Treatment with polyamine oxidase inhibitor reduces microglial activation and limits vascular injury in ischemic retinopathy

Retinal vascular injury is a major cause of vision impairment in ischemic retinopathies. Insults such as hyperoxia, oxidative stress and inflammation contribute to this pathology. Previously, we showed that hyperoxia-induced retinal neurodegeneration is associated with increased polyamine oxidation. Here, we are studying the involvement of polyamine oxidases in hyperoxia-induced injury and death of retinal vascular endothelial cells. New-born C57BL6/J mice were exposed to hyperoxia (70% O2) from postnatal day (P) 7 to 12 and were treated with the polyamine oxidase inhibitor MDL 72527 or vehicle starting at P6. Mice were sacrificed after different durations of hyperoxia and their retinas were analyzed to determine the effects on vascular injury, microglial cell activation, and inflammatory cytokine profiling. The results of this analysis showed that MDL 72527 treatment significantly reduced hyperoxia-induced retinal vascular injury and enhanced vascular sprouting as compared with the vehicle controls. These protective effects were correlated with significant decreases in microglial activation as well as levels of inflammatory cytokines and chemokines. In order to model the effects of polyamine oxidation in causing microglial activation in vitro, studies were performed using rat brain microvascular endothelial cells treated with conditioned-medium from rat retinal microglia stimulated with hydrogen peroxide. Conditioned-medium from activated microglial cultures induced cell stress signals and cell death in microvascular endothelial cells. These studies demonstrate the involvement of polyamine oxidases in hyperoxia-induced retinal vascular injury and retinal inflammation in ischemic retinopathy, through mechanisms involving cross-talk between endothelial cells and resident retinal microglia.

Endoplasmic reticulum stress-regulated CXCR3 pathway mediates inflammation and neuronal injury in acute glaucoma

Acute glaucoma is a leading cause of irreversible blindness in East Asia. The mechanisms underlying retinal neuronal injury induced by a sudden rise in intraocular pressure (IOP) remain obscure. Here we demonstrate that the activation of CXCL10/CXCR3 axis, which mediates the recruitment and activation of inflammatory cells, has a critical role in a mouse model of acute glaucoma. The mRNA and protein expression levels of CXCL10 and CXCR3 were significantly increased after IOP-induced retinal ischemia. Blockade of the CXCR3 pathway by deleting CXCR3 gene significantly attenuated ischemic injury-induced upregulation of inflammatory molecules (interleukin-1β and E-selectin), inhibited the recruitment of microglia/monocyte to the superficial retina, reduced peroxynitrite formation, and prevented the loss of neurons within the ganglion cell layer. In contrast, intravitreal delivery of CXCL10 increased leukocyte recruitment and retinal cell apoptosis. Inhibition of endoplasmic reticulum (ER) stress with chemical chaperones partially blocked ischemic injury-induced CXCL10 upregulation, whereas induction of ER stress with tunicamycin enhanced CXCL10 expression in retina and primary retinal ganglion cells. Interestingly, deleting CXCR3 attenuated ER stress-induced retinal cell death. In conclusion, these results indicate that ER stress-medicated activation of CXCL10/CXCR3 pathway has an important role in retinal inflammation and neuronal injury after high IOP-induced ischemia.

Arginase 2 deficiency reduces hyperoxia-mediated retinal neurodegeneration through the regulation of polyamine metabolism

Hyperoxia treatment has been known to induce neuronal and glial death in the developing central nervous system. Retinopathy of prematurity (ROP) is a devastating disease in premature infants and a major cause of childhood vision impairment. Studies indicate that, in addition to vascular injury, retinal neurons are also affected in ROP. Using an oxygen-induced retinopathy (OIR) mouse model for ROP, we have previously shown that deletion of the arginase 2 (A2) significantly reduced neuro-glial injury and improved retinal function. In the current study, we investigated the mechanism of A2 deficiency-mediated neuroprotection in the OIR retina. Hyperoxia treatment has been known to induce neuronal death in neonates. During the hyperoxia phase of OIR, a significant increase in the number of apoptotic cells was observed in the wild-type (WT) OIR retina compared with A2-deficient OIR. Mass spectrometric analysis showed alterations in polyamine metabolism in WT OIR retina. Further, increased expression level of spermine oxidase was observed in WT OIR retina, suggesting increased oxidation of polyamines in OIR retina. These changes were minimal in A2-deficient OIR retina. Treatment using the polyamine oxidase inhibitor, N, N'-bis (2, 3-butadienyl)-1, 4-butanediamine dihydrochloride, significantly improved neuronal survival during OIR treatment. Our data suggest that retinal arginase is involved in the hyperoxia-induced neuronal degeneration in the OIR model, through the regulation of polyamine metabolism.

The role of arginase I in diabetes-induced retinal vascular dysfunction in mouse and rat models of diabetes

AIMS/HYPOTHESIS

A reduction in retinal blood flow occurs early in diabetes and is likely to be involved in the development of diabetic retinopathy. We hypothesise that activation of the arginase pathway could have a role in the vascular dysfunction of diabetic retinopathy.

METHODS

Experiments were performed using a mouse and rat model of streptozotocin (STZ)-induced diabetes for in vivo and ex vivo analysis of retinal vascular function. For in vivo studies, mice were infused with the endothelial-dependent vasodilator acetylcholine (ACh) or the endothelial-independent vasodilator sodium nitroprusside (SNP), and vasodilation was assessed using a fundus microscope. Ex vivo assays included pressurised vessel myography, western blotting and arginase activity measurements.

RESULTS

ACh-induced retinal vasodilation was markedly impaired in diabetic mice (40% of control values), whereas SNP-induced dilation was not altered. The diabetes-induced vascular dysfunction was markedly blunted in mice lacking one copy of the gene encoding arginase I and in mice treated with the arginase inhibitor 2(S)-amino-6-boronohexanoic acid. Ex vivo studies performed using pressure myography and central retinal arteries isolated from rats with STZ-induced diabetes showed a similar impairment of endothelial-dependent vasodilation that was partially blunted by pretreatment of the isolated vessels with another arginase inhibitor, (S)-2-boronoethyl-L-cysteine. The diabetes-induced vascular alterations were associated with significant increases in both arginase I protein levels and total arginase activity.

CONCLUSIONS/INTERPRETATION

These results indicate that, in the mouse and rat model, diabetes-induced increases in arginase I were involved in the diabetes-induced impairment of retinal blood flow by a mechanism involving vascular endothelial cell dysfunction.

Oxidative species increase arginase activity in endothelial cells through the RhoA/Rho kinase pathway

BACKGROUND AND PURPOSE

NO produced by endothelial NOS is needed for normal vascular function. During diabetes, aging and hypertension, elevated levels of arginase can compete with NOS for available l-arginine, reducing NO and increasing superoxide (O(2) (.-)) production via NOS uncoupling. Elevated O(2) (.-) combines with NO to form peroxynitrite (ONOO(-)), further reducing NO. Oxidative species increase arginase activity, but the mechanism(s) involved are not known. Our study determined the mechanism involved in peroxynitrite and hydrogen peroxide-induced enhancement in endothelial arginase activity. We hypothesized that oxidative species increase arginase activity through PKC-activated RhoA/Rho kinase (ROCK) pathway.

EXPERIMENTAL APPROACH

Arginase activity/expression was analysed in bovine aortic endothelial cells (BAEC) treated with an ONOO(-) generator (SIN-1) or H(2) O(2). Pretreatment with inhibitors of Rho kinase (Y-27632) or PKC (Gö6976) was used to investigate the mechanism involved in arginase activation.

KEY RESULTS

Exposure to SIN-1 (25 µM, 24 h) or H(2) O(2) (25 µM, 8 h) increased arginase I expression and arginase activity (35% and 50%, respectively), which was prevented by ROCK inhibitor, Y-27632, PKC inhibitor, Gö6976 or siRNA to p115-Rho GEF. There was an early activation of p115-Rho GEF (SIN-1, 2 h; H(2) O(2), 1 h) and Rho A (SIN-1, 4 h; H(2) O(2), 1 h) that was prevented by using the PKC inhibitor. Exposure to SIN-1 and H(2) O(2 ) also reduced NOS activity, which was blocked by pretreatment with p115-RhoGEF siRNA.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that the oxidative species ONOO(-) and H(2) O(2) increase arginase activity/expression through PKC-mediated activation of RhoA/Rho kinase pathway.

