredoxin interacting protein (TXNIP) andpathogenesis of diabetic retinopathyrdquo Journal of Clinical ampExperimental Ophthalmology vol 4 2013

[32] C D A Stehouwer J Lambert A J M Donker and V W MVan Hinsbergh ldquoEndothelial dysfunction and pathogenesis ofdiabetic angiopathyrdquo Cardiovascular Research vol 34 no 1 pp55ndash68 1997

[33] M Brownlee ldquoBiochemistry and molecular cell biology ofdiabetic complicationsrdquo Nature vol 414 no 6865 pp 813ndash8202001

[34] X Jiang L Yang and Y Luo ldquoAnimal models of diabeticretinopathyrdquo Current Eye Research vol 40 no 8 pp 761ndash7712015

[35] A K W Lai and A C Y Lo ldquoAnimal models of diabeticretinopathy summary and comparisonrdquo Journal of DiabetesResearch vol 2013 Article ID 106594 29 pages 2013

[36] M Yoshioka T Kayo T Ikeda and A Koizumi ldquoA novel locusMody4 distal to D7Mit189 on chromosome 7 determines early-onset NIDDM in nonobese C57BL6 (Akita) mutant micerdquoDiabetes vol 46 no 5 pp 887ndash894 1997

[37] A J Barber D A Antonetti T S Kern et al ldquoThe Ins2Akitamouse as a model of early retinal complications in diabetesrdquoInvestigative Ophthalmology amp Visual Science vol 46 no 6 pp2210ndash2218 2005

[38] M J Gastinger A R Kunselman E E Conboy S K Bronsonand A J Barber ldquoDendrite remodeling and other abnormalitiesin the retinal ganglion cells of Ins2Akita diabetic micerdquo Inves-tigative Ophthalmology and Visual Science vol 49 no 6 pp2635ndash2642 2008

[39] J R Hombrebueno M Chen R G Penalva and H Xu ldquoLossof synaptic connectivity particularly in second order neuronsis a key feature of diabetic retinal neuropathy in the Ins2Akitamouserdquo PLoS ONE vol 9 no 5 Article ID e97970 2014

[40] Y Ha Y DunMThangaraju et al ldquoSigma receptor 1modulatesendoplasmic reticulum stress in retinal neuronsrdquo InvestigativeOphthalmology and Visual Science vol 52 no 1 pp 527ndash5402011

[41] J Wang T Takeuchi S Tanaka et al ldquoA mutation in theinsulin 2 gene induces diabetes with severe pancreatic 120573-cell dysfunction in the Mody mouserdquo The Journal of ClinicalInvestigation vol 103 no 1 pp 27ndash37 1999

[42] E P Rakoczy I S Ali Rahman N Binz et al ldquoCharacterizationof a mouse model of hyperglycemia and retinal neovascular-izationrdquo The American Journal of Pathology vol 177 no 5 pp2659ndash2670 2010

[43] J Wisniewska-Kruk I Klaassen I M C Vogels et al ldquoMolec-ular analysis of blood-retinal barrier loss in the Akimba mousea model of advanced diabetic retinopathyrdquo Experimental EyeResearch vol 122 pp 123ndash131 2014

[44] P Bogdanov L Corraliza J A Villena et al ldquoThe dbdbmousea useful model for the study of diabetic retinal neurodegenera-tionrdquo PLoS ONE vol 9 no 5 Article ID e97302 2014

[45] I S Samuels B A Bell A Pereira J Saxon and N S PeacheyldquoEarly retinal pigment epithelium dysfunction is concomitantwith hyperglycemia in mouse models of type 1 and type 2diabetesrdquo Journal of Neurophysiology vol 113 no 4 pp 1085ndash1099 2015

[46] A K H Cheung M K L Fung A C Y Lo et al ldquoAldosereductase deficiency prevents diabetes-induced blood-retinalbarrier breakdown apoptosis and glial reactivation in theretina of dbdb micerdquo Diabetes vol 54 no 11 pp 3119ndash31252005

