Effects of ischemic preconditioning and bevacizumab on apoptosis and vascular permeability following retinal ischemia-reperfusion injury

PURPOSE

Using transient ischemia followed by reperfusion (IR) to model ischemic retinal disease, this study compares the effects of ischemic preconditioning (IPC) and therapies targeting vascular endothelial growth factor (VEGF) and tumor necrosis factor (TNF)-α on retinal apoptosis, vascular permeability, and mRNA expression.

METHODS

Rats were subjected to 30 or 45 minutes of retinal ischemia followed by reperfusion for up to 48 hours. Neurodegeneration was quantified by caspase-3 (DEVDase) activity and by measuring nucleosomal DNA content (cell death ELISA). Vascular leakage was quantified by the Evans Blue dye method. A set of IR-responsive mRNAs was identified by whole-genome microarray and confirmed by RT-PCR analyses. VEGF protein was measured by Western blot analysis. IPC was accomplished with 10 minutes of ischemia 24 hours before IR. VEGF and TNFα signaling was inhibited by intravitreal injection of bevacizumab or etanercept, respectively.

RESULTS

IR caused significant retinal cell apoptosis and vascular permeability after 4 and 48 hours. Whereas IR decreased VegfA mRNA, VEGF protein was significantly increased. IPC effectively inhibited neurodegeneration, bevacizumab effectively inhibited vascular permeability, and etanercept failed to affect either outcome. IPC significantly altered the IR responses of 15 of 33 IR-responsive mRNAs, whereas bevacizumab had no significant effect on these mRNAs.

CONCLUSIONS

IR provides an acute model of ischemic retinopathy that includes neurodegeneration and VEGF-dependent vascular permeability and is amenable to rapid drug therapy testing. The distinct effects of IPC and bevacizumab demonstrate that the apoptotic and vascular responses to IR may be separated and that therapeutics targeting each pathologic endpoint may be warranted in treating ischemic retinal diseases.

Glucocorticoid Regulation of Endothelial Cell Tight Junction Gene Expression: Novel Treatments for Diabetic Retinopathy

Loss of blood-retinal barrier (BRB) integrity and vascular permeability characterizes diabetic retinopathy, and new therapies to reverse or prevent vascular permeability are needed to treat this debilitating disease. Glucocorticoids are currently under investigation for use as a local therapeutic treatment for diabetic retinopathy. This review examines the changes that occur to barrier properties in diabetic retinopathy and the potential to use glucocorticoids to restore vascular barrier properties in the retina. Glucocorticoids are useful in preserving the integrity of the blood-brain barrier in the treatment of brain tumors, and these steroids show similar effects on the retinal vasculature suggesting their potential usefulness in treating diabetic retinopathy. Recent progress has been made toward the goal of elucidating the precise mechanism underlying the protective effects of glucocorticoids on the retinal vasculature. Glucocorticoids may act by both suppressing inflammation and by directly affecting the endothelial cells by regulating phosphorylation, organization, and content of tight junction proteins. Further work will advance our understanding of glucocorticoid regulation of barrier properties allowing the ultimate goal of developing a specific and safe therapy to treat or prevent loss of the blood-neural barrier in a number of diseases, including brain tumors and diabetic retinopathy.

Occludin phosphorylation and ubiquitination regulate tight junction trafficking and vascular endothelial growth factor-induced permeability

Vascular endothelial growth factor (VEGF) alters tight junctions (TJs) and promotes vascular permeability in many retinal and brain diseases. However, the molecular mechanisms of barrier regulation are poorly understood. Here we demonstrate that occludin phosphorylation and ubiquitination regulate VEGF-induced TJ protein trafficking and concomitant vascular permeability. VEGF treatment induced TJ fragmentation and occludin trafficking from the cell border to early and late endosomes, concomitant with increased occludin phosphorylation on Ser-490 and ubiquitination. Furthermore, both co-immunoprecipitation and immunocytochemistry demonstrated that VEGF treatment increased the interaction between occludin and modulators of intracellular trafficking that contain the ubiquitin interacting motif, including Epsin-1, epidermal growth factor receptor pathway substrate 15 (Eps15), and hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs). Inhibiting occludin phosphorylation by mutating Ser-490 to Ala suppressed VEGF-induced ubiquitination, inhibited trafficking of TJ proteins, and prevented the increase in endothelial permeability. In addition, an occludin-ubiquitin chimera disrupted TJs and increased permeability without VEGF. These data demonstrate a novel mechanism of VEGF-induced occludin phosphorylation and ubiquitination that contributes to TJ trafficking and subsequent vascular permeability.

Phosphorylation site mapping of endogenous proteins: a combined MS and bioinformatics approach

We present a novel approach that combines MALDI-TOF profile analysis and bioinformatics-based inclusion criteria to comprehensively predict phosphorylation sites on a single protein of interest from limiting sample. It is technologically difficult to unambiguously identify phosphorylated residues, as many physiologically important phosphorylation sites are of too low abundance in vivo to be unambiguously assigned by mass spectrometry. Conversely, phosphorylation site prediction algorithms, while increasingly accurate, nevertheless overestimate the number of phosphorylation sites. In this study, we show that MODICAS, an MS data management and analysis tool, can be effectively merged with the bioinformatics attributes of residue conservation and phosphosite prediction to generate a short list of putative phosphorylation sites that can be subsequently verified by additional methodologies such as phosphospecific antibodies or mutational analysis. Therefore, the combination of MODICAS driven MS data analysis with bioinformatics-based filtering represents a substantial increase in the ability to putatively identify physiologically relevant phosphosites from limited starting material.

Efficacy of interleukin-13 receptor-targeted liposomal doxorubicin in the intracranial brain tumor model

Human glioblastoma tumors selectively express receptors for interleukin 13 (IL-13). In a previous study, we showed that liposomes, when conjugated with IL-13, will deliver chemotherapeutics to a subcutaneous glioma tumor model in mice much more effectively than conventional unconjugated liposomes. Based on this observation, we developed an intracranial brain tumor model in nude mice using human U87 glioma cells. Mice receiving weekly i.p. injections of 15 mg/kg of doxorubicin encapsulated in IL-13-conjugated liposomes had a 5-fold reduction in the intracranial tumor volume over 6 weeks and four of seven animals survived >200 days after tumor implantation. In contrast, the animals receiving unconjugated liposomes with the same doxorubicin concentration did not survive beyond 35 days and there was no evidence of tumor size reduction. The presence of liposomes with doxorubicin in the tumor was shown by taking advantage of the selective expression of IL-13 receptors on the tumor cells and the endogenous fluorescence of doxorubicin. There was no increase in the indices of toxicity in animals receiving the doxorubicin-containing liposomes. Finally, a model of the blood-brain barrier was used to show that the nanovesicles do not harm the endothelial cells yet maintain their toxicity to astrocytoma cells. This approach is necessary to show the efficacy of this targeting platform for tumors in which the blood-brain barrier is not compromised and as a potential use of the nanovesicle system as a surveillance mechanism to prevent recurrence. These data show that IL-13 targeted nanovesicles are a viable option for the treatment of brain tumors.

Identification and analysis of occludin phosphosites: a combined mass spectrometry and bioinformatics approach

The molecular function of occludin, an integral membrane component of tight junctions, remains unclear. VEGF-induced phosphorylation sites were mapped on occludin by combining MS data analysis with bioinformatics. In vivo phosphorylation of Ser490 was validated and protein interaction studies combined with crystal structure analysis suggest that Ser490 phosphorylation attenuates the interaction between occludin and ZO-1. This study demonstrates that combining MS data and bioinformatics can successfully identify novel phosphorylation sites from limiting samples.

