Elia J. Duh

Vascular Endothelial Growth Factor–Induced Retinal Permeability Is Mediated by Protein Kinase C In Vivo and Suppressed by an Orally Effective β-Isoform–Selective Inhibitor

Increased vascular permeability and excessive neovas-cularization are the hallmarks of endothelial dysfunction, which can lead to diabetic macular edema and proliferative diabetic retinopathy in the eye. Vascular endothelial growth factor (VEGF) is an important mediator of ocular neovascularization and a known vasopermeability factor in nonocular tissues. In these studies, we demonstrate that intravitreal injection of VEGF rapidly activates protein kinase C (PKC) in the retina at concentrations observed clinically, inducing membrane translocation of PKC isoforms α, βII, and δ and > threefold increases in retinal vasopermeability in vivo. The effect of VEGF on retinal vascular permeability appears to be mediated predominantly by the β-isoform of PKC with >95% inhibition of VEGF-induced permeability by intravitreal or oral administration of a PKC β-isoform-selective inhibitor that did not inhibit histamine-mediated effects. These studies represent the first direct demonstration that VEGF can increase intraocular vascular permeability through activation of PKC in vivo and suggest that oral pharmacological therapies involving PKC β-isoform-selective inhibitors may prove efficacious for the treatment of VEGF-asso-ciated ocular disorders such as diabetic retinopathy.

Pigment Epithelium-Derived Factor Inhibits Retinal and Choroidal Neovascularization

In this study, we investigated whether overexpression of pigment epithelium‐derived factor (PEDF) by gene transfer can inhibit neovascularization by testing its effect in three different models of ocular neovascularization. Intravitreous injection of an adenoviral vector encoding PEDF resulted in expression of PEDF mRNA in the eye measured by RT‐PCR and increased immunohistochemical staining for PEDF protein throughout the retina. In mice with laser‐induced rupture of Bruchs membrane, choroidal neovascularization was significantly reduced after intravitreous injection of PEDF vector compared to injection of null vector or no injection. Subretinal injection of the PEDF vector resulted in prominent staining for PEDF in retinal pigmented epithelial cells and strong inhibition of choroidal neovascularization. In two models of retinal neovascularization (transgenic mice with increased expression of vascular endothelial growth factor (VEGF) in photoreceptors and mice with oxygen‐induced ischemic retinopathy), intravitreous injection of null vector resulted in decreased neovascularization compared to no injection, but intravitreous injection of PEDF vector resulted in further inhibition of neovascularization that was statistically significant. These data suggest that sustained increased intraocular expression of PEDF by gene therapy might provide a promising approach for treatment of ocular neovascularization.

Inducible Expression of Vascular Endothelial Growth Factor in Adult Mice Causes Severe Proliferative Retinopathy and Retinal Detachment

Transgenic mice with vascular endothelial growth factor (VEGF) driven by the rhodopsin promoter (rho/VEGF mice) develop neovascularization that originates from the deep capillary bed of the retina and grows into the subretinal space. In rho/VEGF mice, VEGF expression in photoreceptors begins between postnatal days 5 and 7, the period when the deep capillary bed is developing. An important question is whether or not the developmental stage of the deep capillary bed is critical for occurrence of neovascularization. Also, although rho/VEGF mice are extremely useful for the study of ocular neovascularization, there are some applications for which the early onset of VEGF expression is a disadvantage. In this study, we used the reverse tetracycline transactivator (rtTA) inducible promoter system coupled to either the rhodopsin or interphotoreceptor retinoid-binding protein (IRBP) promoter to control the time of onset of VEGF transgene expression in photoreceptors. In the absence of doxycycline, adult double-transgenic rho/rtTA-TRE/VEGF or IRBP/rtTA-TRE/VEGF mice showed little VEGF transgene expression and no phenotype. The addition of doxycycline to the drinking water resulted in prominent transgene expression and evidence of neovascularization within 3 to 4 days. Like rho/VEGF mice, the neovascularization originated from the deep capillary bed of the retina, but it was more extensive and caused outer retinal folds followed by total retinal detachment. Real-time polymerase chain reaction and enzyme-linked immunosorbent assay demonstrated that the mice with inducible expression of VEGF that developed retinal detachment had much higher ocular levels of VEGF mRNA and protein compared to rho/VEGF mice that manifest a much milder phenotype. These data demonstrate that regardless of developmental stage of the vascular bed, increased expression of VEGF in the retina is sufficient to cause neovascularization, and high levels of expression cause severe neovascularization and traction retinal detachment. Mice with inducible expression of VEGF in the retina provide a valuable new model of ocular neovascularization.