[47] A Ly M F Scheerer S Zukunft et al ldquoRetinal proteomealterations in a mouse model of type 2 diabetesrdquo Diabetologiavol 57 no 1 pp 192ndash203 2014


[48] D M Sherry M M Wang J Bates and L J FrishmanldquoExpression of vesicular glutamate transporter 1 in the mouseretina reveals temporal ordering in development of rod vs coneandONvsOFF circuitsrdquo Journal of ComparativeNeurology vol465 no 4 pp 480ndash498 2003

[49] P Villacampa V Haurigot and F Bosch ldquoProliferativeretinopathies animal models and therapeutic opportunitiesrdquoCurrent Neurovascular Research vol 12 no 2 pp 189ndash198 2015

[50] J Ruberte E Ayuso M Navarro et al ldquoIncreased ocular levelsof IGF-1 in transgenic mice lead to diabetes-like eye diseaserdquoThe Journal of Clinical Investigation vol 113 no 8 pp 1149ndash11572004

[51] P Villacampa A Ribera S Motas et al ldquoInsulin-like growthfactor I (IGF-I)-induced chronic gliosis and retinal stress leadto neurodegeneration in a mouse model of retinopathyrdquo TheJournal of Biological Chemistry vol 288 no 24 pp 17631ndash176422013

[52] A Chronopoulos S Roy E Beglova K Mansfield L Wacht-man and S Roy ldquoHyperhexosemia-induced retinal vascularpathology in a novel primate model of diabetic retinopathyrdquoDiabetes vol 64 no 7 pp 2603ndash2608 2015

[53] P Osaadon X J Fagan T Lifshitz and J Levy ldquoA review of anti-VEGF agents for proliferative diabetic retinopathyrdquo Eye vol 28no 5 pp 510ndash520 2014

[54] J A Wells A R Glassman A R Ayala et al ldquoAfliberceptbevacizumab or ranibizumab for diabetic macular edemardquoTheNew England Journal of Medicine vol 372 no 13 pp 1193ndash12032015

[55] M J Elman A Ayala N M Bressler et al ldquoIntravitrealRanibizumab for diabetic macular edema with prompt versusdeferred laser treatment 5-year randomized trial resultsrdquoOph-thalmology vol 122 no 2 pp 375ndash381 2015

[56] S B Bressler T Almukhtar A Bhorade et al ldquoRepeated intrav-itreous ranibizumab injections for diabetic macular edema andthe risk of sustained elevation of intraocular pressure or theneed for ocular hypotensive treatmentrdquo JAMA Ophthalmologyvol 133 no 5 pp 589ndash597 2015

[57] P A Campochiaro R Channa B B Berger et al ldquoTreatmentof diabetic macular edema with a designed ankyrin repeatprotein that binds vascular endothelial growth factor a phaseIII studyrdquo American Journal of Ophthalmology vol 155 no 4pp 697ndashe2 2013

[58] R Simo and C Hernandez ldquoNovel approaches for treatingdiabetic retinopathy based on recent pathogenic evidencerdquoProgress in Retinal and Eye Research vol 48 pp 160ndash180 2015

[59] M I Nawaz M Abouammoh H A Khan A S AlhomidaM F Alfaran and M S Ola ldquoNovel drugs and their targets inthe potential treatment of diabetic retinopathyrdquoMedical ScienceMonitor vol 19 no 1 pp 300ndash308 2013

[60] L Wang N Wang H Tan Y Zhang and Y Feng ldquoProtectiveeffect of a Chinese Medicine formula He-Ying-Qing-Re For-mula on diabetic retinopathyrdquo Journal of Ethnopharmacologyvol 169 pp 295ndash304 2015

[61] H Huang J He D Johnson et al ldquoDeletion of placental growthfactor prevents diabetic retinopathy and is associated with aktactivation and HIF1120572-VEGF pathway inhibitionrdquo Diabetes vol64 no 1 pp 200ndash212 2015

[62] S Babapoor-Farrokhran K Jee B Puchner et alldquoAngiopoietin-like 4 is a potent angiogenic factor and anovel therapeutic target for patients with proliferative diabeticretinopathyrdquo Proceedings of the National Academy of Sciences ofthe United States vol 112 no 23 pp E3030ndashE3039 2015