Novel potential mechanisms for diabetic macular edema: leveraging new investigational approaches

This article evaluates the current knowledge of the molecular mechanisms by which diabetes ocular and systemic inflammation induce breakdown of the blood-retinal barrier resulting in macular edema. We also summarize the relationship between molecular targets and the use of therapeutic inhibitors in preclinical studies and clinical trials. Further studies are needed to understand the regulation of normal blood-retinal barrier physiology and the relationship between events in animal models of diabetic retinopathy and humans with diabetes.

Whole genome assessment of the retinal response to diabetes reveals a progressive neurovascular inflammatory response

BACKGROUND

Despite advances in the understanding of diabetic retinopathy, the nature and time course of molecular changes in the retina with diabetes are incompletely described. This study characterized the functional and molecular phenotype of the retina with increasing durations of diabetes.

RESULTS

Using the streptozotocin-induced rat model of diabetes, levels of retinal permeability, caspase activity, and gene expression were examined after 1 and 3 months of diabetes. Gene expression changes were identified by whole genome microarray and confirmed by qPCR in the same set of animals as used in the microarray analyses and subsequently validated in independent sets of animals. Increased levels of vascular permeability and caspase-3 activity were observed at 3 months of diabetes, but not 1 month. Significantly more and larger magnitude gene expression changes were observed after 3 months than after 1 month of diabetes. Quantitative PCR validation of selected genes related to inflammation, microvasculature and neuronal function confirmed gene expression changes in multiple independent sets of animals.

CONCLUSION

These changes in permeability, apoptosis, and gene expression provide further evidence of progressive retinal malfunction with increasing duration of diabetes. The specific gene expression changes confirmed in multiple sets of animals indicate that pro-inflammatory, anti-vascular barrier, and neurodegenerative changes occur in tandem with functional increases in apoptosis and vascular permeability. These responses are shared with the clinically documented inflammatory response in diabetic retinopathy suggesting that this model may be used to test anti-inflammatory therapeutics.

Occludin independently regulates permeability under hydrostatic pressure and cell division in retinal pigment epithelial cells

PURPOSE

The aim of this study was to determine the function of the tight junction protein occludin in the control of permeability, under diffusive and hydrostatic pressures, and its contribution to the control of cell division in retinal pigment epithelium.

METHODS

Occludin expression was inhibited in the human retinal pigment epithelial cell line ARPE-19 by siRNA. Depletion of occludin was confirmed by Western blot, confocal microscopy, and RT-PCR. Paracellular permeability of cell monolayers to fluorescently labeled 70 kDa dextran, 10 kDa dextran, and 467 Da tetramethylrhodamine (TAMRA) was examined under diffusive conditions or after the application of 10 cm H2O transmural pressure. Cell division rates were determined by tritiated thymidine incorporation and Ki67 immunoreactivity. Cell cycle inhibitors were used to determine whether changes in cell division affected permeability.

RESULTS

Occludin depletion increased diffusive paracellular permeability to 467 Da TAMRA by 15%, and permeability under hydrostatic pressure was increased 50% compared with control. Conversely, depletion of occludin protein with siRNA did not alter diffusive permeability to 70 kDa and 10 kDa RITC-dextran, and permeability to 70 kDa dextran was twofold lower in occludin-depleted cells under hydrostatic pressure conditions. Occludin depletion also increased thymidine incorporation by 90% and Ki67-positive cells by 50%. Finally, cell cycle inhibitors did not alter the effect of occludin siRNA on paracellular permeability.

CONCLUSIONS

The data suggest that occludin regulates tight junction permeability in response to changes in hydrostatic pressure. Furthermore, these data suggest that occludin also contributes to the control of cell division, demonstrating a novel function for this tight junction protein.

Glucocorticoids induce transactivation of tight junction genes occludin and claudin-5 in retinal endothelial cells via a novel cis-element

Tight junctions between vascular endothelial cells help to create the blood-brain and blood-retinal barriers. Breakdown of the retinal tight junction complex is problematic in several disease states including diabetic retinopathy. Glucocorticoids can restore and/or preserve the endothelial barrier to paracellular permeability, although the mechanism remains unclear. We show that glucocorticoid treatment of primary retinal endothelial cells increases content of the tight junction proteins occludin and claudin-5, co-incident with an increase in barrier properties of endothelial monolayers. The glucocorticoid receptor antagonist RU486 reverses both the glucocorticoid-stimulated increase in occludin content and the increase in barrier properties. Transcriptional activity from the human occludin and claudin-5 promoters increases in retinal endothelial cells upon glucocorticoid treatment, and is dependent on the glucocorticoid receptor (GR) as demonstrated by siRNA. Deletion analysis of the occludin promoter reveals a 205bp sequence responsible for the glucocorticoid response. However, this region does not possess a canonical glucocorticoid response element and does not bind to the GR in a chromatin immunoprecipitation (ChIP) assay. Mutational analysis of this region revealed a novel 40bp occludin enhancer element (OEE), containing two highly conserved regions of 10 and 13 base pairs, that is both necessary and sufficient for glucocorticoid-induced gene expression in retinal endothelial cells. These data suggest a novel mechanism for glucocorticoid induction of vascular endothelial barrier properties through increased occludin and claudin-5 gene expression.

Vascular permeability in ocular disease and the role of tight junctions

Vascular permeability is closely linked with angiogenesis in a number of pathologies. In the retina, the normally well-developed blood-retinal barrier is altered in a host of eye diseases preceding or commensurate with angiogenesis. This review examines the literature regarding the tight junction complex that establishes the blood-retinal barrier focusing on the transmembrane proteins occludin and the claudin family and the membrane associated protein zonula occludens. The changes observed in these proteins associated with vascular and epithelial permeability is discussed. Finally, novel literature addressing the link between the tight junction complex and angiogenesis is considered.

VEGF activation of protein kinase C stimulates occludin phosphorylation and contributes to endothelial permeability

PURPOSE

VEGF is a potent permeabilizing factor that contributes to the pathogenesis of diabetic retinopathy and brain tumors. VEGF-induced vascular permeability in vivo and in cell culture requires PKC activity, but the mechanism by which PKC regulates barrier properties remains unknown. This study was conducted to examine how VEGF and diabetes alter occludin phosphorylation and endothelial cell permeability.

METHODS

Chemical PKC inhibitors and activators were used to treat primary retinal endothelial cells in culture. In vitro kinase assays and Western blot analysis of two-dimensional (2D) and one-dimensional (1D) gel retardation assays were used to analyze occludin phosphorylation. Endothelial cell permeability was determined by measuring the flux of 70-kDa dextran through a cell monolayer in culture. Exogenous expression of a dominant negative PKCbetaII mutant (S217A) was used to assess the PKC dependence of VEGF-induced occludin phosphorylation and endothelial permeability. Occludin phosphorylation was also determined in retinas of streptozotocin-induced diabetic rats.

RESULTS

VEGF stimulated the phosphorylation of occludin in primary retinal endothelial cells. Chemical inhibitors of PKC activity blocked the VEGF-induced increase in occludin phosphorylation, as assessed by 2D gel and gel retardation in Western blot analysis, and blocked part of the VEGF-induced monolayer permeability to 70-kDa dextran. Expression of a dominant negative PKCbetaII mutant blocked VEGF-induced occludin phosphorylation and endothelial permeability. Finally, elevated occludin phosphorylation was observed in the retina of diabetic animals.

