Attenuation of retinal vascular development and neovascularization in transgenic mice over-expressing thrombospondin-1 in the lens

Thrombospondins in the tumor microenvironment

Many cancers begin with the formation of a small nest of transformed cells that can remain dormant for years. Thrombospondin-1 (TSP-1) initially promotes dormancy by suppressing angiogenesis, a key early step in tumor progression. Over time, increases in drivers of angiogenesis predominate, and vascular cells, immune cells, and fibroblasts are recruited to the tumor mass forming a complex tissue, designated the tumor microenvironment. Numerous factors, including growth factors, chemokine/cytokine, and extracellular matrix, participate in the desmoplastic response that in many ways mimics wound healing. Vascular and lymphatic endothelial cells, and cancer-associated pericytes, fibroblasts, macrophages and immune cells are recruited to the tumor microenvironment, where multiple members of the TSP gene family promote their proliferation, migration and invasion. The TSPs also affect the immune signature of tumor tissue and the phenotype of tumor-associated macrophages. Consistent with these observations, expression of some TSPs has been established to correlate with poor outcomes in specific types of cancer.

Thrombospondin-1 in vascular development, vascular function, and vascular disease

Angiogenesis is vital to developmental, regenerative and repair processes. It is normally regulated by a balanced production of pro- and anti-angiogenic factors. Alterations in this balance under pathological conditions are generally mediated through up-regulation of pro-angiogenic and/or downregulation of anti-angiogenic factors, leading to growth of new and abnormal blood vessels. The pathological manifestation of many diseases including cancer, ocular and vascular diseases are dependent on the growth of these new and abnormal blood vessels. Thrompospondin-1 (TSP1) was the first endogenous angiogenesis inhibitor identified and its anti-angiogenic and anti-inflammatory activities have been the subject of many studies. Studies examining the role TSP1 plays in pathogenesis of various ocular diseases and vascular dysfunctions are limited. Here we will discuss the recent studies focused on delineating the role TSP1 plays in ocular vascular development and homeostasis, and pathophysiology of various ocular and vascular diseases with a significant clinical relevance to human health.

Genetic regulation of THBS1 methylation in diabetic retinopathy

BACKGROUND

Diabetic retinopathy (DR) is a common and serious microvascular complication of diabetes mellitus (DM), but its pathological mechanism, especially the formation mechanism of new blood vessels remains unclear. Thrombospondin-1 (THBS1) is a potent endogenous inhibitor of angiogenesis and it was found over expressed in DR in our previous study. Our study aimed to determine whether overexpression of THBS1 is associated with its promoter methylation level, and whether methylation of THBS1 is regulated by genetic variants in DR.

METHODS

Patients diagnosed with DR and DM patients without retinal problems were included in the case-control study. DNA methylation detection of THBS1 by bisulfite sequencing and genotyping of specific SNPs by MassARRAY analysis were performed in the patients recruited from 2019-2020. Real time quantitative PCR was performed to obtain mRNA expression of THBS1 in the patients recruited from August to October 2022. The differentially methylated CpG loci of THBS1 were identified by logistic regression, and associations between 13 SNPs and methylation levels of CpG loci were tested by methylation quantitative trait loci (meQTLs) analysis. Mediation analysis was applied to determine whether CpG loci were intermediate factors between meQTLs and DR.

RESULTS

150 patients diagnosed with DR and 150 DM patients without retinal complications were enrolled in the first recruitment, seven DR patients and seven DM patients were enrolled in the second recruitment. The patients with DR showed promoter hypomethylation of THBS1 (P value = 0.002), and six out of thirty-nine CpG sites within two CpG islands (CGIs) showed hypomethylation(P value < 0.05). THBS1 mRNA expression in peripheral blood was significantly higher in DR patients than in DM patients. Five out of thirteen cis-meQTLs were identified to be associated with CpG sites: rs13329154, rs34973764 and rs5812091 were associated with cis-meQTLs of CpG-4 (P value=0.0145, 0.0095, 0.0158), rs11070177 and rs1847663 were associated with cis-meQTLs of CpG-2 and CpG-3 respectively (P value=0.0201, 0.0275). CpG-4 methylation significantly mediated the effect of the polymorphism rs34973764 on DR (B=0.0535, Boot 95%CI: 0.004~0.1336).

CONCLUSION

THBS1 overexpression is related to THBS1 hypomethylation in patients with DR. DNA methylation may be genetically controlled in DR.

Targeted Thrombospondin-1 Expression in Ocular Vascular Development and Neovascularization

Tight regulation of positive and negative regulators of angiogenesis is essential, particularly in the eye where their dysregulation can lead to vision loss. Thrombospondin-1 (TSP1) is a matricellular protein that negatively regulates angiogenesis and inflammation in the eye. It aids ocular vascular homeostasis such that its loss contributes to increased retinal vascular density and pathologic ocular neovascularization. Our previous studies demonstrated that mice globally lacking TSP1 expression had increased retinal vascular density, decreased hyperoxia-induced retinal vessel loss, and increased choroidal neovascularization. Here we determined the impact to the ocular vasculature of endothelial cell, pericyte, or astrocyte loss of TSP1 expression. Only lack of TSP1 expression in endothelial cells was sufficient to increase choroidal neovascularization with mice lacking expression in pericytes or astrocytes not demonstrating a significant impact. Although the global TSP1 knockout mice demonstrated increased retinal vascular density, individual cell type loss of TSP1 resulted in decreased retinal endothelial cell numbers before and/or after vascular maturation in a cell type specific fashion. Retinas from mice lacking TSP1 expression in endothelial cells, pericytes or astrocytes were not protected from retinal vessel regression in response to hyperoxia as we previously observed in the global knockout. Thus, modulation of TSP1 expression in individual cell types demonstrates a response that is unique to the role TSP1 plays in that cell type of interest, and their coordinated activity is critical for vision.

Role of Smad4 from ocular surface ectoderm in retinal vasculature development

AIM

To investigate how signals from lens regulate retinal vascular development and neovascularization.