[63] Y Jiang Q Zhang and J J Steinle ldquoIntravitreal injectionof IGFBP-3 restores normal insulin signaling in diabetic ratretinardquo PLoS ONE vol 9 no 4 Article ID e93788 2014

[64] H Huang W Li J He P Barnabie D Shealy and S AVinores ldquoBlockade of tumor necrosis factor alpha preventscomplications of diabetic retinopathyrdquo Journal of ClinicalampExperimental Ophthalmology vol 5 no 6 article 384 2014

[65] J He H Wang Y Liu W Li D Kim and H Huang ldquoBlockadeof vascular endothelial growth factor receptor 1 prevents inflam-mation and vascular leakage in diabetic retinopathyrdquo Journal ofOphthalmology vol 2015 Article ID 605946 11 pages 2015

[66] Y Chen Y Hu M Lin et al ldquoTherapeutic effects of PPAR120572agonists on diabetic retinopathy in type 1 diabetes modelsrdquoDiabetes vol 62 no 1 pp 261ndash272 2013

[67] A C Keech P Mitchell P A Summanen et al ldquoEffectof fenofibrate on the need for laser treatment for diabeticretinopathy (FIELD study) a randomised controlled trialrdquoTheLancet vol 370 no 9600 pp 1687ndash1697 2007

[68] R Simo and C Hernandez ldquoFenofibrate for diabetic retinopa-thyrdquoThe Lancet vol 370 no 9600 pp 1667ndash1668 2007

[69] N Sharma J L Ooi J Ong et al ldquoThe use of fenofibrate in themanagement of patients with diabetic retinopathy an evidence-based reviewrdquo Australian Family Physician vol 44 no 6 pp367ndash370 2015

[70] M Tikhonenko T A Lydic Y Wang et al ldquoRemodeling ofretinal fatty acids in an animal model of diabetes a decreasein long-chain polyunsaturated fatty acids is associated with adecrease in fatty acid elongases Elovl2 and Elovl4rdquoDiabetes vol59 no 1 pp 219ndash227 2010

[71] P Sapieha J Chen A Stahl et al ldquoOmega-3 polyunsaturatedfatty acids preserve retinal function in type 2 diabetic micerdquoNutrition and Diabetes vol 2 article e36 2012

[72] N Lois R VMcCarter C OrsquoNeill R J Medina and AW StittldquoEndothelial progenitor cells in diabetic retinopathrdquo Frontiersin Endocrinology vol 5 article 44 2014

[73] F Xiong X Du J Hu T Li S Du and Q Wu ldquoAltered retinalmicroRNA expression profiles in early diabetic retinopathy anin silico analysisrdquo Current Eye Research vol 39 no 7 pp 720ndash729 2014

[74] M A Ruiz B Feng and S Chakrabarti ldquoPolycomb repressivecomplex 2 regulates MiR-200b in retinal endothelial cellspotential relevance in diabetic retinopathyrdquo PLoS ONE vol 10no 4 Article ID e0123987 2015

[75] R Mortuza B Feng and S Chakrabarti ldquomiR-195 regulatesSIRT1-mediated changes in diabetic retinopathyrdquo Diabetologiavol 57 no 5 pp 1037ndash1046 2014

[76] E Di Marco J C Jha A Sharma J L Wilkinson-Berka K AJandeleit-Dahm and J B de Haan ldquoAre reactive oxygen speciesstill the basis for diabetic complicationsrdquo Clinical Science vol129 no 2 pp 199ndash216 2015

[77] M Zhang C An Y Gao R K Leak J Chen and F ZhangldquoEmerging roles of Nrf2 and phase II antioxidant enzymes inneuroprotectionrdquo Progress in Neurobiology vol 100 no 1 pp30ndash47 2013

[78] R C Taylor G Acquaah-Mensah M Singhal D Malhotraand S Biswal ldquoNetwork inference algorithms elucidate Nrf2regulation of mouse lung oxidative stressrdquo PLoS ComputationalBiology vol 4 no 8 Article ID e1000166 2008