Vascular endothelial growth factor in eye disease

Collectively, angiogenic ocular conditions represent the leading cause of irreversible vision loss in developed countries. In the US, for example, retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration are the principal causes of blindness in the infant, working age and elderly populations, respectively. Evidence suggests that vascular endothelial growth factor (VEGF), a 40kDa dimeric glycoprotein, promotes angiogenesis in each of these conditions, making it a highly significant therapeutic target. However, VEGF is pleiotropic, affecting a broad spectrum of endothelial, neuronal and glial behaviors, and confounding the validity of anti-VEGF strategies, particularly under chronic disease conditions. In fact, among other functions VEGF can influence cell proliferation, cell migration, proteolysis, cell survival and vessel permeability in a wide variety of biological contexts. This article will describe the roles played by VEGF in the pathogenesis of retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration. The potential disadvantages of inhibiting VEGF will be discussed, as will the rationales for targeting other VEGF-related modulators of angiogenesis.

Biotechnology and the chicken B cell line DT40

Protein optimization is a major focus of the biotech and pharmaceutical industry. Various in vitro technologies have been developed to accelerate protein evolution and to achieve protein optimization of functional characteristics such as substrate specificity, enzymatic activity and thermostability. The chicken B cell line DT40 diversifies its immunoglobulin (Ig) gene by gene conversion and somatic hypermutation. This machinery can be directed to almost any gene inserted into the Ig locus. Enormously diverse protein libraries of any gene of interest can be quickly generated in DT40 by utilizing random shuffling of complex genetic domains (gene conversion) and by the introduction of novel non-templated genetic information (random mutagenesis). The unique characteristics of the chicken cell line DT40 make it a powerful in-cell diversification system to improve proteins of interest within living cells. One essential advantage of the DT40 protein optimization approach is the fact that variants are generated within an in-cell system thus allowing the direct screening for desired features in the context of intracellular networks. Utilizing specially designed selection strategies, such as the powerful fluorescent protein technology, enables the reliable identification of protein variants exhibiting the most desirable traits. Thus, DT40 is well positioned as a biotechnological tool to generate optimized proteins by applying a powerful combination of gene specific hypermutation, gene conversion and mutant selection.

Homocysteine induces endothelial dysfunction via inhibition of arginine transport

Hyperhomocysteinemia is an independent risk factor for cardiovascular diseases. High levels of plasma homocysteine (HCY) increase oxidative stress and reduce endothelial-dependent relaxation. We determined whether hyperhomocysteinemia-induced endothelial dysfunction is mediated through inhibition of cellular transport of L-arginine. In endothelial cells, HCY had a biphasic effect on arginine transport. HCY treatment for 6 hr increased L-arginine uptake by 34%; however, uptake was decreased by 25% after 24 h. HCY caused membrane hyperpolarization during both 6 and 24 h incubation periods, indicating that the negative charge facilitating arginine uptake was maintained. HCY significantly reduced expression of cellular arginine transporter protein (CAT-1) after 24 h treatment; whereas endothelial nitric oxide synthase (eNOS) protein levels and basal eNOS activity were not altered. Nevertheless, nitric oxide (NO) formation was significantly decreased. The antioxidant ascorbic acid prevented the effect of HCY on arginine transport. HCY induced formation of the peroxynitrite biomarker nitrotyrosine, which was blocked by supplemental L-arginine. HCY treatment of aortic rings caused decreased vasorelaxation to acetylcholine, which was prevented by supplemental arginine. In conclusion, HCY decreased NO formation and induced endothelial dysfunction without altering protein level or basal activity of eNOS, but through decreases in function and protein expression of the CAT-1 transporter. Reduced arginine supply may lead to eNOS uncoupling and generation of superoxide, contributing to HCY-induced oxidative stress.

Peroxynitrite mediates VEGF's angiogenic signal and functionviaa nitration‐independent mechanism in endothelial cells

The modulation of angiogenic signaling by reactive oxygen species (ROS) is an emerging area of interest in cellular and vascular biology research. We provide evidence here that peroxynitrite, the powerful oxidizing and nitrating free radical, is critically involved in transduction of the VEGF signal. We tested the hypothesis that VEGF induces peroxynitrite formation, which causes tyrosine phosphorylation and mediates endothelial cell migration and tube formation, by studies of vascular endothelial cells in vitro and in a model of hypoxia-induced neovascularization in vivo. The specific peroxynitrite decomposition catalyst FeTPPs blocked VEGF-induced phosphorylation of VEGFR2 and c-Src and inhibited endothelial cell migration and tube formation. Furthermore, exogenous peroxynitrite mimicked VEGF activity in causing phosphorylation of VEGFR2 and stimulating endothelial cell growth and tube formation in vitro and new blood vessel growth in vivo. The selective nitration inhibitor epicatechin enhanced VEGF's angiogenic function in activating VEGFR2, c-Src, and promoting endothelial cell growth, migration, and tube formation in vitro and retinal neovascularization in vivo. Decomposing peroxynitrite with FeTPPs or blocking oxidation using the thiol donor NAC blocked VEGF's angiogenic functions in vitro and in vivo. In conclusion, peroxynitrite is critically involved in transducing VEGF's angiogenic signal via nitration-independent and oxidation-mediated tyrosine phosphorylation.

Nerve growth factor supplementation reverses the impairment, induced by Type 1 diabetes, of hindlimb post-ischaemic recovery in mice

AIMS/HYPOTHESIS

Type 1 diabetes increases the risk of peripheral ischaemia and impairs recovery once ischaemia occurs, probably because the healing process is hampered by diabetes-induced endothelial dysfunction. In normoglycaemic mice subjected to limb ischaemia, blockade of nerve growth factor (NGF) compromises reparative angiogenesis. In the present study, we evaluated if expressional alterations of endogenous NGF system components are associated with diabetes-related impairment in neovascularisation. In addition, we tested whether the correction of NGF liabilities benefits post-ischaemic healing of Type 1 diabetic animals.

METHODS

Unilateral hindlimb ischaemia was produced in streptozotocin-induced Type 1 diabetic mice. Purified murine NGF (20 microg daily for 14 days) or PBS were injected into ischaemic adductors. Non-diabetic mice given PBS served as controls. Hindlimb blood flow was analysed sequentially for up to 14 days. At necroscopy, adductors were removed for quantification of microvessel density, endothelial cell apoptosis and NGF receptor expression. NGF content was determined by ELISA three days after ischaemia. In vitro, we tested whether NGF protects endothelial cells from apoptosis induced by high glucose and whether vascular endothelial growth factor-A (VEGF-A) is involved in this beneficial effect.

RESULTS

Muscles removed from Type 1 diabetic mice showed reduced NGF content and up-regulation of the NGF p75 receptor. NGF supplementation promoted capillarisation and arteriogenesis, reduced apoptosis, and accelerated blood flow recovery. NGF stimulated VEGF-A production by human endothelial cells incubated in high-glucose medium and conferred resistance against high-glucose-induced apoptosis via a VEGF-A-mediated mechanism.

CONCLUSIONS/INTERPRETATION

NGF protects endothelial cells from apoptosis induced by Type 1 diabetes and facilitates reparative neovascularisation. The findings may open up new therapeutic options for the treatment of diabetic complications.

Roles of superoxide, peroxynitrite, and protein kinase C in the development of tolerance to nitroglycerin

A current hypothesis states that tolerance to nitroglycerin (GTN) involves increased formation of superoxide (O2*-). Studies showing that inhibitors of protein kinase C (PKC) prevent tolerance to GTN suggest the involvement of PKC activation, which can also increase O2*-. We examined the roles of O2*-, peroxynitrite (ONOO-), and PKC activation in GTN tolerance. Pre-exposure of rat aortic rings to GTN (5 x 10(-4) M) for 2 h caused tolerance to the vasodilating effect of GTN, as evidenced by a substantial rightward shift of GTN concentration-relaxation curves. This shift was reduced by treatment of the rings with the antioxidants uric acid, vitamin C, or tempol or the PKC inhibitor chelerythrine. We also found that O2*- generation via xanthine/xanthine oxidase in the bath induced tolerance to GTN. However, responses to nitroprusside were not affected. In vivo tolerance produced in rats by 3-day i.v. infusion of GTN was also almost completely prevented by coinfusion of tempol. In bovine aortic endothelial cells (EC), addition of GTN produced a marked increase in tyrosine nitrosylation, indicating increased ONOO- formation. This action was blocked by prior treatment with uric acid, superoxide dismutase, NG-nitro-L-arginine methyl ester, or chelerythrine. We also demonstrated that GTN translocates the alpha- and epsilonPKC isoforms in EC. However, PKCzeta was not affected by GTN treatment. In conclusion, tolerance to GTN involves enhanced production of O2*- and ONOO- and activation of NO synthase. Furthermore, sustained activation of alpha- and epsilonPKC isozymes in EC by GTN may play a role in development of tolerance.