Transcription factors Sp1 and Sp3 alter vascular endothelial growth factor receptor expression through a novel recognition sequence

Kinase domain receptor (KDR) is a high affinity, endothelial cell-specific, autophosphorylating tyrosine kinase receptor for vascular endothelial growth factor. This transcriptionally regulated receptor is a critical mediator of endothelial cell (EC) growth and vascular development. In this study, we identify a DNA element modulating KDR promoter activity and evaluate the nuclear binding proteins accounting for a portion of the cell-type specificity of the region. KDR promoter luciferase activity was retained within −85/+296 and was 10–30-fold higher in EC than non-EC. Electrophoretic mobility shift assays demonstrated specific nuclear protein binding to −85/−64, and single point mutations suggested important binding nucleotides between −79/−68 with five critical bases between −74/−70 (5′-CTCCT-3′). DNA-protein complexes were displaced by Sp1 consensus sequence oligodeoxynucleotides and supershifted by Sp1- and Sp3-specific antibodies. Sp1 and Sp3 protein in EC nuclear extracts bound the −79/−68 region even when all surrounding classic Sp1 recognition sites were removed. Sp1 protein in nuclear extracts was 4–24-fold higher in EC than non-EC, whereas Sp3 was 3–7-fold higher. Sp1/Sp3 ratios in EC were 2–10-fold higher. Overexpression of Sp1 protein increased KDR promoter activity 3-fold in both EC and non-EC, whereas simultaneous co-expression of Sp3 attenuated this response. An Sp1 consensus sequence ciselement “decoy” reduced EC KDR promoter activity and mRNA expression by 85 and 69%, respectively. An antisense phosphorothioate oligodeoxynucleotide to Sp1 inhibited Sp1 and KDR protein expression by 66 and 68%, respectively, without changing Sp3 protein expression. These data illustrate that Sp1 and Sp3 modulate KDR promoter activity through a novel recognition binding sequence. However, since Sp1-mediated promoter activation is attenuated by Sp3, endothelial selective KDR promoter activity may be partially regulated by variations in the Sp1/Sp3 ratio.

The bZIP transcription factor Nrl stimulates rhodopsin promoter activity in primary retinal cell cultures.

In vitro DNA binding assays and transient transfection analysis with monkey kidney cells have implicated Nrl, a member of the Maf-Nrl subfamily of bZIP transcription factors, and the Nrl response element (NRE) in the regulation of rhodopsin expression. We have now further explored the role of the NRE and surrounding promoter elements. Using the yeast one-hybrid screen with integrated NRE and flanking DNA as bait, the predominant clone obtained was bovine Nrl. Recovery of truncated clones in the screen demonstrated that the carboxyl-terminal half of Nrl, which contains the basic and leucine zipper domains, is sufficient for DNA binding. To functionally dissect the rhodopsin promoter, transient expression studies with primary chick retinal cell cultures were performed. Deletion and mutation analyses identified two positive regulatory sequences: one between −40 and −84 base pairs (bp) and another between −84 and −130 bp. Activity of the −40 to −84 region was shown to be largely due to the NRE. On co-transfection with an NRL expression vector, there were 3-5-fold increases in the activity of rhodopsin promoter constructs containing an intact NRE but little or no effect with rhodopsin promoters containing a mutated or deleted NRE. Nrl was more effective than the related bZIP proteins, c-Fos and c-Jun, in stimulating rhodopsin promoter activity. The −84- to −130-bp region acted synergistically with the NRE to enhance both the level of basal expression and the degree of Nrl-mediated trans-activation. These studies support Nrl as a regulator of rhodopsin expression in vivo, identify an additional regulatory region just upstream of the NRE, and demonstrate the utility of primary retinal cell cultures for characterizing both the cis-acting response elements and trans-acting factors that regulate photoreceptor gene expression.

