Modulation of VEGF-induced retinal vascular permeability by peroxisome proliferator-activated receptor-β/δ

VEGF-A165a and angiopoietin-2 differently affect the barrier formed by retinal endothelial cells

Exposure to VEGF-A165a over several days leads to a persistent dysfunction of the very tight barrier formed by immortalized endothelial cells of the bovine retina (iBREC). Elevated permeability of the barrier is indicated by low cell index values determined by electric cell-substrate impedance measurements, by lower amounts of claudin-1, and by disruption of the homogenous and continuous staining of vascular endothelial cadherin at the plasma membrane. Because of findings that suggest modulation of VEGF-A's detrimental effects on the inner blood-retina barrier by the angiogenic growth factor angiopoietin-2, we investigated in more detail in vitro whether this growth factor indeed changes the stability of the barrier formed by retinal endothelial cells or modulates effects of VEGF-A. In view of the clinical relevance of anti-VEGF therapy, we also studied whether blocking VEGF-A-driven signaling is sufficient to prevent barrier dysfunction induced by a combination of both growth factors. Although angiopoietin-2 stimulated proliferation of iBREC, the formed barrier was not weakened at a concentration of 3 nM: Cell index values remained high and expression or subcellular localization of claudin-1 and vascular endothelial cadherin, respectively, were not affected. Angiopoietin-2 enhanced the changes induced by VEGF-A165a and this was more pronounced at lower concentrations of VEGF-A165a. Specific inhibition of the VEGF receptors with tivozanib as well as interfering with binding of VEGF-A to its receptors with bevacizumab prevented the detrimental effects of the growth factors; dual binding of angiopoietin-2 and VEGF-A by faricimab was marginally more efficient. Uptake of extracellular angiopoietin-2 by iBREC can be efficiently prevented by addition of faricimab which is also internalized by the cells. Exposure of the cells to faricimab over several days stabilized their barrier, confirming that inhibition of VEGF-A signaling is not harmful to this cell type. Taken together, our results confirm the dominant role of VEGF-A165a in processes resulting in increased permeability of retinal endothelial cells in which angiopoietin-2 might play a minor modulating role.

Effectiveness of carbonic anhydrase inhibitor loaded nanoparticles in the treatment of diabetic retinopathy

Diabetic Retinopathy (DRP) is a disease consisting of all the structural and functional changes that develop in the retinal layer of the eye due to diabetes. DRP is the most important cause of blindness between the ages of 20-74 in the world, and the most successful standard treatment option in the treatment of DRP is intravitreal injections. To synthesize acetazolamide loaded nanoparticles to be applied intravitreal treatment of DRP and to examine thein vitroefficacy of the nanoparticles. ACZ loaded PHBV nanoparticles (PHBV-ACZ NPs) formulations were prepared. Nanoparticles with a particle size of 253.20 ± 0.55 nm. A DRP model was established and characterized in HRMEC cells. The effect of the nanoparticles on permeability has been investigated and carrier proteins in BRB due to the development of DRP has been investigated. To establish thein vitroDRP model, HRMEC was stimulated with Recombinant human 165 Vascular Endothelial Growth Factor (VEGF), thereby temporarily reducing the expression levels of endothelial junction proteins, increasing the number of intercellular spaces in the monolayers of HRMECs. It was determined that after the cells were exposed to Carbonic anhydrase inhibitors (CAI) loaded nanoparticles, permeability decreased and protein expression increased.

Assessment of Inner Blood-Retinal Barrier: Animal Models and Methods

Proper functioning of the neural retina relies on the unique retinal environment regulated by the blood-retinal barrier (BRB), which restricts the passage of solutes, fluids, and toxic substances. BRB impairment occurs in many retinal vascular diseases and the breakdown of BRB significantly contributes to disease pathology. Understanding the different molecular constituents and signaling pathways involved in BRB development and maintenance is therefore crucial in developing treatment modalities. This review summarizes the major molecular signaling pathways involved in inner BRB (iBRB) formation and maintenance, and representative animal models of eye diseases with retinal vascular leakage. Studies on Wnt/β-catenin signaling are highlighted, which is critical for retinal and brain vascular angiogenesis and barriergenesis. Moreover, multiple in vivo and in vitro methods for the detection and analysis of vascular leakage are described, along with their advantages and limitations. These pre-clinical animal models and methods for assessing iBRB provide valuable experimental tools in delineating the molecular mechanisms of retinal vascular diseases and evaluating therapeutic drugs.

Selective drug delivery to the retinal cells: Biological barriers and avenues

Retinal drug delivery is a challenging, but important task, because most retinal diseases are still without any proper therapy. Drug delivery to the retina is hampered by the anatomical and physiological barriers resulting in minimal bioavailability after topical ocular and systemic administrations. Intravitreal injections are current method-of-choice in retinal delivery, but these injections show short duration of action for small molecules and low target bioavailability for many protein, gene based drugs and nanomedicines. State-of-art delivery systems are based on prolonged retention, controlled drug release and physical features (e.g. size and charge). However, drug delivery to the retina is not cell-specific and these approaches do not facilitate intracellular delivery of modern biological drugs (e.g. intracellular proteins, RNA based medicines, gene editing). In this focused review we highlight biological factors and mechanisms that form the basis for the selective retinal drug delivery systems in the future. Therefore, we are presenting current knowledge related to retinal membrane transporters, receptors and targeting ligands in relation to nanomedicines, conjugates, extracellular vesicles, and melanin binding. These issues are discussed in the light of retinal structure and cell types as well as future prospects in the field. Unlike in some other fields of targeted drug delivery (e.g. cancer research), selective delivery technologies have been rarely studied, even though cell targeted delivery may be even more feasible after local administration into the eye.