METHODS

Le-Cre transgenic mouse line was employed to inactivate Smad4 in the surface ectoderm selectively. Standard histological and whole-mount retina staining were employed to reveal morphological changes of retinal vasculature in Smad4 defective eye. cDNA microarray and subsequent analyses were conducted to investigate the molecular mechanism underlying the vascular phenotype. Quantitative polymerase chain reaction (qPCR) was carried out to verify the microarrays results.

RESULTS

We found that inactivation of Smad4 specifically on surface ectoderm leads to a variety of retinal vasculature anomalies. Microarray analyses and qPCR revealed that Sema3c, Sema3e, Nrp1, Tie1, Sox7, Sox17, and Sox18 are significantly affected in the knockout retinas at different developmental stages, suggesting that ocular surface ectoderm-derived Smad4 can signal to the retina and regulates various angiogenic signaling in the retina.

CONCLUSION

Our data suggest that the cross-talk between ocular surface ectoderm and retina is important for retinal vasculature development, and Smad4 regulates various signaling associated with sprouting angiogenesis, vascular remodeling and maturation in the retina of mice.

Role of thrombospondin‑1 and thrombospondin‑2 in cardiovascular diseases (Review)

Thrombospondin (TSP)-1 and TSP-2 are matricellular proteins in the extracellular matrix (ECM), which serve a significant role in the pathological processes of various cardiovascular diseases (CVDs). The multiple effects of TSP-1 and TSP-2 are due to their ability to interact with various ligands, such as structural components of the ECM, cytokines, cellular receptors, growth factors, proteases and other stromal cell proteins. TSP-1 and TSP-2 regulate the structure and activity of the aforementioned ligands by interacting directly or indirectly with them, thereby regulating the activity of different types of cells in response to environmental stimuli. The pathological processes of numerous CVDs are associated with the degradation and remodeling of ECM components, and with cell migration, dysfunction and apoptosis, which may be regulated by TSP-1 and TSP-2 through different mechanisms. Therefore, investigating the role of TSP-1 and TSP-2 in different CVDs and the potential signaling pathways they are associated with may provide a new perspective on potential therapies for the treatment of CVDs. In the present review, the current understanding of the roles TSP-1 and TSP-2 serve in various CVDs were summarized. In addition, the interacting ligands and the potential pathways associated with these thrombospondins in CVDs are also discussed.

Negative Regulators of Angiogenesis, Ocular Vascular Homeostasis, and Pathogenesis and Treatment of Exudative AMD

Angiogenesis, the formation of new blood vessels from pre-existing capillaries, is very tightly regulated and normally does not occur except during developmental and reparative processes. This tight regulation is maintained by a balanced production of positive and negative regulators, and alterations under pathological conditions such as retinopathy of prematurity, diabetic retinopathy, and age-related macular degeneration can lead to growth of new and abnormal blood vessels. Although the role of proangiogenic factors such as vascular endothelial growth factor has been extensively studied, little is known about the roles of negative regulators of angiogenesis in the pathogenesis of these diseases. Here, we will discuss the role of thrombospondin-1 (TSP1), one of the first known endogenous inhibitors of angiogenesis, in ocular vascular homeostasis, and how its alterations may contribute to the pathogenesis of age-related macular degeneration and choroidal neovascularization. We will also discuss its potential utility as a therapeutic target for treatment of ocular diseases with a neovascular component.

Tetrahydrobiopterin (BH4) deficiency is associated with augmented inflammation and microvascular degeneration in the retina

Background

Tetrahydrobiopterin (BH4) is an essential cofactor in multiple metabolic processes and plays an essential role in maintaining the inflammatory and neurovascular homeostasis. In this study, we have investigated the deleterious effects of BH4 deficiency on retinal vasculature during development.

Methods

hph-1 mice, which display deficiency in BH4 synthesis, were used to characterize the inflammatory effects and the integrity of retinal microvasculature. BH4 levels in retinas from hph-1 and wild type (WT) mice were measured by LC-MS/MS. Retinal microvascular area and microglial cells number were quantified in hph-1 and WT mice at different ages. Retinal expression of pro-inflammatory, anti-angiogenic, and neuronal-derived factors was analyzed by qPCR. BH4 supplementation was evaluated in vitro, ex-vivo, and in vivo models.

Results

Our findings demonstrated that BH4 levels in the retina from hph-1 mice were significantly lower by ~ 90% at all ages analyzed compared to WT mice. Juvenile hph-1 mice showed iris atrophy, persistent fetal vasculature, significant increase in the number of microglial cells (p < 0.01), as well as a marked degeneration of the retinal microvasculature. Retinal microvascular alterations in juvenile hph-1 mice were associated with a decreased expression in Norrin (0.2-fold) and its receptor Frizzled-4 (FZD4; 0.51-fold), as well as with an augmented expression of pro-inflammatory factors such as IL-6 (3.2-fold), NRLP-3 (4.4-fold), IL-1β (8.6-fold), and the anti-angiogenic factor thrombospondin-1 (TSP-1; 17.5-fold). We found that TSP-1 derived from activated microglial cells is a factor responsible of inducing microvascular degeneration, but BH4 supplementation markedly prevented hyperoxia-induced microglial activation in vitro and microvascular injury in an ex-vivo model of microvascular angiogenesis and an in vivo model of oxygen-induced retinopathy (OIR).

Conclusion

Our findings reveal that BH4 is a key cofactor in regulating the expression of inflammatory and anti-angiogenic factors that play an important function in the maintenance of retinal microvasculature.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-0955-x) contains supplementary material, which is available to authorized users.

Negative regulators of angiogenesis: important targets for treatment of exudative AMD

Angiogenesis contributes to the pathogenesis of many diseases including exudative age-related macular degeneration (AMD). It is normally kept in check by a tightly balanced production of pro- and anti-angiogenic factors. The up-regulation of the pro-angiogenic factor, vascular endothelial growth factor (VEGF), is intimately linked to the pathogenesis of exudative AMD, and its antagonism has been effectively targeted for treatment. However, very little is known about potential changes in expression of anti-angiogenic factors and the role they play in choroidal vascular homeostasis and neovascularization associated with AMD. Here, we will discuss the important role of thrombospondins and pigment epithelium-derived factor, two major endogenous inhibitors of angiogenesis, in retinal and choroidal vascular homeostasis and their potential alterations during AMD and choroidal neovascularization (CNV). We will review the cell autonomous function of these proteins in retinal and choroidal vascular cells. We will also discuss the potential targeting of these molecules and use of their mimetic peptides for therapeutic development for exudative AMD.