Renaming the DSCR1/Adapt78 gene family as RCAN: regulators of calcineurin.

Kelvin J. A. Davies,* Gennady Ermak,* Beverley A. Rothermel, Melanie Pritchard, Joseph Heitman, Joohong Ahnn, Flavio Henrique-Silva, Dana Crawford, Silvia Canaider,** Pierluigi Strippoli,** Paolo Carinci,** Kyung-Tai Min, Deborah S. Fox, Kyle W. Cunningham, Rhonda Bassel-Duby, Eric N. Olson, Zhuohua Zhang, R. Sanders Williams, Hans-Peter Gerber,*** Merce Perez-Riba, Hisao Seo, Xia Cao, Claude B. Klee, Juan Miguel Redondo, Lois J. Maltais, Elspeth A. Bruford, Sue Povey, Jeffery D. Molkentin,**** Frank D. McKeon, Elia J. Duh, Gerald R. Crabtree,§§§§ Martha S. Cyert, Susana de la Luna, and Xavier Estivill

Pigment Epithelium-Derived Factor Contributes to Insulin Resistance in Obesity

Obesity is a major risk factor for insulin resistance; however, the factors linking these disorders are not well defined. Herein, we show that the noninhibitory serine protease inhibitor, pigment epithelium-derived factor (PEDF), plays a causal role in insulin resistance. Adipocyte PEDF expression and serum levels are elevated in several rodent models of obesity and reduced upon weight loss and insulin sensitization. Lean mice injected with recombinant PEDF exhibited reduced insulin sensitivity during hyperinsulinemic-euglycemic clamps. Acute PEDF administration activated the proinflammatory serine/threonine kinases c-Jun terminal kinase and extracellular regulated kinase in both muscle and liver, which corresponded with reduced insulin signal transduction. Prolonged PEDF administration stimulated adipose tissue lipolysis, resulted in ectopic lipid deposition, and reduced insulin sensitivity, while neutralizing PEDF in obese mice enhanced insulin sensitivity. Overall, these results identify a causal role for PEDF in obesity-induced insulin resistance.

TNFα Is Required for Late BRB Breakdown in Diabetic Retinopathy, and Its Inhibition Prevents Leukostasis and Protects Vessels and Neurons from Apoptosis

PURPOSE Blood-retinal barrier [BRB] breakdown, characteristic of diabetic retinopathy (DR), is believed to depend on inflammation and apoptosis. Retinal inflammation is almost completely suppressed in the absence of TNFα, which is also associated with apoptosis. This study was conducted to determine the role of TNFα in these diabetic complications. METHODS Diabetes was induced with streptozotocin in Tnfa knockout (KO) mice, to provide a chemical model of diabetes, and Tnfa (KO) mice were crossed with Ins2(Akita) mice to generate a genetic model, with both models being devoid of TNFα. The BRB was assessed at 1, 1.5, 3, and 6 months. Leukostasis was assessed using FITC-conjugated ConA to label leukocytes. Apoptosis was assessed with TUNEL and activated caspase-3 staining. PECAM1 identified endothelial cells, and SMA identified pericytes. RESULTS At 1 month of diabetes, the absence of TNFα had no effect on DR-associated BRB breakdown, even though it prevented retinal leukostasis, demonstrating that neither TNFα nor inflammation is essential for early BRB breakdown in DR in either model of diabetes. At 3 months of diabetes, BRB breakdown was significantly suppressed and at 6 months, it was completely prevented in the absence of TNFα in both models, showing that TNFα is essential for progressive BRB breakdown. DR-mediated apoptosis in the retina, which appears to involve endothelial cells, pericytes, and neurons, was inhibited in the absence of TNFα in both models. CONCLUSIONS Although neither TNFα nor inflammation is necessary for early BRB breakdown in DR, TNFα is critical for later complications and would be a good therapeutic target for the prevention of the progressive BRB breakdown, retinal leukostasis, and apoptosis associated with DR.