Siponimod As a Novel Inhibitor of Retinal Angiogenesis: In Vitro and In Vivo Evidence of Therapeutic Efficacy

Sphingosine-1-phosphate (S1P) receptors control endothelial cell proliferation, migration, and survival. Evidence of the ability of S1P receptor modulators to influence multiple endothelial cell functions suggests their potential use for antiangiogenic effect. The main purpose of our study was to investigate the potential of siponimod for the inhibition of ocular angiogenesis in vitro and in vivo. We investigated the effects of siponimod on the metabolic activity (thiazolyl blue tetrazolium bromide assay), cell toxicity (lactate dehydrogenase release), basal proliferation and growth factor-induced proliferation (bromodeoxyuridine assay), and migration (transwell migration assay) of human umbilical vein endothelial cells (HUVEC) and retinal microvascular endothelial cells (HRMEC). The effects of siponimod on HRMEC monolayer integrity, barrier function under basal conditions, and tumor necrosis factor alpha (TNF-α)-induced disruption were assessed using the transendothelial electrical resistance and fluorescein isothiocyanate-dextran permeability assays. Siponimod's effect on TNF-α-induced distribution of barrier proteins in HRMEC was investigated using immunofluorescence. Finally, the effect of siponimod on ocular neovascularization in vivo was assessed using suture-induced corneal neovascularization in albino rabbits. Our results show that siponimod did not affect endothelial cell proliferation or metabolic activity but significantly inhibited endothelial cell migration, increased HRMEC barrier integrity, and reduced TNF-α-induced barrier disruption. Siponimod also protected against TNF-α-induced disruption of claudin-5, zonula occludens-1, and vascular endothelial-cadherin in HRMEC. These actions are mainly mediated by sphingosine-1-phosphate receptor 1 modulation. Finally, siponimod prevented the progression of suture-induced corneal neovascularization in albino rabbits. In conclusion, the effects of siponimod on various processes known to be involved in angiogenesis support its therapeutic potential in disorders associated with ocular neovascularization. SIGNIFICANCE STATEMENT: Siponimod is an extensively characterized sphingosine-1-phosphate receptor modulator already approved for the treatment of multiple sclerosis. It inhibited retinal endothelial cell migration, potentiated endothelial barrier function, protected against tumor necrosis factor alpha-induced barrier disruption, and also inhibited suture-induced corneal neovascularization in rabbits. These results support its use for a novel therapeutic indication in the management of ocular neovascular diseases.

Cathepsin S Knockdown Suppresses Endothelial Inflammation, Angiogenesis, and Complement Protein Activity under Hyperglycemic Conditions In Vitro by Inhibiting NF-κB Signaling

Hyperglycemia plays a key role in the development of microvascular complications, endothelial dysfunction (ED), and inflammation. It has been demonstrated that cathepsin S (CTSS) is activated in hyperglycemia and is involved in inducing the release of inflammatory cytokines. We hypothesized that blocking CTSS might alleviate the inflammatory responses and reduce the microvascular complications and angiogenesis in hyperglycemic conditions. In this study, we treated human umbilical vein endothelial cells (HUVECs) with high glucose (HG; 30 mM) to induce hyperglycemia and measured the expression of inflammatory cytokines. When treated with glucose, hyperosmolarity could be linked to cathepsin S expression; however, many have mentioned the high expression of CTSS. Thus, we made an effort to concentrate on the immunomodulatory role of the CTSS knockdown in high glucose conditions. We validated that the HG treatment upregulated the expression of inflammatory cytokines and CTSS in HUVEC. Further, siRNA treatment significantly downregulated CTSS expression along with inflammatory marker levels by inhibiting the nuclear factor-kappa B (NF-κB) mediated signaling pathway. In addition, CTSS silencing led to the decreased expression of vascular endothelial markers and downregulated angiogenic activity in HUVECs, which was confirmed by a tube formation experiment. Concurrently, siRNA treatment reduced the activation of complement proteins C3a and C5a in HUVECs under hyperglycemic conditions. These findings show that CTSS silencing significantly reduces hyperglycemia-induced vascular inflammation. Hence, CTSS may be a novel target for preventing diabetes-induced microvascular complications.

Beovu, but not Lucentis impairs the function of the barrier formed by retinal endothelial cells in vitro

Because rare, but severe adverse effects, i.e. retinal vasculitis or retinal vein occlusion, have been observed after repetitive intravitreal injections of VEGF-A-binding single-chain variable fragment brolucizumab (Beovu), we investigated its possible impact on the barrier formed by immortalized bovine retinal endothelial cells (iBREC) in comparison to that of the VEGF-A-binding Fab fragment ranibizumab (Lucentis). As a measure of stability of the barrier formed by a confluent monolayer of iBREC, we determined the cell index over seven days by continuous electric cell-substrate impedance measurements: Beovu but not Lucentis indeed significantly lowered the cell index, evident about 1.5 days after its addition, pointing to barrier impairment. Early after addition of Beovu, amounts of the integrins α5 and β1-subunits of the fibronectin receptor-had changed in opposite ways, suggesting an effect on cell adhesion due to hindered dimer formation. After exposure for eight days to Beovu, levels of claudin-1-an essential part of the iBREC barrier-were significantly lower, less claudin-1 was located at the plasma membrane after exposure to the VEGF-A antagonist for five days. Beovu did not induce secretion of inflammatory cytokines or VEGF-A. Interestingly, polysorbate-80-component of Beovu-but not polysorbate-20-in Lucentis-slightly, but significantly lowered the cell index, also associated with reduced claudin-1 expression. In summary, our results indicate that Beovu changes the behavior of retinal endothelial cells, thus providing an alternative "non-immunological" explanation for the most relevant of observed side effects.

RNA-Seq Provides Insights into VEGF-Induced Signaling in Human Retinal Microvascular Endothelial Cells: Implications in Retinopathy of Prematurity

The pathophysiology of retinopathy of prematurity (ROP) is postulated to first involve delayed intraretinal vascularization, followed by intravitreal neovascularization (IVNV). Although intravitreal agents that reduce the bioactivity of vascular endothelial growth factor (VEGF) are used to treat IVNV, concerns exist regarding their effects on intraretinal vascularization. In an experimental ROP model, VEGF receptor 2 (VEGFR2) knockdown in retinal endothelial cells reduced IVNV and promoted intraretinal vascularization, whereas knockdown of a downstream effector, signal transducer and activator of transcription 3 (STAT3) in retinal endothelial cells only reduced IVNV. In this study, we tested the hypothesis that the different pathways involved in VEGF-triggered VEGFR2 signaling and VEGF-triggered STAT3 signaling in retinal endothelial cells would allow us to delineate signaling pathways involved in IVNV from those involved in intraretinal vascularization in ROP. To address our hypothesis, we used RNA-sequencing and pathway enrichment analysis to determine changes in the transcriptome of cultured human retinal microvascular endothelial cells (HRMECs). Of the enriched pathways, inactivation of oncostatin M signaling was predicted by either KDR or STAT3 knockdown in the presence of VEGF. Activation of kinetochore metaphase signaling was predicted by KDR knockdown, whereas inactivation was predicted by STAT3 knockdown in the presence of VEGF. Inactivation of signaling by the Rho family of GTPases was predicted by KDR knockdown, but activation was predicted by STAT3 knockdown in the presence of VEGF. Taken together, our data identified unique signaling pathway differences between VEGF-triggered VEGFR2 and VEGF-triggered STAT3 in HRMECs that might have implications in ROP.