Pro-permeability Factors in Diabetic Macular Edema; the Diabetic Macular Edema Treated With Ozurdex Trial

PURPOSE

The Diabetic Macular Edema Treated with Ozurdex (DMEO) Trial measured aqueous pro-permeability factors (PPFs) in diabetic macular edema (DME) patients before and after injection of dexamethasone implant or vascular endothelial growth factor (VEGF)-neutralizing protein and correlated changes in levels with changes in excess foveal thickness (EFT) to identify potential PPFs contributing to DME.

DESIGN

Prospective, randomized crossover clinical trial.

METHODS

Twenty DME patients randomized to dexamethasone implant or VEGF-neutralizing protein had aqueous taps and spectral-domain optical coherence tomography (SDOCT) at baseline and every 4 weeks for 28 weeks. Aqueous levels of 55 vasoactive proteins were measured with protein array. Crossover at week 16 provided changes in protein levels after each intervention in all 20 patients.

RESULTS

After dexamethasone implant there was significant correlation between changes in levels of 13 vasoactive proteins with changes in EFT, including 3 known PPFs: angiopoietin-2 (r = 0.40, P = .001), hepatocyte growth factor (HGF; r = 0.31, P = .02), and endocrine gland-VEGF (EG-VEGF, r = 0.43, P < .001). Reduction of prolactin, insulin-like growth factor binding protein-3, and matrix metalloproteinase-9 correlated with edema reduction after injection of a VEGF-neutralizing protein as well as dexamethasone implant, suggesting their modulation is likely secondary to changes in edema rather than causative.

CONCLUSIONS

Correlation of edema reduction with reduction in the PPFs angiopoietin-2, HGF, and EG-VEGF provides potential insight into the multifactorial molecular mechanism by which dexamethasone implants reduce edema and suggest that additional study is needed to investigate the contributions of these 3 factors to chronic DME.

The role of extracellular matrix in retinal vascular development and preretinal neovascularization

Extracellular matrix (ECM) plays a central role in angiogenesis. ECM degrading enzymes breakdown the pre-existing vascular basement membrane at an early stage of angiogenesis and subsequently degrade stromal ECM as the new vessels invade into tissues. Conversely certain ECM components including collagen, fibronectin or fibrin are required for endothelial cell migration and tube morphogenesis. As the new vessels form they lay down a basement membrane that surrounds the endothelial tubes and is essential for their stability. In the rodent eye the transient expression of fibronectin and matricellular proteins plays a key role in retinal vascular development. In pathological retinal angiogenesis, such as in proliferative diabetic retinopathy, preretinal neovascularization occurs where new blood vessels invade the cortical vitreous gel and these blood vessels require vitreous collagen for their growth. The vitreous is normally anti-angiogenic and contains endogenous ECM inhibitors of angiogenesis including opticin and thombospondins, and ECM fragments such as endostatin. In preretinal neovascularization, the combined anti-angiogenic effects of these molecules are overcome by an excess of growth factors such as vascular endothelial growth factor-A, and new vessels grow into the vitreous with potentially blinding sequelae.

Thrombospondin-2 Expression During Retinal Vascular Development and Neovascularization

PURPOSE

To determine thrombospondin-2 (TSP2) expression and its impact on postnatal retinal vascular development and retinal neovascularization.

METHODS

The TSP2-deficient (TSP2(-/-)) mice and a line of TSP2 reporter mice were used to assess the expression of TSP2 during postnatal retinal vascular development and neovascularization. The postnatal retinal vascularization was evaluated using immunostaining of wholemount retinas prepared at different postnatal days by collagen IV staining and/or TSP2 promoter driven green fluorescent protein (GFP) expression. The organization of astrocytes was evaluated by glial fibrillary acidic protein (GFAP) staining. Retinal vascular densities were determined using trypsin digestion preparation of wholemount retinas at 3- and 6-weeks of age. Retinal neovascularization was assessed during the oxygen-induced ischemic retinopathy (OIR). Choroidal neovascularization (CNV) was assessed using laser-induced CNV.

RESULTS

Using the TSP2-GFP reporter mice, we observed significant expression of TSP2 mRNA in retinas of postnatal day 5 (P5) mice, which increased by P7 and remained high up to P42. Similar results were observed in retinal wholemount preparations, and western blotting for GFP with the highest level of GFP was observed at P21. In contrast to high level of mRNA at P42, the GFP fluorescence or protein level was dramatically downregulated. The primary retinal vasculature developed at a faster rate in TSP2(-/-) mice compared with TSP2(+/+) mice up to P5. However, the developing retinal vasculature in TSP2(+/+) mice caught up with that of TSP2(-/-) mice after P7. No significant differences in retinal vascular density were observed at 3- or 6-weeks of age. TSP2(-/-) mice also exhibited a similar sensitivity to the hyperoxia-mediated vessel obliteration and similar level of neovascularization during OIR as TSP2(+/+) mice. Lack of TSP2 expression minimally affected laser-induced CNV compared with TSP2(+/+) mice.

CONCLUSIONS

Lack of TSP2 expression was associated with enhanced retinal vascularization during early postnatal days but not at late postnatal times, and minimally affected retinal and CNV. However, the utility of TSP2 as a potential therapeutic target for inhibition of ocular neovascularization awaits further investigation.