Endothelial cell dysfunction in a model of oxidative stress

BACKGROUND

We have investigated the role of L-arginine in hyperhomocysteinemia (HHCY). L-arginine is the substrate required for NO production by endothelial NOS (eNOS). When L-arginine is limited, NOS acts principally upon O2 to form superoxide (O2.-). Because HHCY causes formation of reactive oxygen species and reduced endothelial-dependent vasodilation, we hypothesized that HHCY decreases NO formation by limiting the cellular supply of L-arginine.

MATERIAL AND METHODS

Studies with cultured bovine aortic endothelial cells (ECs) determined effects of HCY on transport of [3H] L-arginine. Effects on L-arginine transporter protein CAT-1 and eNOS protein were assessed by immunoblotting. Peroxynitrite formation was evaluated by an immunoassay for nitrotyrosine levels. eNOS activity in forming NO was determined by assay for 3H-L-arginine to 3H-citrulline conversion.

RESULTS

HCY had a depressive effect on arginine transport in ECs. HCY treatment for a 24 hr period decreased arginine uptake by 27%. HCY treatment for 24 hr significantly reduced cellular levels of the CAT-1 arginine transporter protein ( approximately 30%) and increased nitrotyrosine formation, whereas levels of eNOS protein and basal NOS activity were not altered. Nevertheless, total NO production as indicated by citrulline conversion was significantly decreased. Treatment with the antioxidant N-acetylcysteine reversed the HCY effect on arginine transport, suggesting that transporter oxidation may contribute to the endothelial dysfunction.

CONCLUSIONS

The association of HCY-induced decreases in NO formation with decreases in function and expression of the arginine transporter in the absence of alterations in eNOS expression or activity suggests a primary role for arginine transport alterations in HHCY. The action of HCY to reduce arginine uptake may accentuate endothelial dysfunction due to generation of O2.- and peroxynitrite formation, which may cause further oxidative injury.

TGF-beta increases retinal endothelial cell permeability by increasing MMP-9: possible role of glial cells in endothelial barrier function

PURPOSE

To determine transforming growth factor (TGF) beta effects on matrix metalloproteinases (MMPs) as a potential cause of the blood-retinal barrier breakdown at the onset of angiogenesis. Previously, glial cells were shown to play a role in the angiogenesis process and to express the angiogenic regulating factor TGF-beta, which becomes active under hypoxia conditions. Here, the authors demonstrate that retinal endothelial cells express MMP-9 when treated with TGF-beta or cocultured with glial cells and that both TGF-beta and MMP-9 increase endothelial cell permeability.

METHODS

Primary cultures of bovine retinal endothelial (BRE) cells grown on porous membranes were treated with TGF-beta or purified MMP-9, and permeability changes were assayed. The amount and distribution of the tight junction protein occludin also was analyzed by immunocytochemistry and Western blotting. Cell extracts or conditioned media from TGF-beta-treated BRE cells and from glial cell-BRE cocultures were analyzed for MMP-9 content by substrate gel electrophoresis (zymography) or Western blotting.

RESULTS

Both TGF-beta and MMP-9 increased the permeability of BRE monolayers and reduced the levels of the junction protein occludin. The effect of MMP-9 on permeability was rapid, but the TGF-beta-induced permeability required longer incubation and was blocked by anti-TGF-beta and anti-MMP-9 antibodies as well as by TGF-beta latency-associated peptide. Zymography showed that MMP-9 activity, which was very low or absent in untreated BRE cultures, was dramatically increased by TGF-beta as well as by coculturing with either astrocytes or Müller glial cells. Anti-TGF-beta antibody blocked the TGF-beta effect, but not the coculture effect on MMP-9 production.

CONCLUSIONS

These data indicate a direct correlation between TGF-beta-induced MMP-9 activity and increased endothelial cell permeability. Moreover, endothelial cell production of MMP-9 is regulated by glial cells through expression of TGF-beta or by direct cell-to-cell contact. During retinal disease, glial cell production of active TGF-beta may contribute to breakdown of the blood-retinal barrier by stimulating endothelial cell MMP-9 production.

Reduced severity of oxygen-induced retinopathy in eNOS-deficient mice

PURPOSE

Exposure of premature human infants to hyperoxia results in the obliteration of developing retina capillaries, leading to a vision-threatening retinopathy termed retinopathy of prematurity (ROP). The authors hypothesized that this process may be mediated in part by endothelial nitric oxide (NO)-derived oxidants such as peroxynitrite and tested this hypothesis in a mouse model of ROP.

METHODS

Normal mice, mice treated with the nitric oxide synthase (NOS) inhibitor N:(G)-nitro-L-arginine (L-NNA), and knockout mice carrying a homozygous targeted disruption of the gene for endothelial NOS (eNOS) were studied in an experimental model of ROP. Retinas were compared for extent of capillary obliteration in hyperoxia, vascular endothelial growth factor (VEGF) expression, nitrotyrosine formation, and vitreous neovascularization.

RESULTS

Oxygen-induced retinal vaso-obliteration was significantly reduced by L-NNA treatment (43% decrease from controls). The eNOS-deficient mice showed a similar reduction in vaso-obliteration (46% decrease from controls), and vitreous neovascularization was also substantially reduced (threefold decrease). Retinal nitrotyrosine formation, a measure of in situ peroxynitrite modification of proteins, was significantly elevated in normal mice during hyperoxia, in a spatial and temporal pattern consistent with a role in oxygen-induced vaso-obliteration. This was not seen in eNOS-deficient mice. VEGF expression was similar in both groups of mice, although suppression in hyperoxia was slightly blunted in eNOS-deficient mice.

CONCLUSIONS

These data suggest a role for NO and peroxynitrite in the pathogenesis of ROP. Therapies aimed at modulation of eNOS activity may have therapeutic potential for preventing ROP.

Endothelial nitric oxide synthase is a site of superoxide synthesis in endothelial cells treated with glyceryl trinitrate

Tolerance to glyceryl trinitrate (GTN) involves superoxide (O(2)(*-)) production by endothelial cells. Nitric oxide synthase (NOS) produces O(2)(*-) when L-arginine (L-arg) is limited. The purpose of this study was to test the hypothesis that GTN stimulates NOS to increase O(2)(*-) synthesis in endothelial cells when L-arg is limited. Production of O(2)(*-) by bovine aortic endothelial cells (BAEC, passages 3 - 5) was determined by spectrophotometrically measuring superoxide dismutase-inhibited reduction of ferricytochrome C to ferrocytochrome C. Cells were incubated in buffer without L-arg. O(2)(*-) production was measured using BAEC either untreated or treated with L-NAME or L-arg alone or following treatment with GTN (10(-9) to 10(-6) M) for 30 min or DPTA NONOate (10(-7) and 10(-6) M) alone or with GTN or DPTA NONOate after pretreatment with nitro-L-arginine methyl ester (L-NAME), L-arg or their inactive enantiomers, D-NAME or D-arg (all 5 x 10(-4) M) (n=6 - 7/group). L-NAME alone produced a 69% reduction in O(2)(*-) levels. Treatment with L-arg alone had no effect. Cells treated with GTN alone exhibited an increase in O(2)(*-). This effect was prevented by pretreatment with either L-NAME or L-arg, and was unaffected by D-NAME or D-arg. We observed a dose-response relationship in O(2)(*-) production to GTN over a range of 10(-9) to 10(-7) M. The NO donor, DPTA-NONOate, unlike GTN, did not have a significant effect on O(2)(*-) production. In conclusion, endothelial NOS is a site of O(2)(*-) synthesis in endothelial cells activated by GTN.

Vascular endothelial growth factor activates STAT proteins in aortic endothelial cells

Vascular endothelial growth factor (VEGF) intracellular signaling in endothelial cells is initiated by the activation of distinct tyrosine kinase receptors, VEGFR1 (Flt-1) and VEGFR2 (Flk-1/KDR). Because the tyrosine kinase-dependent transcription factors known as STAT (signal transducers and activators of transcription) proteins are important modulators of cell growth responses induced by other growth factor receptors, we have determined the effects VEGF of on STAT activation in BAEC (bovine aortic endothelial cells). Here, we show that VEGF induces tyrosine phosphorylation and nuclear translocation of STAT1 and STAT6. VEGF also stimulates STAT3 tyrosine phosphorylation, but nuclear translocation does not occur. We found that placenta growth factor, which selectively activates VEGFR1, has no effect on the STATs. However, upon VEGF stimulation, STAT1 associates with the VEGFR2 in a tyrosine kinase-dependent manner, indicating that VEGF-induced STAT1 activation is mediated primarily by VEGFR2. Thus, our study shows for the first time that VEGF activates the STAT pathway through VEGFR2. Because the growth-promoting activity of VEGF depends upon VEGFR2 activation, these findings suggest a role for the STATs in the regulation of gene expression associated with the angiogenic effects of VEGF.