CONCLUSIONS

These results strongly suggest that VEGF-induced endothelial permeability requires PKC-dependent phosphorylation of occludin. Regulation of PKC activity and tight junction protein modifications may have therapeutic implications for treatment of diabetic retinopathy and brain tumors.

Pharmacologic manipulation of sphingosine kinase in retinal endothelial cells: implications for angiogenic ocular diseases

PURPOSE

The increased vascular permeability and pathogenic angiogenesis observed in diabetic retinopathy are induced, at least in part, by local inflammation and vascular endothelial growth factor (VEGF). Therefore, inhibition of signaling from VEGF and tumor necrosis factor-alpha (TNFalpha) is a promising approach to the treatment of this disease, as well as ocular diseases with similar etiologies, including age-related macular degeneration. A growing body of evidence demonstrates that sphingosine kinase (SK) plays an important role in cellular proliferation and angiogenesis. This study was undertaken to examine the effects of SK inhibitors on the responses of retinal endothelial cells (RECs) to VEGF and TNFalpha and their therapeutic efficacy in a diabetic retinopathy model.

METHODS

The expression and function of SK in bovine and human RECs were examined by immunoblot analysis. The involvement of SK in mediating responses to VEGF and TNFalpha was examined by using pharmacologic inhibitors of SK in cellular and in vivo assays, including a 3-month streptozotocin-induced diabetic retinopathy model in rats.

RESULTS

SK was present and active in human and bovine RECs, and SK activity in these cells was stimulated by VEGF. Inhibitors of SK blocked VEGF-induced production of sphingosine 1-phosphate and markedly attenuated VEGF-induced proliferation and migration of RECs. In addition, SK inhibitors were shown to block TNFalpha-induced expression of adhesion proteins, suppress VEGF-induced vascular leakage in an in vivo mouse model, and reduce retinal vascular leakage in the rat diabetic retinopathy model.

CONCLUSIONS

Overall, these studies demonstrate that inhibitors of SK attenuate the effects of proliferative and inflammatory stimuli on RECs both in vitro and in vivo, and so could be significant therapeutics in the treatment of diabetic retinopathy.

Diabetic retinopathy: seeing beyond glucose-induced microvascular disease

Diabetic retinopathy remains a frightening prospect to patients and frustrates physicians. Destruction of damaged retina by photocoagulation remains the primary treatment nearly 50 years after its introduction. The diabetes pandemic requires new approaches to understand the pathophysiology and improve the detection, prevention, and treatment of retinopathy. This perspective considers how the unique anatomy and physiology of the retina may predispose it to the metabolic stresses of diabetes. The roles of neural retinal alterations and impaired retinal insulin action in the pathogenesis of early retinopathy and the mechanisms of vision loss are emphasized. Potential means to overcome limitations of current animal models and diagnostic testing are also presented with the goal of accelerating therapies to manage retinopathy in the face of ongoing diabetes.

Diabetes reduces basal retinal insulin receptor signaling: reversal with systemic and local insulin

Diabetic retinopathy is characterized by early onset of neuronal cell death. We previously showed that insulin mediates a prosurvival pathway in retinal neurons and that normal retina expresses a highly active basal insulin receptor/Akt signaling pathway that is stable throughout feeding and fasting. Using the streptozotocin-induced diabetic rat model, we tested the hypothesis that diabetes diminishes basal retinal insulin receptor signaling concomitantly with increased diabetes-induced retinal apoptosis. The expression, phosphorylation status, and/or kinase activity of the insulin receptor and downstream signaling proteins were investigated in retinas of age-matched control, diabetic, and insulin-treated diabetic rats. Four weeks of diabetes reduced basal insulin receptor kinase, insulin receptor substrate (IRS)-1/2-associated phosphatidylinositol 3-kinase, and Akt kinase activity without altering insulin receptor or IRS-1/2 expression or tyrosine phosphorylation. After 12 weeks of diabetes, constitutive insulin receptor autophosphorylation and IRS-2 expression were reduced, without changes in p42/p44 mitogen-activated protein kinase or IRS-1. Sustained systemic insulin treatment of diabetic rats prevented loss of insulin receptor and Akt kinase activity, and acute intravitreal insulin administration restored insulin receptor kinase activity. Insulin treatment restored insulin receptor-beta autophosphorylation in rat retinas maintained ex vivo, demonstrating functional receptors and suggesting loss of ligand as a cause for reduced retinal insulin receptor/Akt pathway activity. These results demonstrate that diabetes progressively impairs the constitutive retinal insulin receptor signaling pathway through Akt and suggests that loss of this survival pathway may contribute to the initial stages of diabetic retinopathy.

A diet-induced mouse model for glutaric aciduria type I

In the autosomal recessive human disease, glutaric aciduria type I (GA-1), glutaryl-CoA dehydrogenase (GCDH) deficiency disrupts the mitochondrial catabolism of lysine and tryptophan. Affected individuals accumulate glutaric acid (GA) and 3-hydroxyglutaric acid (3-OHGA) in the serum and often suffer acute striatal injury in childhood. Prior attempts to produce selective striatal vulnerability in an animal model have been unsuccessful. We hypothesized that acute striatal injury may be induced in GCDH-deficient (Gcdh-/-) mice by elevated dietary protein and lysine. Here, we show that high protein diets are lethal to 4-week-old and 8-week-old Gcdh-/- mice within 2-3 days and 7-8 days, respectively. High lysine alone resulted in vasogenic oedema and blood-brain barrier breakdown within the striatum, associated with serum and tissue GA accumulation, neuronal loss, haemorrhage, paralysis, seizures and death in 75% of 4-week-old Gcdh-/- mice after 3-12 days. In contrast, most 8-week-old Gcdh-/- mice survived on high lysine, but developed white matter lesions, reactive astrocytes and neuronal loss after 6 weeks. Thus, the Gcdh-/- mouse exposed to high protein or lysine may be a useful model of human GA-1 including developmentally dependent striatal vulnerability.

Shear Stress Regulates HUVEC Hydraulic Conductivity by Occludin Phosphorylation

Human umbilical vein endothelial cells (HUVECs) display hydraulic conductivity (L(P)) responses to shear stress that differ markedly from the responses of bovine aortic endothelial cells (BAECs). In HUVECs, 5, 10, and 20 dyn cm(-2) steady shear stress transiently increased L(P) with a return to preshear baseline after a 2-h exposure to shear stress. Pure oscillatory shear stress of 0 +/- 20 dyn cm(-2) (mean+/-amplitude) had no effect on L(P), whereas superposition of oscillatory shear stress on steady shear stress suppressed the effect induced by steady shear stress alone. Shear reversal (amplitude greater than mean) was not necessary for the inhibitory influence of oscillatory shear stress. The transient increase of L(P) by steady shear stress was not affected by incubation with BAPTA-AM (10 microM), suggesting calcium independence of the shear response. Decreasing nitric oxide (NO) concentration with L-NMMA (100 microM), a nitric oxide synthase (NOS) inhibitor, did not inhibit the HUVEC L(P) response to shear stress. At the protein level, 10 dyn cm(-2) shear stress did not affect the total content of occludin, but it did elevate the phosphorylation level transiently. The positive correlation between occludin phosphorylation and hydraulic conductivity parallels observations in BAECs and suggests that occludin phosphorylation may be a general mediator of shear-L(P) responses in diverse endothelial cell types.