Expression of thrombospondin-1 modulates the angioinflammatory phenotype of choroidal endothelial cells

The choroidal circulation plays a central role in maintaining the health of outer retina and photoreceptor function. Alterations in this circulation contribute to pathogenesis of many eye diseases including exudative age-related macular degeneration. Unfortunately, very little is known about the choroidal circulation and its molecular and cellular regulation. This has been further hampered by the lack of methods for routine culturing of choroidal endothelial cells (ChEC), especially from wild type and transgenic mice. Here we describe a method for isolation and culturing of mouse ChEC. We show that expression of thrombospondin-1 (TSP1), an endogenous inhibitor of angiogenesis and inflammation, has a significant impact on phenotype of ChEC. ChEC from TSP1-deficient (TSP1-/-) mice were less proliferative and more apoptotic, less migratory and less adherent, and failed to undergo capillary morphogenesis in Matrigel. However, re-expression of TSP1 was sufficient to restore TSP1-/- ChEC migration and capillary morphogenesis. TSP1-/- ChEC expressed increased levels of TSP2, phosphorylated endothelial nitric oxide synthase (NOS) and inducible NOS (iNOS), a marker of inflammation, which was associated with significantly higher level of NO and oxidative stress in these cells. Wild type and TSP1-/- ChEC produced similar levels of VEGF, although TSP1-/- ChEC exhibited increased levels of VEGF-R1 and pSTAT3. Other signaling pathways including Src, Akt, and MAPKs were not dramatically affected by the lack of TSP1. Together our results demonstrate an important autocrine role for TSP1 in regulation of ChEC phenotype.

Reduced β‑2‑glycoprotein І inhibits hypoxia‑induced retinal angiogenesis in neonatal mice through the vascular endothelial growth factor pathway

β‑2‑glycoprotein I (β2GPI), also known as apolipoprotein H, is a phospholipid‑binding plasma protein consisting of five homologous repeated units. β2GPI downregulates vascular endothelial growth factor (VEGF) signaling pathways and inhibits angiogenesis in vitro. However, the in vivo roles and effectors of reduced β2GPI and β2GPI in retinal angiogenesis are still not fully understood. In this study, an oxygen‑induced retinopathy model was used to investigate the effects of reduced β2GPI and β2GPI, and to monitor the expression of VEGF, VEGF receptor (VEGFR) 1, VEGFR‑2 and hypoxia‑inducible factor 1 (HIF‑1) mRNA and the phosphorylation of extracellular signal‑regulated kinase (ERK) and Akt. The data showed that both β2GPI and reduced β2GPI inhibited retinal angiogenesis and suppressed the expression of VEGF, VEGFR‑1, VEGFR‑2, HIF‑1, phosphorylated- (p‑) ERK and p‑Akt. The effects of reduced β2GPI were significantly stronger than those of β2GPI. In conclusion, this study showed that β2GPI and reduced β2GPI could inhibit retinal angiogenesis by downregulating the expression of VEGF and its downstream targets. This suggests that β2GPI and reduced β2GPI may have potential anti‑angiogenic activity in vivo.

Organ specific optical imaging of mitochondrial redox state in a rodent model of hereditary hemorrhagic telangiectasia-1

Hereditary Hemorrhagic Telangiectasia-1 (HHT-1) is a vascular disease caused by mutations in the endoglin (Eng)/CD105 gene. The objective of this study was to quantify the oxidative state of a rodent model of HHT-1 using an optical imaging technique. We used a cryofluorescence imaging instrument to quantitatively assess tissue metabolism in this model. Mitochondrial redox ratio (FAD/NADH), FAD RR, was used as a quantitative marker of the metabolic status and was examined in the kidneys, and eyes of wild-type and Eng +/- mice. Kidneys and eyes from wild-type P21, 6W, and 10M old mice showed, respectively, a 9% (±2), 24% (±0.4), 15% (±1), and 23% (±4), 33% (±0.6), and 30% (±2) change in the mean FAD RR compared to Eng +/- mice at the same age. Thus, endoglin haploinsufficiency is associated with less oxidative stress in various organs and mitigation of angiogenesis.

Thrombospondin-1 is produced by retinal glial cells and inhibits the growth of vascular endothelial cells

BACKGROUND/AIMS

By the release of antiangiogenic factors, Müller glial cells provide an angiostatic environment in the normal and ischemic retina. We determined whether Müller cells produce thrombospondin-1 (TSP-1), a known inhibitor of angiogenesis.

METHODS

Secretion of TSP-1 by cultured Müller cells was determined with ELISA. Slices of rat retinas and surgically excised retinal membranes of human subjects were immunostained against TSP-1 and the glial marker vimentin. The effects of TSP-1 on the growth of bovine retinal endothelial cells (BRECs) and activation of ERK1/2 were determined with DNA synthesis and migration assays, and Western blotting, respectively.

RESULTS

Cultured Müller cells secrete TSP-1 under normoxic and hypoxic (0.2% O2) conditions. Secretion of TSP-1 was increased in hypoxia compared to normoxia. In rat retinal slices, glial, retinal ganglion, and possibly horizontal cells were stained for TSP-1. Retinal glial cells in preretinal membranes from human subjects with nonhypoxic epiretinal gliosis (macular pucker) and proliferative diabetic retinopathy, respectively, were immunopositive for TSP-1. Exogenous TSP-1 reduced the VEGF-induced proliferation and migration of BRECs and decreased the phosphorylation level of ERK1/2 in BRECs.

CONCLUSION

The data suggest that Müller cells are one major source of TSP-1 in the normal and ischemic retina. Glia-derived TSP1 may inhibit angiogenic responses in the ischemic retina.

Matricellular protein thrombospondins: influence on ocular angiogenesis, wound healing and immuneregulation

Thrombospondins are a family of large multi-domain glycoproteins described as matricelluar proteins based on their ability to interact with a broad range of receptors, matrix molecules, growth factors or proteases, and to modulate array of cellular functions including intracellular signaling, proliferation and migration. Two members of the thrombospondin family, thrombospondin 1 (TSP-1) and thrombospondin 2 (TSP-2) are studied extensively to determine their structure and function. While expressed at low levels in normal adult tissues, their increased expression is seen predominantly in response to cellular perturbations. Despite structural similarities, a notable functional difference between TSP-1 and TSP-2 includes the ability of former to activate of latent TGF-β and its competitive inhibition by the latter. Both these thrombospondins are reported to play important roles in TGF-β rich ocular environment with most reports related to TSP-1. They are expressed by many ocular cell types and detectable in the aqueous and vitreous humor. TSP-1 and TSP-2 influence many cellular interactions in the eye such as angiogenesis, cell migration, wound healing, TGF-β activation and regulation of inflammatory immune responses. Together, these processes are known to contribute to the immune privilege status of the eye. Emerging roles of TSP-1 and TSP-2 in ocular functions and pathology are reviewed here.