Effect of nitrite on endothelial function in isolated lung

Nitrated tyrosine, implicated in protein dysfunction, is increased in various tissues in association with diverse pathological processes. Angiotensin converting enzyme (ACE) is a luminal vascular endothelial enzyme whose dysfunction is an early sign of endothelial injury. ACE contains a tyrosine critical for its enzymatic activity. Others have shown that nitrite exacerbates the ACE dysfunction of cultured endothelial cells in contact with activated polymorphonuclear neutrophils (PMN). We hypothesized that exogenous nitrite would enhance endothelial ACE dysfunction associated with PMN activation in the isolated lung. Rats received lipopolysaccharide (LPS) 2 h prior to isolated lung perfusion with Ficoll containing buffer. Either formyl-Met-Leu-Phe (fMLP, 10(-7) M) or phorbol myristate acetate (PMA, 10(-7) M) was used to activate PMN in lungs treated or not treated with 300-microM nitrite. A first pass indicator dilution method and first order reaction kinetics were used to determine ACE activity, while lung Ficoll content served as an index of vascular permeability. Both fMLP and PMA decreased endothelial ACE activity and increased pulmonary artery pressure, edema and vascular permeability. Exogenous nitrate did not potentiate the decrease in ACE activity, the lung injury or nitrotyrosine immunoreactivity of lung homogenates. In contrast to observations in cultured endothelial cells, our findings in the whole lung are compatible with the speculation of others that the rat lung has an unidentified factor, which minimizes accumulation of nitrated proteins.

Vascular and endothelial actions of inhibitors of substance P amidation

Formation of mature active neuropeptides such as substance P (SP) from their glycine extended precursors entails alpha-amidation of peptide precursors by the sequential enzymatic action of peptidylglycine alpha-monooxygenase (PAM) and peptidylamidoglycolate lyase (PGL). We reported that these two enzymes that can produce mature active neuropeptides are present in cultured bovine aortic endothelial cells (BAECs). We hypothesize that alpha-amidation of peptides occurs in endothelial cells and that these peptides are critically involved in the overall regulation of cardiovascular function. In this study, this hypothesis was tested using specific amidation inhibitors to determine their effects on the actions of SP and its glycine-extended precursor (SP-Gly). We have found that SP and SP-Gly are equipotent in stimulating nitric oxide (NO) release by BAECs. At 10(-5) M, the specific inhibitors of PAM (4-phenyl-3-butenoic acid; PBA) and PGL (5-acetamido-2,4-diketo-6-phenyl-hexanoic acid and its methyl ester) reduced NO basal release by 40, 34, and 45%, respectively. They also reduced the production of NO induced by SP-Gly by 63, 68, and 69%, respectively, but had no effect on NO production in response to either SP or acetylcholine. SP and SP-Gly also were equipotent in relaxing rat aortic segments. The vasorelaxation to SP-Gly was endothelium dependent and inhibited by the NOS antagonist L-nitroarginine methyl ester (L-NAME), but it was not affected by inhibition of prostaglandin synthesis. Inhibitors of both PAM and PGL significantly reduced the vasorelaxing actions of SP-Gly, whereas responses to SP were not affected. A cumulative infusion of PBA into the femoral artery of rabbits, at final concentrations of 2.4, 24, and 240 microM for 20 min each, increased the vascular resistance (VR), indicating the tonic production of vasodilating amidated peptide(s). This effect was maximum at 60 min after infusion (20.5 +/- 4.7 vs. 8.2 +/- 0.7 mm Hg/ml/min; p < 0.05). These results suggest that endothelial cells can produce mature SP from its SP-Gly precursor and that a product of peptide alpha-amidation tonically stimulates endothelial cell NO release to control vascular tone.

Role of L-arginine in the vascular actions and development of tolerance to nitroglycerin

The goal of this work was to test the role of nitric oxide synthase (NOS) and its substrate L-arginine in development of tolerance to nitroglycerin's (GTN) vasodilator actions. GTN's effects on NOS activity and NO formation were tested in cultured bovine aortic endothelial cells (BAECs). The arginine to citrulline conversion assay showed that GTN stimulated NOS basal activity in BAECs by approximately 40%, comparable with acetylcholine (ACh)-treated controls. Both effects were blocked by L-NMMA. Photometric assays showed that both GTN and ACh-stimulated NO formation. Both effects were potentiated by L-arginine and inhibited by L-NAME. L-NAME inhibited ACh responses approximately 80% compared with approximately 40% for GTN responses. The aortic ring assay showed that 2 h pretreatment with GTN caused substantial tolerance to GTN's vasodilating effects as evidenced by a 38 fold rightward shift of the concentration-relaxation curve. In contrast to D-arginine, addition of L-arginine substantially inhibited this effect, reducing the rightward shift to 4.4 fold of control values. GTN tolerance was associated with a 40% reduction in L-arginine tissue levels. GTN had a biphasic effect on BAEC uptake of L-arginine, stimulating uptake at 5 and 15 min, and suppressing uptake after 1 and 4 h In summary, acute GTN treatment stimulates endothelial NOS activity in producing NO and increases cellular uptake of L-arginine. Prolonged GTN exposure reduces GTN's vasodilator actions, decreases L-arginine tissue levels and depresses BAECs uptake of L-arginine. Supplementation of L-arginine reduces development of GTN tolerance. These data indicate that GTN tolerance depends in part on activation of the NOS pathway.

Endothelial nitric oxide synthase interactions with G-protein-coupled receptors

The endothelial nitric oxide synthase (eNOS) is activated in response to stimulation of endothelial cells by a number of vasoactive substances including, bradykinin (BK), angiotensin II (Ang II), endothelin-1 (ET-1) and ATP. In the present study we have used in vitro activity assays of purified eNOS and in vitro binding assays with glutathione S-transferase fusion proteins to show that the capacity to bind and inhibit eNOS is a common feature of membrane-proximal regions of intracellular domain 4 of the BK B2, the Ang II AT1 and the ET-1 ETB receptors, but not of the ATP P2Y2 receptor. Phosphorylation of serine or tyrosine residues in the eNOS-interacting region of the B2 receptor results in a loss of eNOS inhibition due to a decrease in the binding affinity of the receptor domain for the eNOS enzyme. Furthermore, the B2 receptor is transiently phosphorylated on tyrosine residues in cultured endothelial cells in response to BK stimulation. Phosphorylation occurs during the time in which eNOS transiently dissociates from the receptor accompanied by a transient increase in nitric oxide production. Taken together, these data support the hypotheses that eNOS is regulated in endothelial cells by reversible and inhibitory interactions with G-protein-coupled receptors and that these interactions can be modulated by receptor phosphorylation.

Vascular endothelial growth factor signals endothelial cell production of nitric oxide and prostacyclin through flk-1/KDR activation of c-Src

Vascular endothelial growth factor (VEGF) is a potent endothelial cell-specific mitogen that promotes angiogenesis, vascular hyperpermeability, and vasodilation by autocrine mechanisms involving nitric oxide (NO) and prostacyclin (PGI(2)) production. These experiments used immunoprecipitation and immunoassay procedures to characterize the signaling pathways by which VEGF induces NO and PGI(2) formation in cultured endothelial cells. The data showed that VEGF stimulates complex formation of the flk-1/kinase-insert domain-containing receptor (KDR) VEGF receptor with c-Src and that Src activation is required for VEGF induction of phospholipase C gamma1 activation and inositol 1,4,5-trisphosphate formation. Reporter cell assays showed that VEGF promotes a approximately 50-fold increase in NO formation, which peaks at 5-20 min. This effect is mediated by a signaling cascade initiated by flk-1/KDR activation of c-Src, leading to phospholipase C gamma1 activation, inositol 1,4,5-trisphosphate formation, release of [Ca(2+)](i) and nitric oxide synthase activation. Immunoassays of VEGF-induced 6-keto prostaglandin F(1alpha) formation as an indicator of PGI(2) production revealed a 3-4-fold increase that peaked at 45-60 min. The PGI(2) signaling pathway follows the NO pathway through release of [Ca(2+)](i), but diverges prior to NOS activation and also requires activation of mitogen-activated protein kinase. These results suggest that NO and PGI(2) function in parallel in mediating the effects of VEGF.