Impairment of the Retinal Endothelial Cell Barrier Induced by Long-Term Treatment with VEGF-A165 No Longer Depends on the Growth Factor's Presence

As responses of immortalized endothelial cells of the bovine retina (iBREC) to VEGF-A₁₆₅ depend on exposure time to the growth factor, we investigated changes evident after long-term treatment for nine days. The cell index of iBREC cultivated on gold electrodes—determined as a measure of permeability—was persistently reduced by exposure to the growth factor. Late after addition of VEGF-A₁₆₅ protein levels of claudin-1 and CD49e were significantly lower, those of CD29 significantly higher, and the plasmalemma vesicle associated protein was no longer detected. Nuclear levels of β-catenin were only elevated on day two. Extracellular levels of VEGF-A—measured by ELISA—were very low. Similar to the binding of the growth factor by brolucizumab, inhibition of VEGFR2 by tyrosine kinase inhibitors tivozanib or nintedanib led to complete, although transient, recovery of the low cell index when added early, though was inefficient when added three or six days later. Additional inhibition of other receptor tyrosine kinases by nintedanib was similarly unsuccessful, but additional blocking of c-kit by tivozanib led to sustained recovery of the low cell index, an effect observed only when the inhibitor was added early. From these data, we conclude that several days after the addition of VEGF-A₁₆₅ to iBREC, barrier dysfunction is mainly sustained by increased paracellular flow and impaired adhesion. Even more important, these changes are most likely no longer VEGF-A-controlled.

Role of the Endothelium in Neonatal Diseases

In both fetal and neonatal physiologic and pathologic processes in most organs, endothelial cells are known to play critical roles. Although the endothelium is one of the most ubiquitous cell type in the body, the tight adherence to the blood vessel wall has made it difficult to study their diverse function and structure. In this article, we have reviewed endothelial cell origins and explored their heterogeneity in terms of structure, function, developmental changes, and their role in inflammatory and infectious diseases. We have also attempted to evaluate the untapped therapeutic potentials of endothelial cells in neonatal disease. This article comprises various peer-reviewed studies, including ours, and an extensive database literature search from EMBASE, PubMed, and Scopus.

Blocking of VEGF-A is not sufficient to completely revert its long-term effects on the barrier formed by retinal endothelial cells

The VEGF-A-induced functional impairment of the barrier formed by retinal endothelial cells (REC) can be prevented and even - at least temporarily - reverted by trapping the growth factor in a complex with a VEGF-binding protein or by inhibiting the activity of the VEGF receptor 2 (VEGFR2). In an approach to emulate the clinically relevant situation of constant exposure to effectors, we investigated (1) whether prolonged exposure to VEGF-A₁₆₅ for up to six days results in a different type of disturbance of the barrier formed by immortalized bovine REC (iBREC) and (2) whether alterations of the barrier induced by VEGF-A₁₆₅ can indeed be sustainably reverted by subsequent treatment with the VEGF-A-binding proteins ranibizumab or brolucizumab. As a measure of barrier integrity, the cell index (CI) of iBREC cultivated on gold electrodes was monitored continuously. CI values declined shortly after addition of the growth factor and then remained low for more than six days over which considerable amounts of both extra- and intracellular VEGF-A were measured. Interestingly, the specific VEGFR2 inhibitor nintedanib normalized the lowered CI when added to iBREC pre-treated with VEGF-A₁₆₅ for one day, but failed to do so when cells had been exposed to the growth factor for six days. Expression of the tight junction (TJ) protein claudin-5 was unchanged early after addition of VEGF-A₁₆₅ but higher after prolonged treatment, whereas decreased amounts of the TJ-protein claudin-1 remained low, and increased expression of the plasmalemma vesicle-associated protein (PLVAP) remained high during further exposure. After two days, the characteristic even plasma membrane stainings of claudin-1 or claudin-5 appeared weaker or disordered, respectively. After six days the subcellular localization of claudin-5 was similar to that of control cells again, but claudin-1 remained relocated from the plasma membrane. To counteract these effects of VEGF-A₁₆₅, brolucizumab or ranibizumab was added after one day, resulting in recovery of the then lowered CI to normal values within a few hours. However, despite the VEGF antagonist being present, the CI declined again two days later to values that were just slightly higher than without VEGF inhibition during further assessment for several days. At this stage, neither the supernatants nor whole cell extracts from iBREC treated with VEGF-A₁₆₅ and its antagonists contained significant amounts of free VEGF-A. Treatment of VEGF-A₁₆₅-challenged iBREC with ranibizumab or brolucizumab normalized expression of claudin-1 and claudin-5, but not completely that of PLVAP. Interestingly, the characteristic VEGF-A₁₆₅-induced relocalization of claudin-1 from the plasma membrane was reverted within one day by any of the VEGF antagonists, but reappeared despite their presence after further exposure for several days. Taken together, barrier dysfunction induced by VEGF-A₁₆₅ results from deregulated para- and transcellular flow but the precise nature or magnitude of underlying changes on a molecular level clearly depend on the time of exposure, evolving into a stage of VEGF-A₁₆₅-independent barrier impairment. These findings also provide a plausible explanation for resistance to treatment with VEGF-A antagonists frequently observed in clinical practice.

Therapeutic Effects of Fenofibrate Nano-Emulsion Eye Drops on Retinal Vascular Leakage and Neovascularization

Macular edema caused by retinal vascular leakage and ocular neovascularization are the leading causes of severe vision loss in diabetic retinopathy (DR) and age-related macular degeneration (AMD) patients. Oral administration of fenofibrate, a PPARα agonist, has shown therapeutic effects on macular edema and retinal neovascularization in diabetic patients. To improve the drug delivery to the retina and its efficacy, we have developed a nano-emulsion-based fenofibrate eye drop formulation that delivered significantly higher amounts of the drug to the retina compared to the systemic administration, as measured by liquid chromatography-mass spectrometer (LC-MS). The fenofibrate eye drop decreased leukocytes adherent to retinal vasculature and attenuated overexpression of multiple inflammatory factors in the retina of very low-density lipoprotein receptor knockout (Vldlr^-/-) mice, a model manifesting AMD phenotypes, and streptozotocin-induced diabetic rats. The fenofibrate eye drop also reduced retinal vascular leakage in these models. The laser-induced choroidal neovascularization was also alleviated by the fenofibrate eye drop. There were no detectable ocular toxicities associated with the fenofibrate eye drop treatment. These findings suggest that fenofibrate can be delivered efficiently to the retina through topical administration of the nano-emulsion eye drop, which has therapeutic potential for macular edema and neovascularization.