Neonatal systemic inflammation in rats alters retinal vessel development and simulates pathologic features of retinopathy of prematurity

Background

Alteration of retinal angiogenesis during development leads to retinopathy of prematurity (ROP) in preterm infants, which is a leading cause of visual impairment in children. A number of clinical studies have reported higher rates of ROP in infants who had perinatal infections or inflammation, suggesting that exposure of the developing retina to inflammation may disturb retinal vessel development. Thus, we investigated the effects of systemic inflammation on retinal vessel development and retinal inflammation in neonatal rats.

Methods

To induce systemic inflammation, we intraperitoneally injected 100 μl lipopolysaccharide (LPS, 0.25 mg/ml) or the same volume of normal saline in rat pups on postnatal days 1, 3, and 5. The retinas were extracted on postnatal days 7 and 14, and subjected to assays for retinal vessels, inflammatory cells and molecules, and apoptosis.

Results

We found that intraperitoneal injection of LPS impaired retinal vessel development by decreasing vessel extension, reducing capillary density, and inducing localized overgrowth of abnormal retinal vessels and dilated peripheral vascular ridge, all of which are characteristic findings of ROP. Also, a large number of CD11c⁺ inflammatory cells and astrocytes were localized in the lesion of abnormal vessels. Further analysis revealed that the number of major histocompatibility complex (MHC) class II^loCD68^loCD11b^loCD11c^hi cells in the retina was higher in LPS-treated rats compared to controls. Similarly, the levels of TNF-α, IL-1β, and IL-12a were increased in LPS-treated retina. Also, apoptosis was increased in the inner retinal layer where retinal vessels are located.

Conclusions

Our data demonstrate that systemic LPS-induced inflammation elicits retinal inflammation and impairs retinal angiogenesis in neonatal rats, implicating perinatal inflammation in the pathogenesis of ROP.

STAT1-mediated Bim expression promotes the apoptosis of retinal pericytes under high glucose conditions

Hyperglycemia impacts different vascular cell functions and promotes the development and progression of various vasculopathies including diabetic retinopathy. Although the increased rate of apoptosis in pericytes (PCs) has been linked to increased oxidative stress and activation of protein kinase C-δ (PKC-δ) and SHP-1 (Src homology region 2 domain-containing phosphatase-1) tyrosine phosphatase during diabetes, the detailed mechanisms require further elucidation. Here we show that the rate of apoptosis and expression of proapoptotic protein Bim were increased in the retinal PCs of diabetic Akita/+ mice and mouse retinal PCs cultured under high glucose conditions. Increased Bim expression in retinal PCs under high glucose conditions required the sustained activation of signal transducer and activator of transcription 1 (STAT1) through production of inflammatory cytokines. PCs cultured under high glucose conditions also exhibited increased oxidative stress and diminished migration. Inhibition of oxidative stress, PKC-δ or Rho-associated protein kinase I/II was sufficient to protect PCs against apoptosis under high glucose conditions. Furthermore, PCs deficient in Bim expression were protected from high glucose-mediated increased oxidative stress and apoptosis. However, only inhibition of PKC-δ lowered Bim levels. N-acetylcysteine did not affect STAT1 levels, suggesting that oxidative stress is downstream of Bim. PCs cultured under high glucose conditions disrupted capillary morphogenesis of retinal endothelial cells (ECs) in coculture experiments. In addition, conditioned medium prepared from PCs under high glucose conditions attenuated EC migration. Taken together, our results indicate that Bim has a pivotal role in the dysfunction of retinal PCs under high glucose conditions by increasing oxidative stress and death of PCs.

Lack of thrombospondin 1 and exacerbation of choroidal neovascularization

OBJECTIVES

To assess the impact of thrombospondin 1(TSP1) deficiency on choroidal neovascularization (CNV)and to determine whether administration of a TSP1 antiangiogenic mimetic peptide attenuates CNV.

METHODS

The impact of TSP1 deficiency on laser induced CNV was assessed using wild-type (TSP1 +/+) and TSP1-deficient (TSP1 −/−) mice. Three laser burns were placed in each eye of TSP1 +/+ and TSP1 −/− mice to induce CNV. Intravitreal injection of the TSP1 mimetic peptide was performed on days 1 and 7 postlaser in the mice.For quantitative measurements of neovascularization, intercellular adhesion molecule 2 staining was performed at 14 days postlaser of the choroidal-sclera flat mounts. The recruitment of macrophages to the sites of damage was investigated by immunohistochemistry. The CNV area was measured by intercellular adhesion molecule 2 staining and use of ImageJ software.

RESULTS

The TSP1 −/− mice exhibited significantly larger areas of neovascularization on choroidal flat mounts compared with TSP1 +/ mice. This was consistent with enhanced recruitment of macrophages in TSP1 −/− mice compared with TSP1 +/+ mice 3 days postlaser. The development of CNV was significantly attenuated in mice receiving the TSP1 antiangiogenic mimetic peptide compared with those receiving vehicle alone.

CONCLUSIONS

Deficiency of TSP1 contributes to enhanced choroidal neovascularization. This is consistent with the anti-inflammatory and antiangiogenic activity of TSP1. The TSP1 antiangiogenic peptide was effective in attenuation of CNV.

CLINICAL RELEVANCE

Intravitreal injection of TSP1 antiangiogenic mimetic peptides may provide alternative treatment for CNV.

Novel targets against retinal angiogenesis in diabetic retinopathy

Proliferative diabetic retinopathy (PDR), characterized by pathologic retinal angiogenesis, is a major cause of blindness in the USA and globally. Treatments targeting vascular endothelial growth factor (VEGF) have emerged as a beneficial part of the therapeutic armamentarium for this condition, highlighting the utility of identifying and targeting specific pathogenic molecules. There continues to be active research into the molecular players regulating retinal angiogenesis, including pro-angiogenic factors, anti-angiogenic factors, and integrins and matrix proteinases. New insights have been especially prominent regarding molecules which regulate specialized endothelial cells called tip cells, which play a lead role in endothelial sprouting. Together, these research efforts are uncovering new, important molecular regulators of retinal angiogenesis, which provide fertile areas for therapeutic exploration. This review discusses potential molecular targets, with an emphasis towards newer targets.