VEGF induces nuclear translocation of Flk-1/KDR, endothelial nitric oxide synthase, and caveolin-1 in vascular endothelial cells

VEGF increases endothelial cell permeability and growth by a process requiring NOS activity. Because eNOS activity is regulated by its interaction with the caveolar structural protein caveolin-1, we analyzed VEGF effects on structural interactions between eNOS, caveolin-1 and the VEGF receptor Flk-1/KDR. Confocal immunolocalization analysis of the subcellular distribution of Flk-1/KDR, caveolin-1 and eNOS showed that VEGF stimulated the translocation of all three proteins into the nucleus. This result was confirmed by cell fractionation and immunoblotting studies showing that levels of all three proteins within the caveolar compartment declined progressively after 30 and 60 min of VEGF treatment. The pattern was reversed for nuclear fractions. Protein levels were lowest in the control cultures, but increased progressively after 30 and 60 min of treatment. Nuclear translocation of eNOS and Flk-1/KDR within caveolae may represent a mechanism for targeting NO production to the nuclear compartment where it could influence transcription factor activation.

VEGF-induced permeability increase is mediated by caveolae

PURPOSE

To determine the cellular route by which vascular endothelial cell growth factor (VEGF) increases the permeability of cultured retinal endothelial cells and to test whether nitric oxide (NO) production by NO synthase (NOS) is involved in signaling VEGF's permeability enhancing effects.

METHODS

Cultured bovine retinal microvascular endothelial (BRE) cells were used for bioassay of permeability function and its ultrastructural correlates. The role of NOS activity in VEGF's permeability enhancing effects was tested with the use of an NOS inhibitor. Because activity of endothelial NOS (eNOS) is thought to be regulated by its interaction with the caveolar protein caveolin-1, structural relationships between eNOS, caveolin-1, and the VEGF receptor FIk-1/KDR were analyzed with double-label immunofluorescence and cell fractionation procedures.

RESULTS

Bioassays of permeability function and structure demonstrated that VEGF increases permeability of cultured BRE cells by an NOS-dependent process of transcytotic transport in caveolae. Double-label analysis showed that Flk-1/KDR and eNOS colocalize with caveolin-1 in plasma membrane caveolae. Cell fractionation and immunoblot analysis confirmed this effect. Densitometry showed that Flk-1/KDR, eNOS, and caveolin-1 levels were highest in caveolar fractions. Similar results were obtained in studies with bovine aortic endothelial cells.

CONCLUSIONS

These results demonstrate that VEGF increases endothelial cell permeability by an eNOS-dependent mechanism of transcytosis in caveolae. Localization of Flk-1/KDR and eNOS with caveolin-1 suggests that VEGF signaling occurs within the caveolar compartment.

Pravastatin sodium activates endothelial nitric oxide synthase independent of its cholesterol-lowering actions

OBJECTIVES

We tested the hypothesis that pravastatin (PRA) activates endothelial nitric oxide synthase (eNOS).

BACKGROUND

Pravastatin has been found to have clinical benefits beyond those predicted by its actions in reducing plasma low density lipoprotein cholesterol (LDL). Both PRA and simvastatin (SIM) are equally effective in reducing LDL, but only PRA reduces platelet aggregation and is an effective vasodilator. Nitric oxide (NO) also inhibits platelet aggregation and vasodilates.

METHODS

We determined PRA and SIM effects on vasorelaxation in aortic rings and NO production by cultured bovine aortic endothelial cells. Nitric oxide was measured by using a NO electrode and by an assay for conversion of hemoglobin to methemoglobin. Specificity of NOS activation was tested by using the NOS inhibitor nitro-L-arginine methyl ester (L-NAME, 1 mmol/liter) in the presence or absence of excess L-arginine (L-ARG, 1 mmol/liter).

RESULTS

Endothelium-dependent vasorelaxation was maximal with acetylocholine (ACH, 100%), followed by PRA (62.8%) and then SIM (37.1%). Direct measurement of NO confirmed that vasorelaxation is due to NO release and showed that PRA and ACH had similar dose-dependent effects on NO production, while SIM was only 25% to 30% as effective. Methemoglobin assay confirmed these results and demonstrated their specificity for NOS activity. The L-NAME blunted the responses to 45% of initial values. Excess L-ARG reversed this effect and potentiated NO production to 133% of initial levels.

CONCLUSIONS

Both PRA and SIM activate eNOS, but SIM is much less effective. Clinical benefits with PRA not explained by LDL reductions may be the result of an independent action of PRA on eNOS activation.

Inhibition by the JAK/STAT pathway of IFNgamma- and LPS-stimulated nitric oxide synthase induction in vascular smooth muscle cells

Inducible nitric oxide synthase (iNOS) is induced in many cell types by cytokines and lipopolysaccharide (LPS). Cytokine signal transduction is believed to be mediated primarily through the JAK/STAT pathway. We therefore examined the effects of a JAK2-specific inhibitor, an antisense oligonucleotide to JAK2, and electroporation of neutralizing anti-STAT1 and anti-STAT3 antibodies on IFNgamma- and LPS-stimulated induction of iNOS in vascular smooth muscle cells. Unexpectedly, we found that the JAK/STAT pathway suppresses IFNgamma- and LPS-stimulated iNOS induction in these cells. In contrast, the JAK/STAT pathway appears to have a positive role in iNOS induction in RAW 264.7 macrophages.

Effects of hypoxia on glial cell expression of angiogenesis-regulating factors VEGF and TGF-beta

Perivascular glial cells are thought to be involved in physiologic vascularization and also in pathologic angiogenesis in the central nervous system. We have previously shown that astrocytes are a source of transforming growth factor-beta (TGF-beta) and another inhibiting factor, which block endothelial cell growth and induce their apoptosis. Astroglia are also known to express vascular endothelial growth factor (VEGF), which is up-regulated during hypoxia. Here we demonstrate the effects of hypoxia on the expression of both TGF-beta and VEGF by retinal glial cells. Muller cells isolated from rat retina were incubated under hypoxia or normoxia and the resulting conditioned media (H-MCM and N-MCM) were assayed for their effects on growth of bovine retinal capillary endothelial (BRE) and the TGF-beta-sensitive mink lung epithelial CCL cells. The expression and quantities of VEGF and TGF-beta (active vs. latent form) were determined by immuno-adsorption, Western or Northern blotting, and ELISA. N-MCM stimulated BRE cell growth by twofold but inhibited CCL cells under similar assay conditions, whereas H-MCM had a weak stimulating effect on BRE and substantial inhibitory activity on CCL cells. Adsorption of MCM by specific antibodies as well as Western and Northern blot analysis indicated that stimulating and inhibitory activities of MCM are due to the presence of VEGF and TGF-beta, respectively. ELISA revealed that the hypoxia condition converts latent TGF-beta into its active form. In N-MCM, TGF-beta is found predominantly in the latent form, but in hypoxia MCM it is mainly active. Furthermore, it was found that treatment of Muller cells with exogenous TGF-beta under either hypoxia or normoxia increases VEGF expression in a time- and dose-dependent fashion. TGF-beta activation may, therefore, be prerequisite for hypoxia-induced up-regulation of VEGF and stimulation of angiogenesis in vivo.

Modulation of VEGF production by pH and glucose in retinal Müller cells

PURPOSE

To investigate the influence of pH and glucose concentration, both of which represent significant biochemical variables in tissue ischemia, on the production of VEGF protein by retinal Müller cells and C6 glioma cells, under normoxic and hypoxic conditions.

METHODS

Rat retinal Müller cells and C6 glioma cells grown in tissue culture monolayers were studied. The effect of pH (range 7.0-8.0) and glucose concentration (0.6-25 mmol/L) on VEGF protein production, under both normoxic and hypoxic conditions, were evaluated by ELISA analysis of the conditioned media. Establishment of significant cell hypoxia was verified by measurement of lactate release into the conditioned media.

RESULTS

Hypoxia caused a 7.9-fold increase in VEGF production in C6 cells at 24 h, and a 3.4-fold increase in Müller cells after 48 h. Under hypoxic conditions, VEGF protein production was increased further by increasing pH and increasing glucose, and decreased by low pH and low glucose. Varying the glucose concentration or pH of the medium did not result in significant induction of VEGF protein production by either cell type under normoxic conditions.

CONCLUSIONS

Both glucose and pH significantly affected VEGF production induced by low oxygen. However, neither exerted a measurable stimulatory effect on VEGF production in normoxic conditions. Coexisting hypoxia and acidosis or hypoglycemia, as might occur in severe tissue ischemia, may render glial cells incapable of effectively upregulating VEGF synthesis, while alkalosis or hyperglycemia may augment hypoxia-induced VEGF production.