Evaluation of the role of P-glycoprotein in the uptake of paroxetine, clozapine, phenytoin and carbamazapine by bovine retinal endothelial cells

Expression of the drug transport proteins, including P-glycoprotein (Pgp), in the brain vascular endothelium represents a challenge for the effective delivery of drugs for the treatment of several central nervous system (CNS) disorders including depression, schizophrenia and epilepsy. It has been hypothesized that Pgp plays a major role in drug efflux at the blood-brain barrier, and may be an underlying factor in the variable responses of patients to CNS drugs. However, the role of Pgp in the transport of many CNS drugs has not been directly demonstrated. To explore the role of Pgp in drug transport across an endothelial cell barrier derived from the central nervous system, the expression and activity of Pgp in bovine retinal endothelial cells (BRECs) and the effects of representative CNS drugs on Pgp activity were examined. Significant Pgp expression in BRECs was demonstrated by western analyses, and expression was increased by treatment of the cells with hydrocortisone. Intracellular accumulation of the well-characterized Pgp-substrate Taxol was markedly increased by the non-selective transporter inhibitor verapamil and the Pgp-selective antagonist PGP-4008, demonstrating that Pgp is active in these endothelial cells. In contrast, neither verapamil nor PGP-4008 affected the intracellular accumulation of [3H]paroxetine, [14C]phenytoin, [3H]clozapine or [14C]carbamazapine, indicating that these drugs are not substrates for Pgp. Paroxetine, clozapine and phenytoin were shown to be Pgp inhibitors, while carbamazapine did not inhibit Pgp at any concentration tested. These results indicate that Pgp is not likely to modulate patient responses to these drugs.

The Ins2Akita mouse as a model of early retinal complications in diabetes

PURPOSE

This study tested the Ins2(Akita) mouse as an animal model of retinal complications in diabetes. The Ins2(Akita) mutation results in a single amino acid substitution in the insulin 2 gene that causes misfolding of the insulin protein. The mutation arose and is maintained on the C57BL/6J background. Male mice heterozygous for this mutation have progressive loss of beta-cell function, decreased pancreatic beta-cell density, and significant hyperglycemia, as early as 4 weeks of age.

METHODS

Heterozygous Ins2(Akita) mice were bred to C57BL/6J mice, and male offspring were monitored for hyperglycemia, beginning at 4.5 weeks of age. After 4 to 36 weeks of hyperglycemia, the retinas were analyzed for vascular permeability, vascular lesions, leukostasis, morphologic changes of micro- and macroglia, apoptosis, retinal degeneration, and insulin receptor kinase activity.

RESULTS

The mean blood glucose of Ins2(Akita) mice was significantly elevated, whereas the body weight at death was reduced compared with that of control animals. Compared with sibling control mice, the Ins2(Akita) mice had increased retinal vascular permeability after 12 weeks of hyperglycemia (P < 0.005), a modest increase in acellular capillaries after 36 weeks of hyperglycemia (P < 0.0008), and alterations in the morphology of astrocytes and microglia, but no changes in expression of Muller cell glial fibrillary acidic protein. Increased apoptosis was identified by immunoreactivity for active caspase-3 after 4 weeks of hyperglycemia (P < 0.01). After 22 weeks of hyperglycemia, there was a 16.7% central and 27% peripheral reduction in the thickness of the inner plexiform layer, a 15.6% peripheral reduction in the thickness of the inner nuclear layer (P < 0.001), and a 23.4% reduction in the number of cell bodies in the retinal ganglion cell layer (P < 0.005). In vitro insulin receptor kinase activity was reduced (P < 0.05) after 12 weeks of hyperglycemia.

CONCLUSIONS

The retinas of heterozygous male Ins2(Akita) mice exhibit vascular, neural, and glial abnormalities generally consistent with clinical observations and other animal models of diabetes. In light of the relatively early, spontaneous onset of the disease and the popularity of the C57BL/6J inbred strain as a background for the generation and study of other genetic alterations, combining the Ins2(Akita) mutation with other engineered mutations will be of great use for studying the molecular basis of retinal complications of diabetes.

VEGF increases paracellular transport without altering the solvent-drag reflection coefficient

Vascular endothelial growth factor (VEGF) increases microvascular permeability and has been implicated in the development of numerous pathologies including diabetic retinopathy (DR), hypoxia/ischemia, and tumor biology. The transport pathways by which water and solutes cross the endothelium in response to VEGF, however, are not completely understood. We measured, in real time, bovine retinal endothelial cell (BREC) hydraulic conductivity (Lp), 70 kDa dextran permeability (Pe), and the solvent-drag reflection coefficient (sigma) before and after addition of 50 ng/ml VEGF. The diffusional permeability coefficient for dextran (Pd) was measured before pressure gradient application. The sudden application of a 10-cm H2O hydrostatic pressure gradient induced water and solute fluxes that decayed to steady-state values after approximately 2 h. Subsequently, the addition of VEGF significantly increased Lp and Pe by 4.3-fold +/- 0.7-fold and 3.0-fold +/- 0.3-fold, respectively, after 110 min; however, the reflection coefficient remained approximately constant throughout the experiment (approximately 0.8). These observations suggest that water and dextran utilize common paracellular channels across BREC monolayers. Furthermore, the addition of VEGF increases the number or availability of channels but does not alter the selectivity of the monolayer to 70 kDa dextran.

Regulation of tight junctions and loss of barrier function in pathophysiology

The mechanism by which epithelial and endothelial cells interact to form polarized tissue is of fundamental importance to multicellular organisms. Dysregulation of these barriers occurs in a variety of diseases, destroying the normal cellular environments and leading to organ failure. Increased levels of growth factors are a common characteristic of diseases exhibiting tissue permeability, suggesting that growth factors play a direct role in elevating permeability. Of particular concern for this laboratory, increased expression of vascular endothelial growth factor may enhance vascular permeability in diabetic retinopathy, leading to vision impairment and blindness. However, the mechanism by which growth factors increase permeability is unclear. Polarized cells form strong barriers through the development of tight junctions, which are specialized regions of the junctional complex. Tight junctions are composed of three types of transmembrane proteins, a number of peripheral membrane structural proteins, and are associated with a variety of regulatory proteins. Recent data suggest that growth factor-stimulated alterations in tight junctions contribute to permeability in a variety of disease states. The goal of this review was to elucidate potential mechanisms by which elevated growth factors elicit deregulated paracellular permeability via altered regulation of tight junctions, with particular emphasis on the tight junction proteins occludin and ZO-1, protein kinase C signaling, and endocytosis of junctional proteins. Understanding the molecular mechanisms underlying growth factor-mediated regulation of tight junctions will facilitate the development of novel treatments for diseases such as brain tumors, diabetic retinopathy and other diseases with compromised tight junction barriers.

Alterations in expression of angiopoietins and the Tie-2 receptor in the retina of streptozotocin induced diabetic rats

PURPOSE

The angiopoietin (Ang)/Tie-2 system may play a role in vascular integrity and angiogenesis. In this study, we investigated alterations of the gene expression of Ang-1 and Ang-2 in the retinas of streptozotocin (STZ) induced diabetic rats.

METHODS

In situ hybridization, reverse transcriptase polymerase chain reaction (RT-PCR) and western blot analyses were performed to determine the mRNA and protein content for Ang-1 and Ang-2 and the Tie2 receptor in the retinas of STZ diabetic and age matched control rats.