Suppression of thrombospondin-1 expression during uveal melanoma progression and its potential therapeutic utility

OBJECTIVES

To determine whether expression of thrombospondin-1 (TSP1), an endogenous inhibitor of angiogenesis, is downregulated during progression of uveal melanoma and whether administration of TSP1 and/or its antiangiogenic peptides attenuate tumor growth.

METHODS

Tyrosinase-SV40 T-antigens (Tyr Tag) transgenic mice were used for evaluation of TSP1 expression during tumor progression using immunohistological methods. The therapeutic potential of TSP1 on tumor progression was evaluated either by crossing Tyr Tag mice with a line of transgenic mice overexpressing TSP1 in the eye or by administration of TSP1-mimetic peptide with known antiangiogenic, antitumor activity. Tumor areas were measured in histological sections using Optima software (Media Cybernetics, Inc).

RESULTS

The Tyr Tag tumors from 3-week-old mice showed significant TSP1 expression, which was dramatically downregulated in tumors from 12-week-old mice. Furthermore, the development and progression of tumor was significantly delayed in Tyr Tag TSP1 transgenic mice or Tyr Tag mice receiving TSP1-mimetic peptide (100 mg/kg/d).

CONCLUSIONS

Expression of TSP1 was decreased with the angiogenic switch during progression of uveal melanoma, and TSP1 and/or its antiangiogenic peptides were effective in attenuation of tumor growth.

CLINICAL RELEVANCE

Modulation of TSP1 expression and/or activity may be beneficial in treating uveal melanoma.

The vitreous glycoprotein opticin inhibits preretinal neovascularization

PURPOSE

Opticin is an extracellular matrix glycoprotein that the authors discovered in the vitreous humor of the eye. It is synthesized by the nonpigmented ciliary epithelium and secreted into the vitreous cavity and, unusually for an extracellular matrix molecule, high-level synthesis is maintained into adult life. Here the authors investigated the hypothesis that opticin influences vascular development in the posterior segment of the eye and pathologic angiogenesis into the normally avascular, mature (secondary) vitreous.

METHODS

Opticin was localized in murine eyes by immunohistochemistry. An opticin knockout mouse was established and vascular development was compared between knockout and wild-type mice. Wild-type and opticin null mice were compared in the oxygen-induced retinopathy model, a model of pathologic angiogenesis, and this model was also used to assess the effects of intravitreal injection of recombinant opticin into eyes of wild-type mice.

RESULTS

Opticin colocalizes with the collagen type II-rich fibrillar network of the vitreous, the inner limiting lamina, the lens capsule, the trabecular meshwork, and the iris. Analyses of the hyaloid and retinal vasculature showed that opticin has no effect on hyaloid vascular regression or developmental retinal vascularization. However, using the oxygen-induced retinopathy model, the authors demonstrated that opticin knockout mice produce significantly more preretinal neovascularization than wild-type mice, and the intravitreal delivery of excess opticin inhibited the formation of neovessels in wild-type mice.

CONCLUSIONS

A lack of opticin does not influence vascular development, but opticin is antiangiogenic and inhibits preretinal neovascularization.

The potential of nanomedicine therapies to treat neovascular disease in the retina

Neovascular disease in the retina is the leading cause of blindness in all age groups. Thus, there is a great need to develop effective therapeutic agents to inhibit and prevent neovascularization in the retina. Over the past decade, anti-VEGF therapeutic agents have entered the clinic for the treatment of neovascular retinal disease, and these agents have been effective for slowing and preventing the progression of neovascularization. However, the therapeutic benefits of anti-VEGF therapy can be diminished by the need for prolonged treatment regimens of repeated intravitreal injections, which can lead to complications such as endophthalmitis, retinal tears, and retinal detachment. Recent advances in nanoparticle-based drug delivery systems offer the opportunity to improve bioactivity and prolong bioavailability of drugs in the retina to reduce the risks associated with treating neovascular disease. This article reviews recent advances in the development of nanoparticle-based drug delivery systems which could be utilized to improve the treatment of neovascular disease in the retina.

Association of retinal vascular endothelial growth factor with avascular retina in a rat model of retinopathy of prematurity

OBJECTIVE

To study the effects of oxygen fluctuations on rat vascular endothelial growth factor (VEGF), VEGF receptor 1(VEGFR1), and VEGFR2 in a model of retinopathy of prematurity (ROP).

METHODS

Retinas at several postnatal days (p) were analyzed for VEGF splice variants, VEGFR1 and VEGFR2 messenger RNAs (mRNAs) using real-time polymerase chain reaction or for VEGF protein using enzyme-linked immunosorbent assay.

RESULTS

Older developmental age was associated with VEGFR1 (P < .001), VEGF(120) (P < .001), and VEGF(188) (P = .03) mRNA overexpression. Expression of VEGFR2 and VEGF(164) mRNAs were associated with older age (P < .001) or exposure to the ROP model (P = .02 and P < .001, respectively). Expression of VEGF protein was greater at p14, when 30% avascular retina existed in the ROP model, compared with room air, when no avascular retina existed, and at p18, when intravitreous neovascularization existed in the model but not in room air (P < .001 for both).

CONCLUSIONS

Unlike models of oxygen-induced retinopathy that describe ROP before implementation of oxygen regulation, the ROP model re-creates oxygen stresses relevant to preterm infants with severe ROP today. Expression of VEGF(164) and VEGFR2 mRNAs and VEGF protein were increased in association with the ROP model and older developmental age and at time points when not only intravitreous neovascularization but also avascular retina were present in the ROP model and not in room air. Clinical Relevance Regulation of VEGF may have a role in the development of avascular retina and intravitreous neovascularization in some forms of severe ROP.