Amidative peptide processing and vascular function

Substance P (SP), an amidated peptide present in many sensory nerves, is known to affect cardiovascular function, and exogenously supplied SP has been shown to activate nitric oxide synthase (NOS) in endothelial cells. We now report that SP-Gly, the glycine-extended biosynthetic precursor of SP (which is enzymatically processed to the mature amidated SP), causes relaxation of rat aortic strips with an efficacy and potency comparable to that of SP itself. Pretreatment of the aortic strips with 4-phenyl-3-butenoic acid (PBA), an irreversible amidating enzyme inactivator, results in marked inhibition of the vasodilation activity induced by SP-Gly but not of that induced by SP itself. Isolated endothelial cell basal NOS activity is also decreased by pretreatment with PBA, with no evidence of cell death or direct action of PBA on NOS activity. Both bifunctional and monofunctional forms of amidating enzymes are present in endothelial cells, as evidenced by affinity chromatography and Western blot analysis. These results provide evidence for a link between amidative peptide processing, NOS activation in endothelial cells, and vasodilation and suggest that a product of amidative processing provides intrinsic basal activation of NOS in endothelial cells.

Serum opens tight junctions and reduces ZO-1 protein in retinal epithelial cells

We have shown previously that serum inhibits tight junction formation in a retinal epithelial cell culture model for the blood-brain barrier. We have now examined in detail the effects of serum on the tight junctions. Our data show that serum induces a breakdown in tight junction function as indicated by decreased transepithelial electrical resistance and increased permeability. Rat serum had effects similar to those of bovine serum, indicating that the activity is species-independent. The effect is concentration-dependent, reversible, and specific for the apical surface, suggesting the involvement of a specific receptor-ligand interaction. Differences in the time course, response magnitude, and structural manifestations between the serum-induced breakdown and that induced by switching the cultures to a low-calcium medium suggest fundamental differences in their mechanisms. The calcium switch results in an immediate and complete junctional breakdown with cell retraction and perinuclear translocation of both actin and the tight junction protein zonula occludens-1. The serum-induced breakdown occurs slowly, is incomplete, and is manifested structurally by decreases in zonula occludens-1 protein, whereas actin organization is unchanged. Thus, serum induces a specific breakdown in retinal epithelial cell tight junctions that may be mediated by effects on the expression of zonula occludens-1.

Serum inhibits tight junction formation in cultured pigment epithelial cells

PURPOSE

These experiments were designed to characterize tight junction formation by retinal pigment epithelial (RPE) cells in vitro and to compare the effects on this process of hormonally defined medium (HDM) and serum-containing medium.

METHODS

Formation of RPE tight junctions was analyzed in freshly isolated rat RPE cells maintained either in HDM or serum-containing medium. Junctions were evaluated functionally by measuring transepithelial electrical resistance (TER) and permeability and structurally by immunolocalization of the junction-associated actin microfilaments. Calcium dependency of the junctions was determined by reducing media calcium concentration.

RESULTS

RPE cells cultured in serum-free HDM developed calcium-dependent tight junctions, which exhibited TER levels > 150 omega cm2 and low paracellular permeability. Serum-containing media inhibited tight junction formation as indicated by significant reductions in TER and increases in permeability. Junction-associated actin microfilaments and cell density were unchanged.

CONCLUSIONS

Tight junction formation by RPE cells is inhibited by serum. This activity may play an important role in responses of the RPE layer to injury, contributing to the pathologic progression of blood-retinal barrier dysfunction.

Retinal pigment epithelial cells from dystrophic rats form normal tight junctions in vitro

PURPOSE

In the genetically defective Royal College of Surgeons (RCS) rat model for retinal degeneration, a breakdown occurs in the retinal pigment epithelial (RPE) cell tight junctions just as the photoreceptors begin to degenerate. These experiments sought to determine the impact of the RPE genetic defect on this alteration in the RPE cell tight junctions.

METHODS

Retinal pigment epithelial cell cultures prepared from RCS and control rats were treated with hormonally defined medium (HDM), base medium conditioned by RCS or control retinas, or unconditioned base medium. The tight junctions formed by these cultures were assayed functionally by measuring transepithelial electrical resistance and permeability. Junction structure was evaluated by immunolocalization of the tight junction protein zonula occludens 1 and of the junction-associated actin microfilaments.

RESULTS

Retinal pigment epithelial cultures from dystrophic rats formed structurally and functionally normal tight junctions when maintained in hormonally defined medium. The junctions remained stable when the medium bathing the apical surface was switched to base medium preconditioned by normal retinas. In contrast, cultures treated with medium preconditioned by degenerating dystrophic retinas or with unconditioned medium exhibited a breakdown in their tight junctions.

CONCLUSIONS

Retinal pigment epithelial cells isolated from dystrophic RCS rats can form tight junctions normally in vitro. Normal, but not dystrophic, retinas release factors that support RPE tight junctions. Therefore, the junctional abnormality seen in dystrophic rat RPE cells in vivo is probably caused by the loss of trophic factors normally provided by the healthy neural retina rather than by a direct effect of the genetic defect on the tight junctions.

Microglial cells invade the outer retina as photoreceptors degenerate in Royal College of Surgeons rats

PURPOSE

Photoreceptor degeneration is accompanied by the invasion of phagocytic cells into the outer retina of Royal College of Surgeons (RCS) rats. Previous studies suggested that these mononuclear phagocytes were blood-borne macrophages and not retinal pigment epithelial cells nor Müller glia. Thus, immunospecific markers were used to identify these cells and to determine their distribution in the dystrophic retina.

METHODS

Retinas from RCS and control (RCS-rdy+) rats were processed for immunocytochemistry using antibodies against phosphotyrosine, which labels both microglial cells and peripheral macrophages, and ED2, which labels peripheral macrophages only. As a positive control to demonstrate ED2-labeling of peripheral macrophages that enter the retina during injury, experiments were performed using needle-punctured Long Evans rat retinas.

RESULTS

In normal animals, process-bearing, phosphotyrosine-reactive cells were restricted to the inner retinal layers and the outer plexiform layer. In early dystrophic retinas, phosphotyrosine-reactive cells also were observed in the outer retinal layers. The number of phosphotyrosine-labeled cells in the outer retina increased substantially in later stages of dystrophy. ED2-reactive cells were observed in normal or dystrophic retinas only after needle puncture.

CONCLUSIONS

These findings suggest that phagocytic cells during the early stages of dystrophy in RCS rat retinas are derived from resident microglial cells, not from peripheral macrophages. The migration of microglial cells into the outer retina when photoreceptor cells begin to degenerate further suggests that they may play a major role in photoreceptor cell death in the dystrophic retina.

Angiostatic role of astrocytes: suppression of vascular endothelial cell growth by TGF-beta and other inhibitory factor(s)

Our previous in vivo analyses have suggested that astrocytes play a key role in retinal vascularization by inducing endothelial cell differentiation. Here we demonstrate that medium conditioned by cultured rat brain astrocytes (ACM) contains factors, including transforming growth factor-beta (TGF-beta), that inhibit endothelial cell growth. Serum-free medium conditioned for 1-3 days was tested on exponentially growing bovine retinal microvascular endothelial, aortic endothelial, mink lung epithelial CCL-64, and Swiss mouse 3T3 fibroblast cells. The growth of all four cell types was inhibited in a dose- and time-dependent manner. CCL cells, which are used as a model for assaying TGF-beta activity, were more sensitive than the endothelial cells, suggesting that ACM contains TGF-beta. Moreover, acid treatment significantly increased the inhibitory activity of ACM, indicating that TGF-beta in ACM is predominantly in the latent form. Mouse fibroblasts, which are not affected by TGF-beta treatment under the same conditions, were also inhibited by ACM. This suggests that other inhibitory factors in addition to TGF-beta may be involved. Adsorption by an anti-TGF-beta polyclonal antibody column substantially reduced but did not eliminate the inhibitory activity of ACM for CCL and endothelial cells. Western blot analysis of ACM and proteins eluted from the affinity column revealed a 25 kDa band that co-migrates with TGF-beta. Comparative densitometry of the 25 kDa bands on Western blot indicated that the amount of TGF-beta in ACM is not sufficient to account for the total growth-inhibitory activity. These experiments demonstrate directly that rat brain astrocytes express TGF-beta. They also indicate that astrocytes may produce other growth-inhibitory factor(s) yet to be identified.