Evans blue staining to detect deep blood vessels in peripheral retina for observing retinal pathology in early-stage diabetic rats

AIM

To observe and compare the statistical significance of superficial and deep vascular leakage in the pathological changes of the diabetic rats retina after the Evans blue (EB) perfusion, and utilize the modified whole-retina spreading method to make the slides while protecting the periphery of the retina.

METHODS

The Sprague-Dawley (SD) rats were randomly divided into 6 groups. Each group named as the normal groups for 4, 8, and 12wk and the diabetic groups for 4, 8, and 12wk. The EB was injected into the cardiovascular system of the rats at the different time points. The retina of each group was obtained for observation.

RESULTS

The superficial vascular leakage was found in all 6 groups. The size of leakage area of superficial retinal blood vessels was (0.54±0.23)%, (0.65±0.11)%, and (0.58±0.10)% in normal group. No notable leakage was found in the deep blood vessels [(0.03±0.04)%, (0.03±0.05)%, and (0.03±0.05)%]. The deep retinal vascular leakage was found in the peripheral retina of diabetic rats. The size of leakage area of superficial retinal blood vessels in diabetic group were (0.53±0.22)%, (0.69±0.16)%, and (0.52±0.11)%. The leakage areas of deep blood vessels were (0.54±0.50)%, (1.42±0.16)%, and (1.80±0.07)% at 4, 8, and 12wk, respectively. There was a statistically difference of the leakage area between the 8th week and the 4th week of diabetes group (P=0.003). The statistically significant difference between the diabetes and the control groups was noted at 4wk and 8wk (P<0.001).

CONCLUSION

The main retinal pathological changes of early-stage diabetic rats are the vascular leakage of the periphery of deep retina. Diabetic rats modeled after 8wk have semi-quantitative statistical difference compared with the normal rats, thus early intervention treatment research can start at this time point.

Type of culture medium determines properties of cultivated retinal endothelial cells: induction of substantial phenotypic conversion by standard DMEM

Contradictory behavior of microvascular retinal endothelial cells (REC) - a reliable in vitro model to study retinal diseases - have recently been reported which might result from cultivating the cells in standard DMEM not optimized for this cell type. Therefore, we studied DMEM's effects on phenotype and behavior of immortalized bovine REC. Cells were cultivated in endothelial cell growth medium (ECGM) until a confluent monolayer was reached and then further kept for 1-4 days in ECGM, DMEM, or mixes thereof all supplemented with 5% fetal bovine serum, endothelial cell growth supplement, 90 μg/ml heparin, and 100 nM hydrocortisone. Within hours of cultivation in DMEM, the cell index - measured to assess the cell layer's barrier function - dropped to ~5% of the initial value and only slowly recovered, not only accompanied by stronger expression of HSP70 mRNA and secretion of interleukin-6, but also by lower expressions of tight junction proteins claudin-5, claudin-1 or of the marker of cell type conversion caveolin-1. Altered subcellular localizations of EC-typic claudin-5, vascular endothelial cadherin and von Willebrand factor were also observed. Taken together, all experiments with (retinal) EC cultivated in common DMEM need to be interpreted very cautiously and should at least include phenotypic validation.

Small-Molecule Modulation of PPARs for the Treatment of Prevalent Vascular Retinal Diseases

Vascular-related retinal diseases dramatically impact quality of life and create a substantial burden on the healthcare system. Age-related macular degeneration, diabetic retinopathy, and retinopathy of prematurity are leading causes of irreversible blindness. In recent years, the scientific community has made great progress in understanding the pathology of these diseases and recent discoveries have identified promising new treatment strategies. Specifically, compelling biochemical and clinical evidence is arising that small-molecule modulation of peroxisome proliferator-activated receptors (PPARs) represents a promising approach to simultaneously address many of the pathological drivers of these vascular-related retinal diseases. This has excited academic and pharmaceutical researchers towards developing new and potent PPAR ligands. This review highlights recent developments in PPAR ligand discovery and discusses the downstream effects of targeting PPARs as a therapeutic approach to treating retinal vascular diseases.

Transport and fate of aflibercept in VEGF-A165-challenged retinal endothelial cells

Retinal vessels are at least in part involved in clearing of Fc terminus-containing proteins from the vitreous. In vitro, the Fc fusion protein aflibercept is transported through a monolayer of unchallenged immortalized bovine retinal endothelial cells (iBREC), mediated by the neonatal Fc receptor (FcRn), but part of the Fc fusion protein is also degraded. Aflibercept's target VEGF-A not only enhances the permeability of REC by destabilization of tight junctions (TJs) thereby allowing for paracellular flow, it may also lower the intracellular stability of the Fc fusion protein by changing its binding properties to the FcRn. Therefore, we investigated the transport and fate of aflibercept in VEGF-A₁₆₅-challenged iBREC. All cell culture media were supplemented with 5% fetal bovine serum (FBS) as its absence results in accumulation of aflibercept in iBREC due to deregulated expression of transport proteins. Early after exposure of a confluent iBREC monolayer cultivated on gold electrodes to 5% FBS, the cell index (CI) - assessed as a measure of barrier function, cell viability and cell adhesion - transiently declined but recovered again within a few hours to high values. These values remained stable for several days associated with a strong expression of the TJ-protein claudin-1, indicative of a functional barrier formed by the iBREC monolayer. Transient changes of the plasma membrane localizations of claudin-5 and vascular endothelial cadherin - both important for regulation of paracellular flow - accompanied the transient reduction of the CI not prevented by VEGF-binding proteins. Treatment of iBREC with 50 ng/ml VEGF-A₁₆₅ for one day resulted in a strong and persistent decline of the CI associated with a low expression level of the TJ-protein claudin-1; reversion to normal values was complete one day after aflibercept's addition at a final concentration of 250 μg/ml. Expressions of other proteins involved in regulation of paracellular flow or transcellular transport were not significantly changed. More aflibercept passed through the monolayer of iBREC cultivated on permeable membrane inserts pretreated with VEGF-A for one day, but this was not affected by a FcRn-inhibiting antibody. Subcellular localization of aflibercept was hardly changed in VEGF-A-exposed iBREC 3 h after its addition to the cells; inhibition of (non)-lysosomal or proteasomal proteases then only weakly affected the amount of internalized aflibercept. iBREC also internalized VEGF-A which was barely detectable as early as 2 h after addition of aflibercept. In contrast, blocking the tyrosine kinase activity of VEGF receptor(s) did not prevent VEGF-A's uptake. Inhibition of cellular proteases strongly increased the amount of internalized VEGF-A in the absence and presence of the Fc fusion protein. We therefore conclude that a FcRn-mediated transport plays a minor role in aflibercept's passage through a leaky barrier of REC. Even early after addition of aflibercept to VEGF-A-exposed iBREC, the levels of free intracellular VEGF-A are low, as aflibercept likely prevents binding of VEGF-A to its receptor. Interestingly, the growth factor's detrimental effects still persist for nearly one day.