CYP1B1 and endothelial nitric oxide synthase combine to sustain proangiogenic functions of endothelial cells under hyperoxic stress

We have recently shown that deletion of constitutively expressed CYP1B1 is associated with attenuation of retinal endothelial cell (EC) capillary morphogenesis (CM) in vitro and angiogenesis in vivo. This was largely caused by increased intracellular oxidative stress and increased production of thrombospondin-2, an endogenous inhibitor of angiogenesis. Here, we demonstrate that endothelium nitric oxide synthase (eNOS) expression is dramatically decreased in the ECs prepared from retina, lung, heart, and aorta of CYP1B1-deficient (CYP1B1(-/-)) mice compared with wild-type (CYP1B1(+/+)) mice. The eNOS expression was also decreased in retinal vasculature of CYP1B1(-/-) mice. Inhibition of eNOS activity in cultured CYP1B1(+/+) retinal ECs blocked CM and was concomitant with increased oxidative stress, like in CYP1B1(-/-) retinal ECs. In addition, expression of eNOS in CYP1B1(-/-) retinal ECs or their incubation with a nitric oxide (NO) donor enhanced NO levels, lowered oxidative stress, and improved cell migration and CM. Inhibition of CYP1B1 activity in the CYP1B1(+/+) retinal ECs resulted in reduced NO levels and attenuation of CM. In contrast, expression of CYP1B1 increased NO levels and enhanced CM of CYP1B1(-/-) retinal ECs. Furthermore, attenuation of CYP1B1 expression with small interfering RNA proportionally lowered eNOS expression and NO levels in wild-type cells. Together, our results link CYP1B1 metabolism in retinal ECs with sustained eNOS activity and NO synthesis and/or bioavailability and low oxidative stress and thrombospondin-2 expression. Thus CYP1B1 and eNOS cooperate in different ways to lower oxidative stress and thereby to promote CM in vitro and angiogenesis in vivo.

TRAIL-deficient mice exhibit delayed regression of retinal neovascularization

While it is well established that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in various cell types, the role of TRAIL in regulation of retinal neovascularization (NV) has not been described. Here we determined the role of TRAIL in retinal NV during oxygen-induced retinopathy using TRAIL deficient ((-/-)) mice. TRAIL and its receptor, DR5, were expressed in wild-type retinas at all time points evaluated (postnatal days 12, 17, 21, 24) during oxygen-induced retinopathy and in age-matched room air control animals. Localization of TRAIL(+) cells within the neovascular tufts of hyperoxia- exposed wild-type mice suggested TRAIL plays a role in oxygen-induced retinopathy. Retinal vascular development appeared normal in the TRAIL(-/-) mice, except for a small but significant difference in the capillary-free zone surrounding major arteries. A minimal difference in avascularity was observed at postnatal day 12 in the retinas of TRAIL(-/-) mice after hyperoxia-exposure compared with wild-type mice, suggesting that TRAIL does not play a major role in the vaso-obliterative phase of oxygen-induced retinopathy. However, at the peak of NV, TRAIL(-/-) mice had a significant increase in retinal neovascularization. In addition, when NV naturally regresses in wild-type mice, TRAIL(-/-) mice continued to display significantly high levels of NV. This was attributed to a significant decrease in neovascular tuft cells undergoing apoptosis in TRAIL(-/-) mice. Together, these data strongly suggest that TRAIL plays a role in the control of retinal NV.

Modulation of thrombospondin 1 and pigment epithelium-derived factor levels in vitreous fluid of patients with diabetes

OBJECTIVE

To determine the levels of 2 endogenous inhibitors of angiogenesis, thrombospondin 1 (TSP-1) and pigment epithelium-derived factor (PEDF), in the vitreous fluid from patients with and without diabetes.

METHODS

The levels of TSP-1 and PEDF in vitreous samples from diabetic and age-matched nondiabetic patients were determined by Western blot analysis.

RESULTS

We observed significant amounts of TSP-1 and PEDF in the vitreous samples of control eyes. The TSP-1 levels varied in samples from patients with diabetes. In contrast, PEDF levels showed little or no change in vitreous samples from patients with or without diabetes. However, the PEDF protein exhibited variation in its molecular weight among the samples. We consistently observed lower levels of TSP-1 in diabetic patients who expressed the higher-molecular-weight PEDF isoform.

CONCLUSIONS

In diabetes, changes in the TSP-1 level may play a role in shifting the angiogenic balance and contributing to the pathogenesis of diabetic retinopathy. Although the PEDF level did not change, the diabetic samples with the higher-molecular-weight PEDF isoform consistently showed lower levels of TSP-1.

CLINICAL RELEVANCE

The presence of the higher-molecular-weight PEDF isoform may be associated with greater risk of severe diabetic retinopathy.

Thrombospondin-1 modulates VEGF-A-mediated Akt signaling and capillary survival in the developing retina

Microvascular development is often perceived to result from a balance of positive and negative factors that impact signaling for proliferation and survival. The survival signaling that results from hypoxia-induced VEGF-A has been well established, but the factors that antagonize this signaling have been poorly studied. As endogenous inhibitors of angiogenesis, thrombospondins (TSPs) are likely candidates to affect survival signaling. Here we report that TSP1 antagonized microvascular survival to retinal hyperoxia, and Akt signaling in both the retina and in cultured endothelial cells. TSP1 expression is correlated with the association of the CD36 receptor with Src versus Fyn. In the presence of TSP1, CD36 is coprecipitated with Fyn as previously shown by others. However, in the absence of TSP1, there is a preferential association with Src. We now demonstrate that these Src family kinases play an important role in modulating microvascular survival in response to TSP1 by crossing tsp1(-/-) mice to the src(-/-) and fyn(-/-) mice and testing the survival of retinal blood vessels in hyperoxia. We find that tsp1(-/-), fyn(-/-), and double-mutant tsp1(-/-)/fyn(-/-) mice have a similar enhancement of capillary survival in oxygen, whereas in a tsp(-/-) background, the loss of only one allele of src restores the balance in survival and apoptosis to that of wild-type mice. Taken together, we hypothesize that TSP1 antagonizes VEGF-driven Akt survival signaling in part through the recruitment of Fyn to membrane domains containing CD36, but when TSP1 is absent, an opposing Src recruitment contributes to VEGF-driven Akt phosphorylation and capillary survival.