[79] S M Tan and J B de Haan ldquoCombating oxidative stress indiabetic complications with Nrf2 activators how much is toomuchrdquo Redox Report vol 19 no 3 pp 107ndash117 2014


[80] J Jeevanandam M K Danquah S Debnath V S Meka andY S Chan ldquoOpportunities for nano-formulations in type 2diabetes mellitus treatmentsrdquo Current Pharmaceutical Biotech-nology vol 16 no 10 pp 853ndash870 2015

[81] D Yohan and B D Chithrani ldquoApplications of nanoparticles innanomedicinerdquo Journal of Biomedical Nanotechnology vol 10no 9 pp 2371ndash2392 2014

[82] X Cai S A Sezate S Seal and J F McGinnis ldquoSustainedprotection against photoreceptor degeneration in tubbymice byintravitreal injection of nanoceriardquo Biomaterials vol 33 no 34pp 8771ndash8781 2012

[83] X Cai S Seal and J F McGinnis ldquoSustained inhibitionof neovascularization in vldlr-- mice following intravitrealinjection of cerium oxide nanoparticles and the role of theASK1-P38JNK-NF-120581B pathwayrdquo Biomaterials vol 35 no 1 pp249ndash258 2014

[84] L Kong XCai X Zhou et al ldquoNanoceria extend photoreceptorcell lifespan in tubby mice by modulation of apoptosissurvivalsignaling pathwaysrdquo Neurobiology of Disease vol 42 no 3 pp514ndash523 2011

[85] X Cai J Yodoi S Seal and J F McGinnis ldquoNanoceria andthioredoxin regulate a common antioxidative gene network intubby micerdquo Advances in Experimental Medicine and Biologyvol 801 pp 829ndash836 2014

[86] A Liew and T OrsquoBrien ldquoThe potential of cell-based therapy fordiabetes and diabetes-related vascular complicationsrdquo CurrentDiabetes Reports vol 14 no 3 article 469 2014

[87] V Marchetti T U Krohne D F Friedlander and M Fried-lander ldquoStemming vision loss with stem cellsrdquo The Journal ofClinical Investigation vol 120 no 9 pp 3012ndash3021 2010

[88] Y Huang V Enzmann and S T Ildstad ldquoStem cell-basedtherapeutic applications in retinal degenerative diseasesrdquo StemCell Reviews and Reports vol 7 no 2 pp 434ndash445 2011

[89] S D Schwartz J-P Hubschman G Heilwell et al ldquoEmbryonicstem cell trials for macular degeneration a preliminary reportrdquoThe Lancet vol 379 no 9817 pp 713ndash720 2012

[90] S D Schwartz C D Regillo B L Lam et al ldquoHuman embry-onic stem cell-derived retinal pigment epithelium in patientswith age-related macular degeneration and Stargardtrsquos maculardystrophy follow-up of two open-label phase 12 studiesrdquo TheLancet vol 385 no 9967 pp 509ndash516 2015

[91] G C Davey S B Patil A OrsquoLoughlin and T OrsquoBrienldquoMesenchymal stem cell-based treatment for microvascularand secondary complications of diabetes mellitusrdquo Frontiers inEndocrinology vol 5 article 86 2014

[92] R Megaw and B Dhillon ldquoStem cell therapies in the manage-ment of diabetic retinopathyrdquo Current Diabetes Reports vol 14no 7 article 498 2014

[93] T Asahara T Murohara A Sullivan et al ldquoIsolation of putativeprogenitor endothelial cells for angiogenesisrdquo Science vol 275no 5302 pp 964ndash967 1997

[94] G Rajashekhar A Ramadan C Abburi et al ldquoRegenerativetherapeutic potential of adipose stromal cells in early stagediabetic retinopathyrdquo PLoS ONE vol 9 no 1 Article ID e846712014

[95] T A Mendel E B D Clabough D S Kao et al ldquoPericytesderived from adipose-derived stem cells protect against retinalvasculopathyrdquo PLoS ONE vol 8 no 5 Article ID e65691 2013