RESULTS

Using in situ hybridization analysis, Ang-1, Ang-2, and Tie2 mRNA expression was observed in the ganglion cell layer (GCL) and the inner nuclear layer (INL). While Ang-2 mRNA expression did not changed after 2 weeks, 1 month, or 3 months of STZ induced diabetes, it was increased in the GCL and slightly elevated in the INL after 6 months of diabetes. In contrast, Ang-1 and Tie2 mRNA expression was stable at every timepoint during 6 months of STZ induced diabetes. RT-PCR and western blot analyses confirmed the increase of Ang-2 expression after 6 months of diabetes. Furthermore, double staining of alpha-smooth muscle actin (alphaSMA) and Ang-2 mRNA demonstrated that the SMA positive cells surrounding Ang-2-expressing cells were decreased in the GCL.

CONCLUSIONS

Diabetes increases Ang-2 expression in the GCL accompanied by a reduction of alphaSMA positive perivascular cells. These changes may suggest a role for Ang-2 in the mechanism of pericyte loss in diabetic retinopathy.

Mechanisms and regulation of transferrin and iron transport in a model blood-brain barrier system

For peripheral iron to reach the brain, it must transverse the blood-brain barrier. In order for the brain to obtain iron, transferrin receptors are present in the vascular endothelial cell to facilitate movement of transferrin bound iron into the brain parenchyma. However, a number of significant voids exist in our knowledge about transport of iron into the brain. These gaps in our knowledge are significant not only because iron is an essential neurotrophic factor but also because the system for delivery of iron into the brain is being viewed as an opportunity to circumvent the blood-brain barrier for delivery of neurotoxins to tumors or trophic factors in neurodegenerative diseases. In this study, we have used fluorescein-transferrin-59Fe in a bovine retinal endothelial cell culture system to determine the mechanism of transferrin-iron transport and to test the hypothesis that the iron status of the endothelial cells would influence iron transport. Our results indicated that iron is transported across endothelial cells both bound to and not bound to transferrin. The ratio of non-transferrin-bound iron to transferrin-bound iron transported is dependent upon the iron status of the cells. Blocking acidification of endosomes led to a significant decrease in transport of non-transferrin-bound iron but not transferrin-bound iron. Blocking pinocytosis had no effect on either transferrin or iron transcytosis. These results indicate that there is both transferrin-mediated and non-transferrin-mediated transcytosis of iron and that the process is influenced by the iron status of the cells. These data have considerable implications for common neurodegenerative diseases that are associated with excess brain iron accumulation and the numerous neurological complications associated with brain iron deficiency.

Insulin Promotes Rat Retinal Neuronal Cell Survival in a p70S6K-dependent Manner

The purpose of this study was to examine the role of the ribosomal protein S6 protein kinase (p70S6K), a protein synthesis regulator, in promoting retinal neuronal cell survival. Differentiated R28 rat retinal neuronal cells were used as an experimental model. Cells were maintained in Dulbecco's modified Eagle's medium supplemented with 10% newborn calf serum, and during the period of experimentation were exposed either to the absence or presence of 10 nm insulin. Insulin treatment induced p70S6K, mTOR, and Akt phosphorylation, effects that were completely prevented by the PI3K inhibitor, LY294002. Insulin-induced phosphorylation of p70S6K and mTOR was prevented by the mTOR inhibitor, rapamycin. Apoptosis, induced by serum deprivation and evaluated by Hoechst staining, was inhibited by insulin treatment in R28 cells, but not in L6 muscle cells. This effect of insulin was also largely prevented by rapamycin. Inhibition of p70S6K activity by exogenous expression of a dominant negative mutant of p70S6K prevented insulin-induced cell survival, whereas, overexpression of wild type p70S6K or expression of a rapamycin resistant form of the kinase enhanced the effect of insulin on survival. Enhanced cell survival under the latter condition was accompanied by increased p70S6K activity and phosphorylation. Rapamycin did not inhibit insulin induced p70S6K phosphorylation and activity in cells transfected with the rapamycin-resistant mutant. Together, these results suggest that p70S6K plays a key role in insulin stimulated retinal neuronal cell survival.

A transmural pressure gradient induces mechanical and biological adaptive responses in endothelial cells

A sudden increase in the transmural pressure gradient across endothelial monolayers reduces hydraulic conductivity ( Lp), a phenomenon known as the sealing effect. To further characterize this endothelial adaptive response, we measured bovine aortic endothelial cell (BAEC) permeability to albumin and 70-kDa dextran, Lp, and the solvent-drag reflection coefficients (σ) during the sealing process. The diffusional permeability coefficients for albumin (1.33 ± 0.18 × 10–6cm/s) and dextran (0.60 ± 0.16 × 10–6cm/s) were measured before pressure application. The effective permeabilities (measured when solvent drag contributes to solute transport) of albumin and dextran ( Pealband Pedex) were measured after the application of a 10 cmH2O pressure gradient; during the first 2 h of pressure application, Pealb, Pedex, and Lpwere significantly reduced by 2.0 ± 0.3-, 2.1 ± 0.3-, and 3.7 ± 0.3-fold, respectively. Immunostaining of the tight junction (TJ) protein zonula occludens-1 (ZO-1) was significantly increased at cell-cell contacts after the application of transmural pressure. Cytochalasin D treatment significantly elevated transport but did not inhibit the adaptive response, whereas colchicine treatment had no effect on diffusive permeability but inhibited the adaptive response. Neither cytoskeletal inhibitor altered σ despite significantly elevating both Lpand effective permeability. Our data suggest that BAECs actively adapt to elevated transmural pressure by mobilizing ZO-1 to intercellular junctions via microtubules. A mechanical (passive) component of the sealing effect appears to reduce the size of a small pore system that allows the transport of water but not dextran or albumin. Furthermore, the structures of the TJ determine transport rates but do not define the selectivity of the monolayer to solutes (σ).

Mapping the Blood Vessels with Paracellular Permeability in the Retinas of Diabetic Rats

PURPOSE

Diabetic retinopathy increases the permeability of the blood-retinal barrier, but the specific vessels that become permeable have not been identified. Both transcellular and paracellular pathways of vascular solute flux have been proposed. This study was conducted to test the hypothesis that paracellular flux contributes to increased retinal vascular permeability after VEGF treatment or diabetes, and to map the types of vessels that became permeable.

METHODS

Regions of paracellular flux were identified by perfusion with fluorescent concanavalin A (ConA). Rats were injected intravitreally with VEGF or made diabetic with streptozotocin (STZ). After specified times, the rats were perfused with fixative followed by ConA, which binds to the basement membrane but not the luminal surface of endothelial cells. With this approach, ConA labels only blood vessels with paracellular permeability. Retinas were also labeled by immunofluorescence for the tight junction proteins occludin and claudin-5 and examined by confocal microscopy.

RESULTS

ConA labeling increased in the superficial arterioles and postcapillary venules, 2 weeks after the onset of diabetes. After 1 month, ConA labeling dramatically increased and extended to the capillaries of the outer plexiform layer. There was an inverse relationship between occludin immunoreactivity and ConA binding, but no change in claudin-5 immunoreactivity was detected. Injection of VEGF gave similar results.

CONCLUSIONS

Diabetes and VEGF increase paracellular vascular permeability in the retina, associated with redistribution of occludin. This permeability begins in the superficial arterioles and postcapillary venules and progresses to the capillary bed.