Attenuation of proliferation and migration of retinal pericytes in the absence of thrombospondin-1

Perivascular supporting cells, including vascular smooth muscle cells (VSMCs) and pericytes (PCs), provide instructive signals to adjacent endothelial cells helping to maintain vascular homeostasis. These signals are provided through direct contact and by the release of soluble factors by these cells. Thrombospondin (TSP)1 is a matricellular protein and an autocrine factor for VSMCs. TSP1 activity, along with that of PDGF, regulates VSMC proliferation and migration. However, the manner in which TSP1 and PDGF impact retinal PC function requires further investigation. In the present study, we describe, for the first time, the isolation and culture of retinal PCs from wild-type (TSP1(+/+)) and TSP1-deficient (TSP1(-/-)) immortomice. We showed that these cells express early and mature markers of PCs, including NG2, PDGF receptor-beta, and smooth muscle actin as well as desmin, calbindin, and mesenchymal stem cell markers. These cells were successfully passaged and maintained in culture for several months without significant loss of expression of these markers. TSP1(+/+) PCs proliferated at a faster rate compared with TSP1(-/-) PCs. In addition, TSP1(+/+) PCs, like VSMCs, responded to PDGF-BB with enhanced migration and proliferation. In contrast, TSP1(-/-) PCs failed to respond to the promigratory and proliferative activity of PDGF-BB. This may be attributed, at least in part, to the limited interaction of PDGF-BB with TSP1 in null cells, which is essential for PDGF proliferative and migratory action. We observed no significant differences in the rates of apoptosis in these cells. TSP1(-/-) PCs were also less adherent, expressed increased levels of TSP2 and fibronectin, and had decreased amounts of N-cadherin and alpha(v)beta(3)-integrin on their surface. Thus, TSP1 plays a significant role in retinal PC proliferation and migration impacting retinal vascular development and homeostasis.

CYP1B1 expression promotes the proangiogenic phenotype of endothelium through decreased intracellular oxidative stress and thrombospondin-2 expression

Reactive species derived from cell oxygenation processes play an important role in vascular homeostasis and the pathogenesis of many diseases including retinopathy of prematurity. We show that CYP1B1-deficient (CYP1B1(-/-)) mice fail to elicit a neovascular response during oxygen-induced ischemic retinopathy. In addition, the retinal endothelial cells (ECs) prepared from CYP1B1(-/-) mice are less adherent, less migratory, and fail to undergo capillary morphogenesis. These aberrant cellular responses were completely reversed when oxygen levels were lowered or an antioxidant added. CYP1B1(-/-) ECs exhibited increased oxidative stress and expressed increased amounts of the antiangiogenic factor thrombospondin-2 (TSP2). Increased lipid peroxidation and TSP2 were both observed in retinas from CYP1B1(-/-) mice and were reversed by administration of an antioxidant. Reexpression of CYP1B1 in CYP1B1(-/-) ECs resulted in down-regulation of TSP2 expression and restoration of capillary morphogenesis. A TSP2 knockdown in CYP1B1(-/-) ECs also restored capillary morphogenesis. Thus, CYP1B1 metabolizes cell products that modulate intracellular oxidative stress, which enhances production of TSP2, an inhibitor of EC migration and capillary morphogenesis. Evidence is presented that similar changes occur in retinal endothelium in vivo to limit neovascularization.

Thrombospondin-1 expression and localization in the developing ovine lung

Fetal lung growth is critically dependent on the degree to which the lungs are expanded by liquid, although the mechanisms involved are unknown. As thrombospondin-1 (TSP-1) can regulate cell proliferation, attachment, spreading and angiogenesis, we investigated the effects of alterations in fetal lung expansion on TSP-1 expression in sheep. TSP-1 mRNA levels were investigated using Northern blot analysis and in situ hybridization, whereas the protein levels were determined by immunohistochemistry. Early growth response 1 (EGR1) mRNA levels were measured by quantitative real-time PCR. TSP-1 was expressed in type-II alveolar epithelial cells and fibroblasts and its mRNA levels increased from 100.0 +/- 14.0% in control fetuses to 347.5 +/- 73.6% at 36 h of increased lung expansion (P < 0.05), and were reduced to 39.4 +/- 6.1% of control levels (100.0 +/- 20.4%) at 20 days of decreased lung expansion (P < 0.05). The percentage of cells positive for TSP-1 mRNA increased from 1.9 +/- 0.4% to 5.2 +/- 0.8% at 36 h of increased fetal lung expansion (P < 0.01). The proportion of tissue stained positive for TSP-1 protein doubled at 36 h of increased lung expansion (23.3 +/- 2.2%) compared to controls (11.7 +/- 3.2%; P < 0.05). Conversely, at 20 days of decreased lung expansion, the percentage of tissue that stained positive for TSP-1 was halved (25.7 +/- 3.2%) compared to controls (39.8 +/- 3.3%; P < 0.05). The increase in TSP-1 expression may be due to increased mRNA levels of the transcription factor EGR1 at 36 h of increased lung expansion (2.7 +/- 0.7-fold of control levels (1.0 +/- 0.2); P < 0.05). Given the known functions of TSP-1 and its localization within the lung, we speculate that TSP-1 may have a significant role in regulating fetal lung growth.

Ocular neovascularization: Implication of endogenous angiogenic inhibitors and potential therapy

Ocular neovascularization (NV) is the primary cause of blindness in a wide range of ocular diseases, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). The exact mechanism underlying the pathogenesis of ocular NV is not yet well understood, and as a consequence, there is no satisfactory therapy for ocular NV. In the last 10 years, a number of studies provided increasing evidence demonstrating that the imbalance between angiogenic stimulating factors and angiogenic inhibitors is a major contributor to the angiogenesis induced by various insults, such as hypoxia or ischemia, inflammation and tumor. The angiogenic inhibitors alone or in combination with other existing therapies are, therefore, believed to be promising in the treatment of ocular NV in the near future. This article reviews recent progress in studies on the mechanisms and treatment of ocular NV, focusing on the implication and therapeutic potential of endogenous angiogenic inhibitors in ocular NV.