Nanoparticle diffusion in, and microrheology of, the bovine vitreous ex vivo.

Intravitreal injection of biodegradable nanoparticles (NP) holds promise for gene therapy and drug delivery to the back of the eye. In some cases, including gene therapy, NP need to diffuse rapidly from the site of injection in order to reach targeted cell types in the back of the eye, whereas in other cases it may be preferred for the particles to remain at the injection site and slowly release drugs that may then diffuse to the site of action. We studied the movements of polystyrene (PS) NP of various sizes and surface chemistries in fresh bovine vitreous. PS NP as large as 510nm rapidly penetrated the vitreous gel when coated with polyethylene glycol (PEG), whereas the movements of NP 1190nm in diameter or larger were highly restricted regardless of surface chemistry owing to steric obstruction. PS NP coated with primary amine groups (NH2) possessed positively charged surfaces at the pH of bovine vitreous (pH=7.2), and were immobilized within the vitreous gel. In comparison, PS NP coated with COOH (possessing negatively charged surfaces) in the size range of 100-200nm and at particle concentrations below 0.0025% (w/v) readily diffused through the vitreous meshwork; at higher concentrations (~0.1% w/v), these nanoparticles aggregated within vitreous. Based on the mobility of different sized PEGylated PS NP (PS-PEG), we estimated the average mesh size of fresh bovine vitreous to be ~550±50nm. The bovine vitreous behaved as an impermeable elastic barrier to objects sized 1190nm and larger, but as a highly permeable viscoelastic liquid to non-adhesive objects smaller than 510nm in diameter. Guided by these studies, we next sought to examine the transport of drug- and DNA-loaded nanoparticles in bovine vitreous. Biodegradable NP with a diameter of 227nm, composed of a poly(lactic-co-glycolic acid) (PLGA)-based core coated with poly(vinyl alcohol) rapidly penetrated vitreous. Rod-shaped, highly-compacted CK30PEG10k/DNA with PEG coating (neutral surface charge; hydrodynamic diameter ~60nm) also diffused rapidly within vitreous. These findings will help guide the development of nanoparticle-based therapeutics for the treatment of vision-threatening ocular diseases.

Different effects of angiopoietin-2 in different vascular beds: new vessels are most sensitive

In this study, we used double transgenic mice with inducible expression of angiopoietin‐2 (Ang2) to investigate the role of Ang2 in the retinal and choroidal circulations and in three models of ocular neovascularization (NV). Mice with induced expression of Ang2 ubiquitously, or specifically in the retina, survived and appeared grossly normal. They also had normal‐appearing retinal and choroidal circulations, demonstrating that high levels of Ang2 did not induce regression of mature retinal or choroidal vessels. When Ang2 expression was induced soon after birth, there was increased density of the deep capillary bed on postnatal day (P) 11 that returned to normal by P18, the time that retinal vascular development is usually completed. In mice with ischemic retinopathy, induction of Ang2 during the ischemic period resulted in a significant increase in retinal NV, but induction of Ang2 at a later time point when ischemia (and vascular endothelial growth factor [VEGF]) was less, hastened regression of NV. In triple transgenic mice that coexpressed VEGF and Ang2, the increased expression of Ang2 inhibited VEGF‐induced NV in the retina. Increased expression of Ang2 also resulted in regression of choroidal neovascularization. These data suggest that ocular neovascularization, but not mature retinal or choroidal vessels, is sensitive to Ang2; a high Ang2/VEGF ratio promotes regression, while high Ang2 in the setting of hypoxia and/or concomitantly high Ang2 and VEGF stimulate neovascularization.