Characterization of insulin signaling in rat retina in vivo and ex vivo

Insulin receptor (IR) signaling cascades have been studied in many tissues, but retinal insulin action has received little attention. Retinal IR signaling and activity were investigated in vivo in rats that were freely fed, fasted, or injected with insulin by phosphotyrosine immunoblotting and by measuring kinase activity. A retina explant system was utilized to investigate the IR signaling cascade, and immunohistochemistry was used to determine which retinal cell layers respond to insulin. Basal IR activity in the retina was equivalent to that in brain and significantly greater than that of liver, and it remained constant between freely fed and fasted rats. Furthermore, IR signaling increased in the retina after portal vein administration of supraphysiological doses of insulin. Ex vivo retinas responded to 10 nM insulin with IR beta-subunit (IRbeta) and IR substrate-2 (IRS-2) tyrosine phosphorylation and AktSer473 phosphorylation. The retina expresses mRNA for all three Akt isoforms as determined by in situ hybridization, and insulin specifically increases Akt-1 kinase activity. Phospho-AktSer473 immunoreactivity increases in retinal nuclear cell layers with insulin treatment. These results demonstrate that the retinal IR signaling cascade to Akt-1 possesses constitutive activity, and that exogenous insulin further stimulates this prosurvival pathway. These findings may have implications in understanding normal and dysfunctional retinal physiology.

Platelet-derived growth factor mediates tight junction redistribution and increases permeability in MDCK cells

Increased tissue permeability is a common characteristic of a number of diseases such as pulmonary edema, inflammatory bowel disease, several kidney diseases, diabetic retinopathy, and tumors. We hypothesized that growth factors increase permeability by redistribution of tight junction proteins away from the cell border. To investigate mechanisms of growth factor-mediated permeability, we examined the effect of platelet derived growth factor (PDGF) on Madin-Darby canine kidney (MDCK) cell tight junction protein distribution and on permeability. PDGF altered the cellular distribution of occludin and ZO-1 from the cell border to the cytoplasm and increased permeability to 70 kDa dextran in a concentration-dependent manner. Treatment of MDCK cells with PDGF prior to fixation allowed binding of the lectin concanavalin A to the basement membrane of fixed cells, while binding was prevented in untreated control monolayers, implying that PDGF induced the formation of a paracellular transport pathway. Cell fractionation experiments with PDGF-treated cells revealed a novel occludin-containing low-density, detergent resistant subcellular structure, which increased in the buoyant fractions relative to occludin in the pellet in a time- and concentration-dependent manner. Immunocytochemistry revealed that a pool of internalized occludin co-labels with the early endosome marker, EEA1, suggesting that PDGF may stimulate occludin to enter an endosomal pathway. PDGF may act as a permeabilizing agent by moving tight junction proteins away from the cell border in discrete microdomains, and the effects of PDGF on permeability and tight junction protein distribution may model the regulation of epithelial and endothelial barrier properties by other peptide growth factors.

Diabetic retinopathy: more than meets the eye

Retinal microvascular dysfunction in diabetes is a major component of diabetic retinopathy. This review highlights recent observations regarding the cellular anatomy that contributes to the blood-retinal barrier and its breakdown, the alterations of macroglial, neuronal, and microglial cells in diabetes, and how these changes lead to loss of vision. In addition, the effects of systemic pathophysiologic influences, including metabolic control, blood pressure, and fluid volume on the formation of diabetic macular edema are discussed. Finally, an overview of inflammatory mechanisms and responses in the retina in diabetes is provided. Together, these new observations provide a broader clinical and research perspective on diabetic retinal vascular dysfunction than previously considered, and provide new avenues for improved treatments to prevent loss of vision.

Hydrocortisone decreases retinal endothelial cell water and solute flux coincident with increased content and decreased phosphorylation of occludin

Corticosteroids provide an effective treatment to reduce edema for conditions in which the blood-brain or blood-retinal barrier is compromised. However, little is known about the mechanism by which these hormones affect endothelial cell function. We hypothesized that hydrocortisone would reduce transport of water and solutes across bovine retinal endothelial cell (BREC) monolayers coincident with changes to the tight junction protein occludin. Treatment of BREC with 103 nm hydrocortisone for two days significantly decreased water and solute transport across cell monolayers. Immunoblot analysis of occludin extracted in SDS or urea based buffers revealed a 1.65- or 2.57-fold increase in content, respectively. A similar two-fold increase in occludin mRNA was observed by real-time PCR. Immunocytochemistry revealed hydrocortisone dramatically increased both occludin and ZO-1 staining at the cell border. Additionally, 4 h of hydrocortisone treatment significantly reduced occludin phosphorylation. To our knowledge, this is the first example of a regulated decrease in occludin phosphorylation associated with increased barrier properties. In conclusion, hydrocortisone directly affects retinal endothelial cell barrier properties coincident with changes in occludin content, phosphorylation and tight junction assembly. Localized hydrocortisone therapy may be developed as a treatment option for patients suffering from retinal edema due to diabetes.

Excessive hexosamines block the neuroprotective effect of insulin and induce apoptosis in retinal neurons

In addition to microvascular abnormalities, neuronal apoptosis occurs early in diabetic retinopathy, but the mechanism is unknown. Insulin may act as a neurotrophic factor in the retina via the phosphoinositide 3-kinase/Akt pathway. Excessive glucose flux through the hexosamine biosynthetic pathway (HBP) is implicated in the development of insulin resistance in peripheral tissues and diabetic complications such as nephropathy. We tested whether increased glucose flux through the HBP perturbs insulin action and induces apoptosis in retinal neuronal cells. Exposure of R28 cells, a model of retinal neurons, to 20 mm glucose for 24 h attenuated the ability of 10 nm insulin to rescue them from serum deprivation-induced apoptosis and to phosphorylate Akt compared with 5 mm glucose. Glucosamine not only impaired the neuroprotective effect of insulin but also induced apoptosis in R28 cells in a dose-dependent fashion. UDP-N-acetylhexosamines (UDP-HexNAc), end products of the HBP, were increased approximately 2- and 15-fold after a 24-h incubation in 20 mm glucose and 1.5 mm glucosamine, respectively. Azaserine, a glutamine:fructose-6-phosphate amidotransferase inhibitor, reversed the effect of 20 mm glucose, but not that of 1.5 mm glucosamine, on attenuation of the ability of insulin to promote cell survival and phosphorylate Akt as well as accumulation of UDP-HexNAc. Glucosamine also impaired insulin receptor processing in a dose-dependent manner but did not decrease ATP content. By contrast, in L6 muscle cells, glucosamine impaired insulin receptor processing but did not induce apoptosis. These results suggest that the excessive glucose flux through the HBP may direct retinal neurons to undergo apoptosis in a bimodal fashion; i.e. via perturbation of the neuroprotective effect of insulin mediated by Akt and via induction of apoptosis possibly by altered glycosylation of proteins. The HBP may be involved in retinal neurodegeneration in diabetes.

Insulin rescues retinal neurons from apoptosis by a phosphatidylinositol 3-kinase/Akt-mediated mechanism that reduces the activation of caspase-3

The ability of insulin to protect neurons from apoptosis was examined in differentiated R28 cells, a neural cell line derived from the neonatal rat retina. Apoptosis was induced by serum deprivation, and the number of pyknotic cells was counted. p53 and Akt were examined by immunoblotting after serum deprivation and insulin treatment, and caspase-3 activation was examined by immunocytochemistry. Serum deprivation for 24 h caused approximately 20% of R28 cells to undergo apoptosis, detected by both pyknosis and activation of caspase-3. 10 nm insulin maximally reduced the amount of apoptosis with a similar potency as 1.3 nm (10 ng/ml) insulin-like growth factor 1, which acted as a positive control. Insulin induced serine phosphorylation of Akt, through the phosphatidylinositol (PI) 3-kinase pathway. Inhibition of PI 3-kinase with wortmannin or LY294002 blocked the ability of insulin to rescue the cells from apoptosis. SN50, a peptide inhibitor of NF-kappaB nuclear translocation, blocked the rescue effect of insulin, but neither insulin or serum deprivation induced phosphorylation of IkappaB. These results suggest that insulin is a survival factor for retinal neurons by activating the PI 3-kinase/Akt pathway and by reducing caspase-3 activation. The rescue effect of insulin does not appear to be mediated by NF-kappaB or p53. These data suggest that insulin provides trophic support for retinal neurons through a PI 3-kinase/Akt-dependent pathway.