Investigating the Role of PPARβ/δ in Retinal Vascular Remodeling Using Pparβ/δ-Deficient Mice

Peroxisome proliferator-activated receptor (PPAR)β/δ is a member of the nuclear receptor superfamily of transcription factors, which plays fundamental roles in cell proliferation and differentiation, inflammation, adipogenesis, and energy homeostasis. Previous studies demonstrated a reduced choroidal neovascularization (CNV) in Pparβ/δ-deficient mice. However, PPARβ/δ's role in physiological blood vessel formation and vessel remodeling in the retina has yet to be established. Our study showed that PPARβ/δ is specifically required for disordered blood vessel formation in the retina. We further demonstrated an increased arteriovenous crossover and wider venous caliber in Pparβ/δ-haplodeficient mice. In summary, these results indicated a critical role of PPARβ/δ in pathological angiogenesis and blood vessel remodeling in the retina.

VEGF receptor 2 inhibitor nintedanib completely reverts VEGF-A165-induced disturbances of barriers formed by retinal endothelial cells or long-term cultivated ARPE-19 cells

Various severe ocular diseases are associated with an elevated intravitreal expression of VEGF-A which increases the permeability of retinal endothelial cells (REC) or retinal pigment epithelial (RPE) cells in vivo and in vitro. Inhibition of VEGF receptor 2 (VEGFR2) is sufficient to completely prevent VEGF-A₁₆₅-induced dysfunctions of barriers formed by long-term cultivated, immortal human ARPE-19 cells or immortalized bovine retinal endothelial cells (iBREC). Extended exposure to VEGF-A could result in additional activation of other growth factor receptors, potentially promoting synergistic effects of corresponding factors on various cellular processes including angiogenesis. Based on these observations, we investigated whether blocking of VEGFR2 is also sufficient to revert VEGF-A-induced changes of the barriers consisting of iBREC (i.e. inner blood-retina barrier) or ARPE-19 cells (i.e. outer blood-retina barrier) in vitro. Alterations of confluent monolayers' properties induced by treatment with VEGF-A₁₆₅ for one day followed by addition of small molecule inhibitors of the VEGFR2 were determined by continuous cell index (CI) measurements using the microelectronic biosensor system for cell-based assays xCELLigence. VEGF-A₁₆₅ induced a long-lasting drop of the otherwise high CI of iBREC accompanied by reduced expression of the tight junction (TJ) protein claudin-1 and subtle changes of the plasma membrane localizations of TJ-protein claudin-5 and of vascular endothelial cadherin. Blocking mainly VEGFR2 with 10 nM nintedanib, 10 nM tivozanib or 500 nM ZM323881 efficiently reverted these changes within one day; higher concentrations of nintedanib or additional inhibition of neuropilin-1 were not superior. Interestingly, the CI of short-term cultivated, confluent ARPE-19 cells slightly increased in the presence of VEGF-A₁₆₅, but was not changed by nintedanib. In contrast, VEGF-A₁₆₅ markedly reduced the transepithelial electrical resistance of ARPE-19 cells cultivated on porous membrane inserts for three weeks, which was also accompanied by a significant loss of the then strongly plasma membrane-expressed TJ-protein ZO-1. These alterations were completely reverted within one day by 10 nM nintedanib of which higher concentrations were not superior. None of the inhibitors tested diminished the strong barrier properties of iBREC or long-term cultivated ARPE-19 cells. Taken together, inhibition of VEGFR2 efficiently reverts VEGF-A₁₆₅-induced barrier disturbances of both cell types forming and regulating the inner and outer blood-retina barrier. As synergistic actions of growth factors seem to play only a minor role in inducing a barrier dysfunction, specific inhibition of VEGFR2 could be an interesting option to treat VEGF-A-induced macular edema without obvious effects on vitality and functions of REC and RPE cells.

Long-Acting FGF21 Inhibits Retinal Vascular Leakage in In Vivo and In Vitro Models

The aim of the current study was to investigate the impact of long-acting fibroblast growth factor 21 (FGF21) on retinal vascular leakage utilizing machine learning and to clarify the mechanism underlying the protection. To assess the effect on retinal vascular leakage, C57BL/6J mice were pre-treated with long-acting FGF21 analog or vehicle (Phosphate Buffered Saline; PBS) intraperitoneally (i.p.) before induction of retinal vascular leakage with intravitreal injection of mouse (m) vascular endothelial growth factor 164 (VEGF164) or PBS control. Five hours after mVEGF164 injection, we retro-orbitally injected Fluorescein isothiocyanate (FITC) -dextran and quantified fluorescence intensity as a readout of vascular leakage, using the Image Analysis Module with a machine learning algorithm. In FGF21- or vehicle-treated primary human retinal microvascular endothelial cells (HRMECs), cell permeability was induced with human (h) VEGF165 and evaluated using FITC-dextran and trans-endothelial electrical resistance (TEER). Western blots for tight junction markers were performed. Retinal vascular leakage in vivo was reduced in the FGF21 versus vehicle- treated mice. In HRMECs in vitro, FGF21 versus vehicle prevented hVEGF-induced increase in cell permeability, identified with FITC-dextran. FGF21 significantly preserved TEER compared to hVEGF. Taken together, FGF21 regulates permeability through tight junctions; in particular, FGF21 increases Claudin-1 protein levels in hVEGF-induced HRMECs. Long-acting FGF21 may help reduce retinal vascular leakage in retinal disorders and machine learning assessment can help to standardize vascular leakage quantification.