Astrocytes modulate retinal vasculogenesis: effects on endothelial cell differentiation

In the developing retina, the microvessels form by differentiation of endothelial precursor cells in a process referred to as vasculogenesis. Experiments using in vivo and in vitro model systems were designed to determine the specific influence of astrocytes on this process. Immunolocalization analyses of retinal vasculogenesis in vivo showed that astrocytes spread within the nerve fiber layer of the neonatal rat retina just ahead of the forming vessels. Then, endothelial precursor cells align themselves in register with the astrocytes. In contact with astrocytes, precursor cells differentiate as vascular endothelium, as indicated by lumen formation and patency to red blood cells. Experiments in vitro using cell culture and conditioned medium approaches showed that cell-cell contact between rat brain astrocytes and bovine retinal endothelial cells results in release of soluble factors, inhibiting endothelial cell growth and inducing morphological differentiation in capillary-like structures. Thus, it is suggested that astrocytes lay down the pattern for vasculogenesis and induce the elongation and alignment of endothelial precursor cells into a prevascular meshwork. In contact with astrocytes, precursor cells differentiate as vascular endothelium. Furthermore, this cell-cell contact with astrocytes apparently inhibits endothelial cell growth and stimulates their elongation, alignment, and morphogenic differentiation by means of the release or activation of soluble, growth factor-like substances.

Astrocytes modulate retinal vasculogenesis: effects on fibronectin expression

Vasculogenesis is the formation of blood-vessels by differentiation of vascular precursor cells. Experiments using retinal models were designed to test the hypothesis that astrocytes influence this process by effects on the composition of the extracellular matrix. Retinal vasculogenesis was studied in relation to the migration of astrocytes and expression of the extracellular matrix proteins laminin and fibronectin by in vivo experiments in neonatal rats. The results show that astrocytes spread into the retina just ahead of the newly formed vessels, where they probably initiate vasculogenesis. They also establish that fibronectin, but not laminin, is expressed in the zone of vasculogenesis immediately prior to vessel formation. Increased amounts of fibronectin mRNA indicate that fibronectin is synthesized by cells within this same region during this same time period. Later, as the new vessels form, differentiation of endothelial cells is correlated with the appearance of pericytes in the vessel wall and laminin in the vascular basement membrane. In vitro experiments using conditioned medium approaches showed that astrocytes stimulate endothelial cell fibronectin expression. Taken together with the in vivo observations these in vitro results suggest that fibronectin expression is an essential component in the initiation of retinal vasculogenesis. This study is the first indication that astrocytes influence the fibronectin component of the extracellular matrix during retinal vasculogenesis and that expression of fibronectin precedes that of laminin in this process.

Isolation and culture of retinal microglia

In the Royal College of Surgeons rat, the migration of phagocytic cells into the subretinal space accompanies photoreceptor cell death during the early stages of retinal dystrophy. These are followed closely by cellular alterations in the retinal pigment epithelium, Müller cells, and outer retinal vessels. We have identified the phagocytic cells as microglia and hypothesized that they may be involved in the above cellular changes. Thus, we developed procedures for their isolation and growth. Our study shows that retina-derived microglia (1) are positive for microglial markers Griffonia simplicifolia isolectin B4, Mac-1 alpha, phosphotyrosine, and vimentin; (2) are highly phagocytic; and (3) respond to macrophage colony stimulating factor by proliferating. This culture system would provide a valuable tool in studying mechanisms of cellular alterations in retinal disease.

Cultured Müller cells have high levels of epidermal growth factor receptors

High levels of epidermal growth factor (EGF)-receptors have been reported in membrane homogenates of bovine retinas, but the biologic function and tissue target of EGF in the retina have not been established fully. Because EGF participation has been suggested in the mechanisms of wound healing and Müller cells undergo changes after retinal injury, the authors studied EGF receptor expression and functional role of this substance in cultured Müller cells. These cells (isolated from normal rats) were tested for the glial cell markers: vimentin, S-100 protein, and carbonic anhydrase C. These markers were found to be positive through all passages used in the experiments. The 125I-EGF binding in Müller cells was highly specific, concentration dependent, and saturable. Compared with 3T3 fibroblasts, Müller cells bound threefold more EGF. Binding kinetics and Scatchard analyses showed the higher level of binding was related to the greater number of receptors on these cells (Müller cells, 2.4 x 10(5) receptors/cell; 3T3 fibroblasts, 7.1 x 10(4) receptors/cell) rather than a change in affinity of the receptors to bind the ligand. Nonlinear-regression analyses suggested the presence of two classes of affinity sites. The high level of EGF-receptor expression in Müller cells was confirmed by western blot analyses that showed increased reactivity of the approximately 170-kilodalton receptor band to a monoclonal anti-EGF receptor antibody. Moreover, EGF treatment of Müller cells resulted in two- to threefold increase in DNA synthesis, as evidenced by 3H-thymidine uptake studies. These findings support a functional role for EGF in Müller cell proliferation in retinal disease.

Photoreceptor-specific activity of the human interphotoreceptor retinoid-binding protein (IRBP) promoter in transgenic mice

In order to define the cellular specificity of the interphotoreceptor retinoid-binding protein (IRBP) promoter in the retina, we linked the human IRBP promoter to the beta-galactosidase (lacZ) gene and made five lines of transgenic mice. In three of the five transgenic mouse lines, retinas showed positive staining upon incubation with 5-bromo-4-chloro-3-indolyl-beta-D-galactoside (X-gal). Mice from one line (OVE278B) showed positive X-gal staining throughout the retina except for the most peripheral regions. Interestingly, the staining was heterogeneous throughout the retina. Heavily stained regions were interspersed with lightly stained areas. Mice in two other lines showed highly mosaic X-gal staining patterns. Histological examination demonstrated that staining was confined to photoreceptor cells in all three expressing families. Furthermore, electron microscopy showed that the promoter is active in both rod and cone cells. Our results demonstrate that the human IRBP promoter can be used to obtain photoreceptor-specific gene expression in transgenic mice.

Vasoactive intestinal polypeptide-containing nerve fibers are increased in abundance in the choroid of dystrophic RCS rats

As photoreceptor degeneration progresses in Royal College of Surgeons (RCS) rats, a variety of morphological and physiological alterations occur in the outer retina. Since the choriocapillaris responds to changes in the outer retina in other retinopathies, we examined the possibility that changes in the choroidal vasculature also occur in RCS rats. The choroidal and choriocapillary vessels in RCS and control (RCS-rdy+) rats were examined during the period after which photoreceptor loss and retinal vascular changes had occurred (7-mos to 28-mos). Light microscopic (LM) morphometry and electron microscopic (EM) examination showed no significant differences between these groups in the number, size or morphology of these vessels. However, EM image analysis revealed that nerve fibers and bundles were twice as abundant in the RCS choroid than in the control. Using immunohistochemical techniques at the LM level combined with image analysis we found that vasoactive intestinal polypeptide positive (VIP+) fibers were significantly increased in the RCS choroid compared with control choroid. In contrast, the abundance of immunoreactive fibers labelled for substance P and dopamine beta hydroxylase appeared similar in both the control and RCS choroid. Since VIP is a potent vasodilator, the increased abundance of nerve fibers in the RCS choroid in conjunction with the unaltered number and size of these vessels suggests that choroidal blood flow may be increased. It is uncertain whether this increase is a response to the outer retinal pathology or contributes to it.

Freeze-fracture and lanthanum studies of the retinal microvasculature in diabetic rats

To see whether or not blood-retinal barrier breakdown during diabetes was associated with breakdown of the endothelial cell tight junctions or with other membrane alterations in the cells comprising the wall of the retinal microvasculature, streptozotocin-induced diabetic rat retinas were studied using lanthanum tracer and freeze-fracture electron microscopic morphometry. This study showed that endothelial cell tight junction permeability to lanthanum and luminal surface area were normal in these diabetic rats. However, freeze-fracture morphometry showed several alterations in the diabetic retinal microvessels. First, the endothelial cell membranes had abnormally large (80-120 nm) plasmalemmal vesicles not evident in the control retinas, suggesting that membrane turnover was abnormal. Second, endothelial cell P-face membranes at the blood front contained more larger particles than those in the control rats (P less than 0.05), implying an alteration in endothelial cell luminal membrane composition. Third, endothelial cell P-face membranes in areas of close apposition with pericyte membranes showed abnormal areas of particle clearing not seen in the control animals, suggesting a change in pericyte-endothelial cell interactions. Finally, pericyte membranes facing the neural retina contained increased numbers of plasmalemmal vesicles compared with control membranes (P less than 0.05). Moreover, the association of these vesicles with collagen fibrils in the extracellular space suggested an alteration in extracellular matrix turnover.