Shear stress regulates occludin content and phosphorylation

Previous studies determined that shear stress imposed on bovine aortic endothelial cell (BAEC) monolayers increased the hydraulic conductivity (L(P)); however, the mechanism by which shear stress increases L(P) remains unknown. This study tested the hypothesis that shear stress regulates paracellular transport by altering the expression and phosphorylation state of the tight junction protein occludin. The effect of shear stress on occludin content was examined by Western blot analysis. Ten dyn/cm(2) significantly reduced occludin content in a time-dependent manner such that after a 3 h exposure to shear, occludin content decreased to 44% of control. Twenty dyn/cm(2) decreased occludin content to 50% of control and increased L(P) by 4.7-fold after 3 h. Occludin expression and L(P) depend on tyrosine kinase activity because erbstatin A (10 microM) attenuated both the shear-induced decrease in occludin content and increase in L(P). Shear stress increased occludin phosphorylation after 5 min, 15 min, and 3 h exposures. The shear-induced increase in occludin phosphorylation was attenuated with dibutyryl (DB) cAMP (1 mM), a reagent previously shown to reverse the shear-induced increase in L(P). We conclude that shear stress rapidly (< or = 5 min) increases occludin phosphorylation and significantly decreases the expression of occludin over 1-4 h. Alterations in the occludin phosphorylation state and occludin total content are potential mechanisms by which shear stress increases L(P).

Effect of VEGF on retinal microvascular endothelial hydraulic conductivity: the role of NO

PURPOSE

Vascular endothelial growth factor (VEGF) increases microvascular permeability in vivo and has been hypothesized to play a role in plasma leakage in diabetic retinopathy. Few controlled studies have been conducted to determine the mechanism underlying the effect of VEGF on transport properties (e.g., hydraulic conductivity [Lp]). This study was conducted to determine the effect of VEGF on bovine retinal microvascular endothelial LP and the role of nitric oxide (NO) and the guanylate cyclase/guanosine 3', 5'-cyclic monophosphate/protein kinase G (GC/cGMP/PKG) pathway downstream of NO in mediating the VEGF response.

METHODS

Bovine retinal microvascular endothelial cells (BRECs) were grown on porous polycarbonate filters, and water flux across BREC monolayers in response to a pressure differential was measured to determine endothelial LP RESULTS: VEGF (100 ng/ml) increased endothelial LP: within 30 minutes of addition and by 13.8-fold at the end of 3 hours of exposure. VEGF stimulated endothelial monolayers to release NO and incubation of the BRECs with the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA; 100 microM) significantly attenuated the VEGF-induced LP increase. It was observed that incubation of the monolayers with the GC inhibitor LY-83583 (10 microM) did not alter the VEGF-mediated LP: response. Addition of the cGMP analogue 8-br-cGMP (1 mM) did not change the baseline LP over 4 hours. Also, the PKG inhibitor KT5823 (1 microM) did not inhibit the response of BREC LP to VEGF.

CONCLUSIONS

These experiments indicate that VEGF elevates hydraulic conductivity in BRECs through a signaling mechanism that involves NO but not the GC/cGMP/PKG pathway.

Altered expression of retinal occludin and glial fibrillary acidic protein in experimental diabetes. The Penn State Retina Research Group

PURPOSE

To investigate how diabetes alters vascular endothelial cell tight junction protein and glial cell morphology at the blood-retinal barrier (BRB).

METHODS

The distribution of the glial marker, glial fibrillary acidic protein (GFAP), and the endothelial cell tight junction protein occludin were explored by immunofluorescence histochemistry in flatmounted retinas of streptozotocin (STZ)-diabetic and age-matched control rats, and in BB/Wor diabetes-prone and age-matched diabetes-resistant rats.

RESULTS

GFAP immunoreactivity was limited to astrocytes in control retinas. Two months of STZ-diabetes reduced GFAP immunoreactivity in astrocytes and increased GFAP immunoreactivity in small groups of Müller cells. After 4 months of STZ-induced diabetes, all Müller cells had intense GFAP immunoreactivity, whereas there was virtually none in the astrocytes. BB/Wor diabetic rats had similar changes in GFAP immunoreactivity. Occludin immunoreactivity in normal rats was greatest in the capillary bed of the outer plexiform layer and arterioles of the inner retina but much less intense in the postcapillary venules. Diabetes reduced occludin immunoreactivity in the capillaries and induced redistribution from continuous cell border to interrupted, punctate immunoreactivity in the arterioles. Forty-eight hours of insulin treatment reversed the pattern of GFAP and occludin immunoreactivity in the STZ-diabetic rats.

CONCLUSIONS

Diabetes alters GFAP expression in retinal glial cells, accompanied by reduction and redistribution of occludin in endothelial cells. These changes are consistent with the concept that altered glial-endothelial cell interactions at the BRB contribute to diabetic retinopathy.

Diabetes reduces glutamate oxidation and glutamine synthesis in the retina. The Penn State Retina Research Group

Retinas of diabetic individuals develop early functional changes measurable by electrophysiological and psychometric testing. Using a rat model of diabetes, we previously identified diabetes-induced alterations in metabolism of the neurotransmitter glutamate which may ultimately lead to accumulation of glutamate in the retina (Diabetes, 47: 815, 1998). We therefore investigated the function of enzymes that mediate the synthesis and breakdown of glutamate in retinas from rats made diabetic by injection of streptozotocin. De novo synthesis of nitrogen-containing amino acids including glutamate, glutamine and aspartate was assessed by measuring the rate of carbon fixation in freshly dissected retinas, and was unchanged by diabetes. In contrast, the oxidation of glutamate was significantly reduced in retinas from diabetic rats (62%, P < 0.05). Furthermore, diabetic retinas were less susceptible to inhibition of glutamate oxidation by the transaminase inhibitor aminoxyacetate (80%, N.S.), compared to the significant decrease seen in control rats (61%, P < 0.001). The activity and content of glutamine synthetase were also significantly reduced in retinas from rats diabetic for 2-6 months [range of 48% (P < 0.005) to 83% (P < 0.05) compared to control]. The activity of glutamine synthetase was normalized by acute injections of insulin, but not by reducing blood sugar levels with injections of phlorizin. These results indicate two enzymatic abnormalities in the glutamate metabolism pathway in the retina during diabetes: transamination to alpha-ketoglutarate and amination to glutamine. The reduced flux through these pathways may be associated with the accumulation of glutamate. These results are also consistent with the possibility that some of the glial changes in the retina during diabetes may be caused by hypoinsulinemia rather than hyperglycemia.

Retinal neurodegeneration: early pathology in diabetes

Normal vision depends on the normal function of retinal neurons, so vision loss in diabetes must ultimately be explained in terms of altered neuronal function. However to date relatively little attention has been paid to the impact of diabetes on the neural retina. Instead, the focus of most research has been primarily on retinal vascular changes, with the assumption that they cause altered neuronal function and consequently vision loss. An increasing body of evidence suggests that alterations in neuronal function and viability may contribute to the pathogenic mechanisms of diabetic retinopathy beginning shortly after the onset of diabetes. This view arises from neurophysiological, psychometric, histopathological and biochemical observations in humans and experimental animals. The collective evidence from past and recent studies supports the hypothesis that neurodegeneration, together with functional changes in the vasculature, is an important component of diabetic retinopathy. The authors invite other investigators to include the neural retina as a component of their studies so that the pathogenesis of diabetic retinopathy can be understood more clearly.