Sitagliptin and the Blood-Retina Barrier: Effects on Retinal Endothelial Cells Manifested Only after Prolonged Exposure

Inhibitors of dipeptidyl peptidase-4 (DPP-4) are widely used to treat diabetes mellitus, but data concerning their effects on the barrier stability of retinal endothelial cells (REC) in vivo and in vitro are inconsistent. Therefore, we studied whether the barrier properties of immortalized endothelial cells of the bovine retina (iBREC) were affected by the inhibitors of DPP-4 sitagliptin (10-1000 nM) and diprotin A (1-25 μM). Their effects were also investigated in the presence of VEGF-A₁₆₅ because diabetic patients often develop macular edema caused by VEGF-A-induced permeability of REC. To detect even transient or subtle changes of paracellular and transcellular flow as well as adhesion of the cells to the extracellular matrix, we continuously monitored the cell index (CI) of confluent iBREC grown on gold electrodes. Initially, the CI remained stable but started to decline significantly and persistently at 40 h or 55 h after addition of sitagliptin or diprotin A, respectively. Both inhibitors did not modulate, prevent, or revert the persistent VEGF-A₁₆₅-induced reduction of the CI. Interestingly, sitagliptin and diprotin A increased the expression of the tight-junction protein claudin-1 which is an important component of a functional barrier formed by iBREC. In contrast, expressions of CD29-a subunit of the fibronectin receptor-or of the tetraspanin CD9 were lower after extended treatment with the DPP-4 inhibitors; less of the CD9 was seen at the plasma membrane after prolonged exposure to sitagliptin. Because both associated proteins are important for adhesion of iBREC to the extracellular matrix, the observed low CI might be caused by weakened attachment of the cells. From our results, we conclude that extended inhibition of DPP-4 destabilizes the barrier formed by microvascular REC and that DPP-4 inhibitors like sitagliptin do not counteract or enhance a VEGF-A₁₆₅-induced barrier dysfunction as frequently observed in DME.

Molecular genetic mechanisms of influence of laser radiation with 577 nm wavelength in a microimpulse mode on the condition of the retina

THE AIM OF THE STUDY

was to investigate the molecular genetic mechanisms of the influence of laser radiation with 577 nm wavelength in a microimpulse mode on the retina in the experimental conditions after the intravitreal injection of VEGF.

MATERIALS AND METHODS

The study was performed on 4-5 week-old male mice of the line C57BL/6J. The animals were divided into 4 groups of 5 mice in each group, one eye was excremental, the contralateral eye remained intact. In the first group, intravitreal injection of PBS was performed; in the second group, intravitreal injection of 50 ng/ml of recombinant VEGF₁₆₅ in 2 μL of phosphate-buffered saline (PBS) was performed; in the third and fourth groups, a day after the intravitreal injection of recombinant VEGF₁₆₅, laser radiation with wavelength 577 nm was applied in the micropulse and continuous modes, respectively. Tissue samples (neuroepithelium, pigment epithelium) for the microarray transcription analysis in the animals from group 1 and 2 were taken 2 days after the injection of PBS and VEGF, in the animals from group 3 and 4 - a day after the retina was exposed to laser radiation.

RESULTS AND CONCLUSION

Molecular genetic mechanisms of the influence of laser radiation with wavelength 577 nm in a microimpulse mode on the retina in experimental conditions were studied and the genes that significantly changed the level of expression (the genes that take part in the regulation of neoangiogenesis, structural cell functions, processes of cells proliferation, transcription, differentiation, transmembrane transport, signaling, synaptic transmission, etc.) were identified.

PPARδ: A Potential Therapeutic Target for the Treatment of Metabolic Hypertension

High blood pressure and its associated cardiovascular diseases have been major risks for public health. Multiple metabolic risk factors can cause the vascular dysfunction and vascular lesion, and the hypertension due to metabolic disturbances was defined as metabolic hypertension. The members of a subfamily of the nuclear receptors, peroxisome proliferator-activated receptors (PPARs), were found to be key regulators of metabolism and vascular function. We provide up-to-date knowledge on the role of subtype PPARδ in the regulation of metabolism and vascular function and the effect of its intervention on the metabolic hypertension management. We hope to give some insights into the development of more effective treatments of metabolic hypertension and its main complications.

Intravitreal Stanniocalcin-1 Enhances New Blood Vessel Growth in a Rat Model of Laser-Induced Choroidal Neovascularization

Purpose

The purpose of this study was to investigate the impact of stanniocalcin-1 (STC-1), a photoreceptor-protective glycoprotein, on the development of choroidal neovascularization (CNV) in relation to VEGF and its main receptor (VEGFR2) expression after laser injury.

Methods

In rats, CNV was induced by laser photocoagulation in both eyes, followed by intravitreal injection of STC-1 in the right eye and vehicle or denatured STC-1 injection in the left eye as control. Two weeks after laser injury, fundus autofluorescence (FAF) imaging and fundus fluorescein angiography (FFA) were performed. Fluorescein leakage from CNV was graded using a defined scale system. The size of CNV was quantified with spectral domain optical coherence tomography (SD-OCT), fluorescein-labeled choroid-sclera flat mounts, and hematoxylin-eosin staining. Protein expressions were evaluated by Western blot.

Results

Photocoagulation produced a well-circumscribed area of CNV. With STC-1 treatment, CNV lesions assessed by FAF were increased by 50% in both intensity and area. The CNV lesions were also increased with SD-OCT, flat-mount, and histologic analyses. FFA disclosed enhanced fluorescein leakage in CNV lesions in STC-1 treated eyes. The STC-1 protein was detected in the choroidal tissue and its level was increased with CNV lesions in correlation with VEGF and VEGFR2 expressions. Intravitreal administration of STC-1 significantly increased choroidal expression of both VEGF and VEGFR2 proteins.

Conclusions

Chorodial tissue expresses STC-1, which seemingly acts as a stress response protein by enhancing pathological new blood vessel growth in laser-induced CNV. It is likely that STC-1 promotes CNV development via VEGF signaling.