[96] E R Burnight P D Hsu D Ochoa et al ldquoUsing RNA-mediated genome editing to create an animal model of retinaldystrophy for analysis of in vivo CRISPRCAS9 treatment

efficacyrdquo Investigative Ophthalmology amp Visual Science vol 56abstract 3589 2015 The ARVO Annual Meeting

[97] E M Stone ldquoGene editing for gene- and cell based treatment ofinherited retinal diseaserdquo Investigative Ophthalmology amp VisualScience abstract 7 2015 The ARVO Annual Meeting

[98] S H Tsang ldquoPersonalized medicine patient specific stem cellsmouse models and therapy for retinal degenerationsrdquo TheARVO Annual Meeting Denver Colo USA abstract 8 May2015

[99] K J Wahlin C Kim J Maruotti et al ldquoGene-edited humanpluripotent stem cell derived 3D retinas for modeling photore-ceptor development and diseaserdquo Investigative OphthalmologyampVisual Science vol 56 abstract 3596 2015TheARVOAnnualMeeting

Submit your manuscripts athttpwwwhindawicom

Stem CellsInternational

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


MEDIATORSINFLAMMATION

of


Behavioural Neurology

EndocrinologyInternational Journal of



Disease Markers


BioMed Research International

OncologyJournal of



Oxidative Medicine and Cellular Longevity


PPAR Research

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Immunology ResearchHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

ObesityJournal of



Computational and Mathematical Methods in Medicine

OphthalmologyJournal of


Diabetes ResearchJournal of



Research and TreatmentAIDS


Gastroenterology Research and Practice


Parkinsonrsquos Disease

Evidence-Based Complementary and Alternative Medicine

Volume 2014Hindawi Publishing Corporationhttpwwwhindawicom


including increased vascular permeability edema inflamma-tory cell infiltration tissue destruction neovascularizationproinflammatory cytokines and chemokines in the retina [310] Some of the potential risk factors leading to the pathologyof other neovascular diseases also contribute to the pathologyof DR

Diabetes is the number one risk factor for the develop-ment of DR Type 1 diabetes (juvenile diabetes in whichno insulin is made) is more likely to develop vision lossthan type 2 diabetes (adult onset diabetes with insufficientinsulin synthesis) In addition race (Hispanic and AfricanAmericans) smoking hyperglycemia (high blood sugar)hypertension (high blood pressure) and hyperlipidemia(high cholesterol) or dyslipidemia are also high risk factors[11 12] Vascular endothelial growth factor (VEGF) elevationinduces a decrease in the tight-junction proteins and break-down of the BRB [13] an increase of leukostasis within retinalvessels [14] inflammation [15 16] upregulation of ICAM-1(intercellular adhesionmolecule-1) expression an increase inall NOS (nitric oxide synthase) isoforms [17] and ametabolicimbalance in inorganic phosphate [18] all of which have beenreported to contribute to DR pathology Multiple intercon-necting biochemical pathways including an increased polyolpathway elevated hexosamine biosynthesis pathway (HBP)activation of protein kinase C (PKC) hemodynamic changesand advanced glycation end product (AGE) formation [5 614 19] have also been found to play key roles in developmentof DR RhoA is a small guanosine-51015840-triphosphate-bindingprotein and acts as aGTPaseTheRhoAmDia-1 (mammaliandiaphanous homolog-1)profiling-1 [20] or RhoAROCK1(Rho-associated coiled-coil-containing protein kinase 1) [21]pathways have been shown to be involved in the pathologyof DR via triggering microvascular endothelial dysfunctionActivation of these pathways leads to the increase of growthfactors such as VEGF and insulin-like growth factor-1 (IGF-1) activation of the renin-angiotensin-aldosterone system(RAAS) subclinical inflammation and capillary occlusion[14] Also increased endoplasmic reticulum (ER) stress andoxidative stress [22] resulting from deregulation of ER andmitochondrial quality control by autophagymitophagy RPEdysfunction genetic variants and epigenetic changes in chro-matin such as DNA methylation histone posttranslationalmodifications affecting gene transcription and regulationby noncoding RNAs [23ndash26] have also been shown to beassociated with DR Interestingly deletion of transforminggrowth factor-120573 (TGF-120573) signaling results in undifferentiatedpericytes that cause retinal changes in structure and functionwhich mimic those of DR [27] Loss of other gene functionssuch as BMP2 (bone morphogenetic protein 2) [28] and Toll-like receptor 4 [29] has been implicated in the pathogenesisof DR Activation of the P2X7 receptor a member of ligand-gated membrane ion channels resulted in the formationof large plasma membrane pores that exacerbate the devel-opment of DR through induction of inflammation [30]Recently a prooxidant and proapoptotic thioredoxin inter-acting protein (TXNIP) was shown to be highly upregulatedin DR and by high glucose (HG) in retinal cells in cultureTXNIP binds to thioredoxin (Trx) inhibiting its oxidantscavenging and thiol-reducing capacity Hence prolonged