New insights into the pathophysiology of diabetic retinopathy: potential cell-specific therapeutic targets

Diabetic retinopathy, a leading cause of vision impairment, is classically defined by its vascular lesions. This review examines how diabetes affects vascular cells, as well as neurons, macroglia, and microglia. The cellular and clinical elements of diabetic retinopathy have many features of chronic inflammation. Understanding the individual cell-specific and global inflammatory changes in the retina may lead to novel therapeutic approaches to prevent vision loss.

Leucine, glutamine, and tyrosine reciprocally modulate the translation initiation factors eIF4F and eIF2B in perfused rat liver

Leucine, glutamine, and tyrosine, three amino acids playing key modulatory roles in hepatic proteolysis, were evaluated for activation of signaling pathways involved in regulation of liver protein synthesis. Furthermore, because leucine signals to effectors that lie distal to the mammalian target of rapamycin, these downstream factors were selected for study as candidate mediators of amino acid signaling. Using the perfused rat liver as a model system, we observed a 25% stimulation of protein synthesis in response to balanced hyperaminoacidemia, whereas amino acid imbalance due to elevated concentrations of leucine, glutamine, and tyrosine resulted in a protein synthetic depression of roughly 50% compared with normoaminoacidemic controls. The reduction in protein synthesis accompanying amino acid imbalance became manifest at high physiologic concentrations and was dictated by the guanine nucleotide exchange activity of translation initiation factor eIF2B. Paradoxically, this phenomenon occurred concomitantly with assembly of the mRNA cap recognition complex, eIF4F as well as activation of the 70-kDa ribosomal S6 kinase, p70(S6k). Dual and reciprocal modulation of eIF4F and eIF2B was leucine-specific because isoleucine, a structural analog, was ineffective in these regards. Thus, we conclude that amino acid imbalance, heralded by leucine, initiates a liver-specific translational fail-safe mechanism that deters protein synthesis under unfavorable circumstances despite promotion of the eIF4F complex.

Molecular mechanisms of vascular permeability in diabetic retinopathy

Diabetes leads to a wide array of complications in humans, including kidney failure, vascular disease, peripheral nerve degeneration, and vision loss. Diabetic retinopathy causes blindness in more working-age people in the United States than any other disease and contributes greatly to blindness in the young and old as well. The increasing rate of diabetes occurring in our society can only bring about a further decrease in the visual health of this country unless new modalities are discovered to prevent and cure diabetic retinopathy. Breakdown of the blood-retinal barrier and the resultant vascular permeability remains one of the first observable alterations in diabetic retinopathy and strongly correlates with vision loss. In this article, we examine the molecular components that form this blood-retinal barrier and explore how changes in the production of growth factors in the neural parenchyma cause an increase in vascular permeability and contribute to retinal degeneration.

Vascular endothelial growth factor induces rapid phosphorylation of tight junction proteins occludin and zonula occluden 1. A potential mechanism for vascular permeability in diabetic retinopathy and tumors

Vascular endothelial growth factor (VEGF) may have a physiologic role in regulating vessel permeability and contributes to the pathophysiology of diabetic retinopathy as well as tumor development. We set out to ascertain the mechanism by which VEGF regulates paracellular permeability in rats. Intra-ocular injection of VEGF caused a post-translational modification of occludin as determined by a gel shift from 60 to 62 kDa. This event began by 15 min post-injection and was maximal by 45 min. Alkaline phosphatase treatment revealed this modification was caused by a change in occludin phosphorylation. In addition, the quantity of extracted occludin increased 2-fold in the same time frame. The phosphorylation and increased extraction of occludin was recapitulated in retinal endothelial cells in culture after VEGF stimulation. The data presented herein are the first demonstration of a change in the phosphorylation of this transmembrane protein under conditions of increased endothelial permeability. In addition, intra-ocular injection of VEGF also caused tyrosine phosphorylation of ZO-1 as early as 15 min and increased phosphorylation 4-fold after 90 min. In conclusion, VEGF rapidly increases occludin phosphorylation as well as the tyrosine phosphorylation of ZO-1. Phosphorylation of occludin and ZO-1 likely contribute to regulated endothelial paracellular permeability.

Vascular permeability in experimental diabetes is associated with reduced endothelial occludin content: vascular endothelial growth factor decreases occludin in retinal endothelial cells. Penn State Retina Research Group

Blood-retinal barrier (BRB) breakdown is a hallmark of diabetic retinopathy, but the molecular changes that cause this pathology are unclear. Occludin is a transmembrane component of interendothelial tight junctions that may regulate permeability at the BRB. In this study, we examined the effects of vascular endothelial growth factor (VEGF) and diabetes on vascular occludin content and barrier function. Sprague-Dawley rats were made diabetic by intravenous streptozotocin injection, and age-matched animals served as controls. After 3 months, BRB permeability was quantified by intravenous injection of fluorescein isothiocyanate-bovine serum albumin (FITC-BSA), Mr 66 kDa, and 10-kDa rhodamine-dextran (R-D), followed by digital image analysis of retinal sections. Retinal fluorescence intensity for FITC-BSA increased 62% (P < or = 0.05), but R-D fluorescence did not change significantly. Occludin localization at interendothelial junctions was confirmed by immunofluorescence, and relative protein content was determined by immunoblotting of retinal homogenates. Retinal occludin content decreased approximately 35% (P < or = 0.03) in the diabetic versus the control animals, whereas the glucose transporter GLUT1 content was unchanged in rat retinas. Additionally, treatment of bovine retinal endothelial cells in culture with 0.12 nmol/l or 12 nmol/l VEGF for 6 h reduced occludin content 46 and 54%, respectively. These data show that diabetes selectively reduces retinal occludin protein expression and increases BRB permeability. Our findings suggest that the elevated VEGF in the vitreous of patients with diabetic retinopathy increases vascular permeability by downregulating occludin content. Decreased tight junction protein expression may be an important means by which diabetes causes increased vascular permeability and contributes to macular edema.

Neural apoptosis in the retina during experimental and human diabetes. Early onset and effect of insulin

This study determined whether retinal degeneration during diabetes includes retinal neural cell apoptosis. Image analysis of retinal sections from streptozotocin (STZ) diabetic rats after 7.5 months of STZ diabetes identified 22% and 14% reductions in the thickness of the inner plexiform and inner nuclear layers, respectively (P < 0. 001). The number of surviving ganglion cells was also reduced by 10% compared to controls (P < 0.001). In situ end labeling of DNA terminal dUTP nick end labeling (TUNEL) identified a 10-fold increase in the frequency of retinal apoptosis in whole-mounted rat retinas after 1, 3, 6, and 12 months of diabetes (P < 0.001, P < 0. 001, P < 0.01, and P < 0.01, respectively). Most TUNEL-positive cells were not associated with blood vessels and did not colocalize with the endothelial cell-specific antigen, von Willebrand factor. Insulin implants significantly reduced the number of TUNEL-positive cells (P < 0.05). The number of TUNEL-positive cells was also increased in retinas from humans with diabetes. These data indicate that retinal neural cell death occurs early in diabetes. This is the first quantitative report of an increase in neural cell apoptosis in the retina during diabetes, and indicates that neurodegeneration is an important component of diabetic retinopathy.