Palmitic Acid Induces Müller Cell Inflammation that is Potentiated by Co-treatment with Glucose

Chronic hyperglycemia is thought to be the major stimulator of retinal dysfunction in diabetic retinopathy (DR). Thus, many diabetes-related systemic factors have been overlooked as inducers of DR pathology. Cell culture models of retinal cell types are frequently used to mechanistically study DR, but appropriate stimulators of DR-like factors are difficult to identify. Furthermore, elevated glucose, a gold standard for cell culture treatments, yields little to no response from many primary human retinal cells. Thus, the goal of this project was to demonstrate the effectiveness of the free fatty acid, palmitic acid and compare its use alone and in combination with elevated glucose as a stimulus for human Müller cells, a retinal glial cell type that is activated early in DR pathogenesis and uniquely responsive to fatty acids. Using RNA sequencing, we identified a variety of DR-relevant pathways, including NFκB signaling and inflammation, intracellular lipid signaling, angiogenesis, and MAPK signaling, that were stimulated by palmitic acid, while elevated glucose alone did not significantly alter any diabetes-relevant pathways. Co-treatment of high glucose with palmitic acid potentiated the expression of several DR-relevant angiogenic and inflammatory targets, including PTGS2 (COX-2) and CXCL8 (IL-8).

Selection of Reference Genes for Studies of Human Retinal Endothelial Cell Gene Expression by Reverse Transcription-Quantitative Real-Time Polymerase Chain Reaction

Background

Human retinal endothelial cells are employed increasingly for investigations of retinal vascular diseases. Analysis of gene expression response to disease-associated stimuli by reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) is common. However, most reported work does not follow the minimum information for publication of qPCR experiments (MIQE) recommendation that multiple, stably expressed reference genes be used for normalization.

Methods

Two human retinal endothelial cell lines were treated with medium alone or containing stimuli that included: glucose at supraphysiological concentration, dimethyloxalyl-glycine, vascular endothelial growth factor, tumor necrosis factor-α, lipopolysaccharide and Toxoplasma gondii tachyzoites. Biological response of cells was confirmed by measuring significant increase in a stimulus-relevant transcript. Total RNA was reverse transcribed and analyzed by commercial PCR arrays designed to detect 28 reference genes. Stability of reference gene expression, for each and both cell lines, and for each and all conditions, was judged on gene-stability measure (M-value) less than 0.2 and coefficient of variation (CV-value) less than 0.1.

Results

Reference gene expression varied substantially across stimulations and between cell lines. Of 27 detectable reference genes, 11-21 (41-78%) maintained expression stability across stimuli and cell lines. Ranking indicated substantial diversity in the most stable reference genes under different conditions, and no reference gene was expressed stably under all conditions of stimulation and for both cell lines. Four reference genes were expressed stably under 5 conditions: HSP90AB1, IPO8, PSMC4 and RPLPO.

Conclusions

We observed variation in stability of reference gene expression with different stimuli and between human retinal endothelial cell lines. Our findings support adherence to MIQE recommendations regarding normalization in RT-qPCR studies of human retinal endothelial cells.

PPARβ/δ selectively regulates phenotypic features of age-related macular degeneration

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a nuclear receptor that regulates differentiation, inflammation, lipid metabolism, extracellular matrix remodeling, and angiogenesis in multiple tissues. These pathways are also central to the pathogenesis of age-related macular degeneration (AMD), the leading cause of vision loss globally. With the goal of identifying signaling pathways that may be important in the development of AMD, we investigated the impact of PPARβ/δ activation on ocular tissues affected in the disease. PPARβ/δ is expressed and can be activated in AMD vulnerable cells, including retinal pigment epithelial (RPE) and choroidal endothelial cells. Further, PPARβ/δ knockdown modulates AMD-related pathways selectively. Specifically, genetic ablation of Pparβ/δ in aged mice resulted in exacerbation of several phenotypic features of early dry AMD, but attenuation of experimentally induced choroidal neovascular (CNV) lesions. Antagonizing PPARβ/δ in both in vitro angiogenesis assays and in the in vivo experimentally induced CNV model, inhibited angiogenesis and angiogenic pathways, while ligand activation of PPARβ/δ, in vitro, decreased RPE lipid accumulation, characteristic of dry AMD. This study demonstrates for the first time, selective regulation of a nuclear receptor in the eye and establishes that selective targeting of PPARβ/δ may be a suitable strategy for treatment of different clinical sub-types of AMD.

Self-Assembled Nanoconstructs Modified with Amplified Aptamers Inhibited Tumor Growth and Retinal Vascular Hyperpermeability via Vascular Endothelial Growth Factor Capturing

Here, nanoconstructs consisting of a DNA-amplified aptamer with a biocompatible polymer backbone for capturing target biomolecules are presented. First, the polymer-DNA nanoconstructs were prepared by hybridization of two complementary single-stranded DNAs that were each conjugated to a dextran polymer backbone. The designed polymer-DNA amplified aptamer nanoconstructs (PA-aNCs) were then prepared by utilizing polymer-DNA nanoconstructs conjugated with an aptamer (PA-NCs) using a rolling circle amplification reaction to amplify the aptamer. These PA-aNCs were successfully applied to alleviate tumor growth and vascular endothelial growth factor (VEGF)-induced retinal vascular hyperpermeability in vivo through the highly effective capture of human VEGF as a target molecule. These PA-aNCs could be used as therapeutic agent for anti-VEGF therapy by efficiently capturing human VEGF.

Epoxygenated Fatty Acids Inhibit Retinal Vascular Inflammation

The objective of the present study was to assess the effect of elevating epoxygenated fatty acids on retinal vascular inflammation. To stimulate inflammation we utilized TNFα, a potent pro-inflammatory mediator that is elevated in the serum and vitreous of diabetic patients. In TNFα-stimulated primary human retinal microvascular endothelial cells, total levels of epoxyeicosatrienoic acids (EETs), but not epoxydocosapentaenoic acids (EDPs), were significantly decreased. Exogenous addition of 11,12-EET or 19,20-EDP when combined with 12-(3-adamantane-1-yl-ureido)-dodecanoic acid (AUDA), an inhibitor of epoxide hydrolysis, inhibited VCAM-1 and ICAM-1 expression and protein levels; conversely the diol product of 19,20-EDP hydrolysis, 19,20-DHDP, induced VCAM1 and ICAM1 expression. 11,12-EET and 19,20-EDP also inhibited leukocyte adherence to human retinal microvascular endothelial cell monolayers and leukostasis in an acute mouse model of retinal inflammation. Our results indicate that this inhibition may be mediated through an indirect effect on NFκB activation. This is the first study demonstrating a direct comparison of EET and EDP on vascular inflammatory endpoints, and we have confirmed a comparable efficacy from each isomer, suggesting a similar mechanism of action. Taken together, these data establish that epoxygenated fatty acid elevation will inhibit early pathology related to TNFα-induced inflammation in retinal vascular diseases.