Pigment epithelial cell changes precede vascular transformations in the dystrophic rat retina

In the Royal College of Surgeons rat with inherited retinal dystrophy, vascularization of the retinal pigment epithelium (RPE) is preceded by migration and proliferation of Müller cell processes in the subretinal space where they contact the RPE. Later, RPE cells envelope subretinal vessels which have lost their perivascular Müller cell sheath. To characterize RPE cell changes and interactions in relation to glial and vascular transformations in retinal dystrophy, we used immunocytochemical techniques and antibodies against cytokeratin (CK) and glial fibrillary acidic protein (GFAP). Prior to the proliferation of Müller cell processes in the dystrophic retina, CK filaments in RPE cells formed a circumferential meshwork with intense cytoplasmic and perinuclear labeling as in control RPE cells. Following entry of Müller cell processes into the membranous debris zone and formation of RPE-Müller cell contact, RPE cells became pleomorphic and extended prominent apical processes in the debris zone. Some CK-reactive RPE cells detached from Bruch's membrane and migrated into the debris zone. Electron microscopic study showed extensive areas of close RPE-Müller cell contact at this time. Obvious junctional specializations of the plasma membranes were not seen but prominent tubulo-vesicular profiles occupied the cytoplasm of altered RPE and Müller cell processes. Following RPE vascularization, hypertrophic CK-positive cells surrounded blood vessels and accompanied them into the inner retina. Electron microscopic analysis showed that RPE-associated vessels were fenestrated and devoid of their perivascular glial sheath. Apparent proliferation of RPE cells and redistribution of CK filaments were observed. Our study shows that RPE cell alterations accompany Müller cell and vascular changes which result in altered RPE-Müller cell and RPE-endothelial cell relationships in the dystrophic rat retina. The altered relationships among RPE, Müller and endothelial cells may result in increased cellular interaction and promote proliferation and transformation of all three cells types in diseased retinas.

The choriocapillaris in spontaneously diabetic rats

During diabetes in rats, the choroid of the eye shows increased permeability to albumin, basement membrane thickening, and decreased anionic charge sites on the abluminal surfaces of the choriocapillary microvessels. In other microvascular beds, permeability differences are correlated with differences in luminal membrane microdomains as indicated by the distribution of luminal membrane anionic charge. To see whether luminal surface charge distribution or other structural features of the choroidal microvasculature become altered during diabetes, we studied spontaneously diabetic and control rats using ultrastructural tracers and morphometric techniques. Rats were injected with horseradish peroxidase and perfused with aldehydes, and then retina-choroid tissue sections were incubated with cationized ferritin, reacted to visualize peroxidase, and prepared for electron microscopic study. The most striking alterations in the diabetic rats were vascular debris and migrating cells resembling vascular cells in the choriocapillaris stroma, suggesting an increase in capillary turnover. In addition, extracellular matrix material was increased, and peroxidase uptake and ferritin binding were low in some vessels of the diabetic rats compared with the controls. Variability was large in the diabetic animals, however, and other vessels remained apparently normal.

An improved method for isolation and culture of pigment epithelial cells from rat retina

An improved method for isolation and culture of pigment epithelial cells from normal rat retinas is described. Following a brief incubation in the neutral protease Dispase, large epithelial sheets can be harvested rapidly. The separation of the choroid from the pigment epithelium prior to retinal detachment greatly reduces the risk of contamination of the cultures with choroidal cells. Growth of pigment epithelial cells on Matrigel-coated microporous filters in hormonally-defined medium allows for development of high levels of transepithelial electrical resistance as well as for preservation of the differentiated pigment epithelial cell phenotype. This method should be useful for studies of pigment epithelial cell permeability and structural differentiation in vitro.

The retinal microvasculature of spontaneously diabetic BB rats: structure and luminal surface properties

Endothelial cell permeability and luminal surface anionic sites were studied in the retinal microvasculature of spontaneously diabetic rats. Horseradish peroxidase (HRP) was used as a tracer of pinocytotic transport, and cationized ferritin (CF) was used as a marker of luminal surface anionic sites. Diabetic and control rats were injected with HRP, and their retinas were fixed. Retinal tissue sections were then incubated in CF, reacted to visualize HRP, and prepared for quantitative electron microscopic analysis. In both control and diabetic rats treated with serotonin and histamine antagonists to prevent HRP-induced vascular changes, the endothelium formed a barrier to the tracer. Pinocytotic uptake was relatively low in most vessels. Reaction product was restricted to pinocytotic vesicles, tubular cisternae, and multivesticular bodies. HRP uptake appeared high in some of the deep capillaries of the diabetic retinas as compared with that of the controls, but the difference was not statistically significant. HRP-induced transendothelial permeability was observed in both control and diabetic rats when serotonin and histamine antagonist pretreatment was omitted. CF studies showed anionic sites in four luminal surface microdomains in control and diabetic endothelial cells. CF-binding, anionic sites were present on the plasma membrane, on all coated vesicles, on some uncoated vesicles, and on most diaphragms of uncoated vesicles. Plasma membrane binding was sparse and patchy in some diabetic vessels, especially in the deep vessels of rats that were not treated with the serotonin and histamine antagonists. However, statistical analysis showed similar numbers of plasma membrane binding sites in diabetic and control rats pretreated with serotonin and histamine antagonists. Our data suggest that the retinal microvasculature in diabetic rats remains normal in terms of permeability and luminal membrane anionic charge.

Müller cell changes precede vascularization of the pigment epithelium in the dystrophic rat retina

In the Royal College of Surgeons rat with inherited retinal dystrophy, photoreceptor cell degeneration is accompanied by retinal pigment epithelial (RPE) cell alterations and Müller cell changes such as increased expression of glial fibrillary acidic protein (GFAP). Vascular changes such as vascularization of the RPE, vascular proliferation, and formation of vitreoretinal membranes (VRMs) are observed later. To study the relationship of Müller cell changes to the vascular alterations in the dystrophic retina, we used immunoperoxidase techniques and antibodies against GFAP and vimentin. Our study showed that during photoreceptor degeneration, Müller cells expressed small amounts of GFAP. As degeneration progressed, GFAP expression increased and morphological alterations occurred in Müller cells. Müller cell apical processes extended and proliferated in the subretinal space and contacted the apical surface of duplicated RPE cells. Later, GFAP reactive fibers surrounded retinal vessels apposed to the RPE. As the vessels became enmeshed within the RPE, the GFAP-positive perivascular processes disappeared. Eventually, the RPE-associated vessels became displaced into the inner retina where VRMs were sometimes observed. Immunoblots showed increased GFAP in dystrophic as compared with control retinas. Studies of vimentin distribution in the dystrophic retina showed results similar to the GFAP study. Moreover, the vimentin study suggested increased number of Müller cell processes in the dystrophic as compared with control retinas. The close temporal and anatomical relationships among Müller cell, RPE, and vascular changes in the dystrophic rat suggest a role for Müller cells in retinal neovascularization and proliferative retinopathy.

Increased vascular density and vitreo-retinal membranes accompany vascularization of the pigment epithelium in the dystrophic rat retina

Observations of vascularization of the retinal pigment epithelium (RPE) and formation of vitreo-retinal membranes (VRMs) in Royal College of Surgeons (RCS) rats with inherited retinal dystrophy suggest that vascular proliferation occurs in this model. To test this hypothesis, we studied the progression of vascular changes in RCS and age-matched control rats using quantitative light microscope morphometry and electron microscopy. At 2 weeks, prior to photoreceptor degeneration, the dystrophic retina is comparable with the control. By 2 months, extensive degeneration of photoreceptor cells results in significant thinning of the dystrophic retina as compared with the control. Signs of vascular degeneration are evident at the electron microscope level--"ghost" vessels consisting of acellular basal lamina surrounded by amorphous electron-dense material; degenerating endothelial cells and pericytes; and abnormal deposits of extracellular matrix (ECM) material around blood vessels. Vascular degeneration is accompanied by glial changes in the form of necrotic perivascular glial processes and abnormal ECM deposits among the altered Muller cell processes. At 2-4 months in the dystrophic retina, numbers of vessel profiles in dystrophic retinas are decreased as compared with controls. However, vascular degeneration is overshadowed by the formation of numerous capillary tufts within the RPE layer, which together with retinal thinning results in increased vessel density. Between 4-12 months, the retinal thickness diminishes further, vascularization of the RPE increases, vitreo-retinal membranes are formed, and vascular density increases. In summary, following an initial period of vascular degeneration, vascularization of the RPE is accompanied by an increase in retinal vessel density and by the formation of vitreo-retinal membranes.