overexpression of TXNIP causes ROSRNS stress mitochon-drial dysfunction inflammation and premature cell deathin DR [31] Collectively hyperglycemia-induced vasculardysfunction and subsequent tissue damage have been pro-posed to act through the following four main pathways [3233] (1) increased polyol pathway flux in which cytosolicredox imbalance occurs with an increased NADHNAD+ratio via the sorbitol pathway resulting in a decrease incytosolic NADPH and cellular functions (2) increased AGEformation in which nonenzymatic glycosylation of proteinsand production of AGEs alter gene expression and AGEs alsoinduce the synthesis of numerous inflammatory cytokines(3) activation of PKC via the formation of intracellulardiacylglycerol (DAG) and AGEs which contributes to thegeneration of ROS which induces VEGF and multiple othergrowth factors and transcription factors and (4) increasedhexosamine pathway flux in which fructose-6-phosphate isconverted to glucosamine-6-phosphate and finally to uridinediphosphate119873-acetyl glucosamineThis modification resultsin changes in gene expression and protein function Howevereach of the four major pathways is linked by overproductionof superoxide and increased generation of ROS [33] whichprovides a common target for potential treatment

3 Animal Models

At present most animal models of DR are rodents miceand rats Based on the experimental approaches to induceDR these models can be classified as chemically inducedspontaneous and genetically created However knowledgeof the molecular mechanisms underlying the initiation anddevelopment of DR is insufficient and largely unknownbecause there are no reliable and appropriate good animalmodels of spontaneous diabetes in which phenotypic char-acteristics exactly mimic the pathogenesis of clinical DRAlthough various traditionally used animal models of DRpresent a number of pathological changes similar to thoseof human DR several pathological characteristics of humanDR such as retinal neovascularization cannot yet be fullymimicked in any existing animal model of DR [34]

31 Chemically Induced Model The commonly used strepto-zotocin (STZ) or alloxan induced DR animal models (ratsor mice) exhibit rapid onset of hyperglycemia (3 days aftertreatment) and some of the symptoms of early DR (type Idiabetes) such as loss of retinal pericytes and capillariesthickening of the vascular basement membrane vascularocclusion and increased vascular permeability [3 34 35]However variability of pathological characteristics such asloss of retinal capillaries ganglion cell death and reductionof retinal function has been reported among different speciesand even within the same species [3 34 35]

32 Akita Mice The Akita (Ins2Akita+minus) mouse a sponta-neous diabetes model for early stage of DR (type I diabetes)is caused by a missense mutation in the diabetogenic Insulin2 gene (Ins2) and is characterized by a rapid onset ofhyperglycemia and hypoinsulinemia and marked reduction










4 Current Therapies

























Acknowledgments


References













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of

























4 Current Therapies

























Acknowledgments


References













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of







































Acknowledgments


References













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of




























Acknowledgments


References













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of























Acknowledgments


References













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of













































































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of












































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of











































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of





















of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of
















Documents

Review Article Diabetic Retinopathy: Animal Models ...downloads.hindawi.com/journals/jdr/2016/3789217.pdf3. Animal Models At present, most animal models of DR are rodents, mice, and