Rethinking Nuclear Receptors as Potential Therapeutic Targets for Retinal Diseases

Collectively, retinal diseases, including age-related macular degeneration, retinitis pigmentosa, and diabetic retinopathy, result in severe vision impairment worldwide. The absence and/or limited availability of successful drug therapies for these blinding disorders necessitates further understanding their pathobiology and identifying new targetable signaling pathways. Nuclear receptors are transcription regulators of many key aspects of human physiology, as well as pathophysiology, with reported roles in development, aging, and disease. Some of the pathways regulated by nuclear receptors include, but are not limited to, angiogenesis, inflammation, and lipid metabolic dysregulation, mechanisms also important in the initiation and development of several retinal diseases. Herein, we present an overview of the biology of three diseases affecting the posterior eye, summarize a growing body of evidence that suggests direct or indirect involvement of nuclear receptors in disease progression, and discuss the therapeutic potential of targeting nuclear receptors for treatment.

NFAT isoforms play distinct roles in TNFα-induced retinal leukostasis

The objective of this study was to determine the role of individual NFAT isoforms in TNFα-induced retinal leukostasis. To this end, human retinal microvascular endothelial cells (HRMEC) transfected with siRNA targeting individual NFAT isoforms were treated with TNFα, and qRT-PCR was used to examine the contribution of each isoform to the TNFα-induced upregulation of leukocyte adhesion proteins. This showed that NFATc1 siRNA increased ICAM1 expression, NFATc2 siRNA reduced CX3CL1, VCAM1, SELE, and ICAM1 expression, NFATc3 siRNA increased CX3CL1 and SELE expression, and NFATc4 siRNA reduced SELE expression. Transfected HRMEC monolayers were also treated with TNFα and assayed using a parallel plate flow chamber, and both NFATc2 and NFATc4 knockdown reduced TNFα-induced cell adhesion. The effect of isoform-specific knockdown on TNFα-induced cytokine production was also measured using protein ELISAs and conditioned cell culture medium, and showed that NFATc4 siRNA reduced CXCL10, CXCL11, and MCP-1 protein levels. Lastly, the CN/NFAT-signaling inhibitor INCA-6 was shown to reduce TNFα-induced retinal leukostasis in vivo. Together, these studies show a clear role for NFAT-signaling in TNFα-induced retinal leukostasis, and identify NFATc2 and NFATc4 as potentially valuable therapeutic targets for treating retinopathies in which TNFα plays a pathogenic role.

RNA-seq identifies a role for the PPARβ/δ inverse agonist GSK0660 in the regulation of TNFα-induced cytokine signaling in retinal endothelial cells

PURPOSE

The peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) is a transcription factor with roles in metabolism, angiogenesis, and inflammation. It has yet undefined roles in retinal inflammation and diabetic retinopathy (DR). We used RNA-seq to better understand the role of the antagonist and inverse agonist of PPARβ/δ, GSK0660, in TNFα-induced inflammation. Understanding the underlying mechanisms of vascular inflammation could lead to new treatments for DR.

METHODS

RNA was isolated from human retinal microvascular endothelial cells treated with a vehicle, TNFα, or TNFα plus GSK0660. RNA-seq was performed with a 50 bp single read protocol. The differential expression was determined using edgeR and gene ontology, and a pathway analysis was performed using DAVID. RNA-seq validation was performed using qRT-PCR using the primers for ANGPTL4, CCL8, NOV, CXCL10, and PDPK1.

RESULTS

TNFα differentially regulated 1,830 transcripts, many of which are involved in the cytokine-cytokine receptor interaction, chemokine signaling, and inflammatory response. Additionally, TNFα highly upregulated genes involved in leukocyte recruitment, including CCL5, CX3CL1, and CXCL10. GSK0660 differentially regulated 273 transcripts in TNFα-treated cells compared to TNFα alone. A pathway analysis revealed the enrichment of cytokine-cytokine receptor signaling. In particular, GSK0660 blocks the TNFα-induced upregulation of CCL8, a chemokine involved in leukocyte recruitment.

CONCLUSIONS

TNFα regulates several genes related to retinal leukostasis in retinal endothelial cells. GSK0660 blocks the effect of TNFα on the expressions of cytokines involved in leukocyte recruitment, including CCL8, CCL17, and CXCL10 and it may therefore block TNFα-induced retinal leukostasis.

Binding of VEGF-A is sufficient to abrogate the disturbing effects of VEGF-B together with VEGF-A on retinal endothelial cells

Purpose

Inhibition of vascular endothelial growth factor (VEGF) is a promising strategy to treat retinal complications of diabetes. In contrast to VEGF-A binding ranibizumab, aflibercept also binds to other members of the VEGF family including VEGF-B, but potential effects of this factor on permeability and angiogenic processes are unclear. Therefore, we studied how VEGF-B variants as single agents or together with VEGF-A₁₆₅ might affect proliferation, migration, or barrier function of retinal endothelial cells (REC). Also investigated was the normalization of REC properties with both VEGF-inhibitors to explore if additional targeting of VEGF-B is relevant.

Methods

Stimulation of proliferation or migration of immortalized bovine REC (iBREC) and disturbance of their barrier by exposure to VEGF-B variants (as single factors or together with VEGF-A₁₆₅) was determined with or without VEGF-binding proteins being added. Permeability of iBREC was assessed by measuring their transendothelial resistance (TER) and expression of the tight junction protein claudin-1.

Results

VEGF-B₁₆₇ and VEGF-B₁₈₆ enhanced proliferation of iBREC but these isoforms did not affect cell migration. Interestingly, ranibizumab completely blocked both migration and proliferation induced by VEGF-A plus VEGF-B. Both VEGF-B variants did also not affect barrier function or claudin-1 expression in a normal or high-glucose environment. Accordingly, binding VEGF-A was enough to normalize a reduced TER and reinstate claudin-1 lost during treatment with this factor in combination with VEGF-B.

Conclusions

Important properties and functions of REC seem not to be affected by any VEGF-B variant and targeting the key factor VEGF-A is sufficient to normalize growth factor-disturbed cells of this type.

Electronic supplementary material

The online version of this article (doi:10.1007/s00417-015-2944-z) contains supplementary material, which is available to authorized users.