Rap1 GTPase activation and barrier enhancement in rpe inhibits choroidal neovascularization in vivo

Decreased CREB phosphorylation impairs embryonic retinal neurogenesis in the Oa1-/- mouse model of Ocular albinism

Mutations in the human Ocular albinism type-1 gene OA1 are associated with abnormal retinal pigment epithelium (RPE) melanogenesis and poor binocular vision resulting from misrouting of ipsilateral retinal ganglion cell (iRGC) axons to the brain. We studied the latter using wild-type (WT) and Oa1-/- mouse eyes. At embryonic stages, the WT RPE-specific Oa1 protein signals through cAMP/Epac1-Erk2-CREB. Following CREB phosphorylation, a pCREB gradient extends from the RPE to the differentiating retinal amacrine and RGCs. In contrast to WT, the Oa1-/- RPE and ventral ciliary-margin-zone, a niche for iRGCs, express less pCREB while their retinas have a disrupted pCREB gradient, indicating Oa1's involvement in pCREB maintenance. Oa1-/- retinas also show hyperproliferation, enlarged nuclei, reduced differentiation, and fewer newborn amacrine and RGCs than WT retinas. Our results demonstrate that Oa1's absence leads to reduced binocular vision through a hyperproliferation-associated block in differentiation that impairs neurogenesis. This may affect iRGC axon's routing to the brain.

Synergic effects of EP2 and FP receptors co-activation on Blood-Retinal Barrier and Microglia

Macular edema (ME) is caused with disruption of the blood-retinal barrier (BRB) followed by fluid accumulation in the subretinal space. Main components of the outer and inner BRB are retinal pigment epithelial (RPE) cells and retinal microvascular endothelial cells, respectively. In addition, glial cells also participate in the functional regulation of the BRB as the member of 'neurovascular unit'. Under various stresses, cells in neurovascular units secrete inflammatory cytokines. Neuroinflammation induced by these cytokines can cause BRB dysfunction by degrading barrier-related proteins and contribute to the pathophysiology of ME. Prostaglandins (PGs) are crucial lipid mediators involved in neuroinflammation. Among PGs, a novel EP2 agonist, omidenepag (OMD) acts on not only the uveoscleral pathway but also the conventional pathway, unlike F prostanoid (FP) receptor agonists. Moreover, the combination use of the EP and the FP agonist is not recommended because of the risk of inflammation. In this study, we investigated effects of OMD and latanoprost acid (LTA), a FP agonist, on BRB and microglia in vitro and in vivo. To investigate the function of outer/inner BRB and microglia, in vitro, ARPE-19 cells, human retinal microvascular endothelial cells (HRMECs), and MG5 cells were used. Cell viability, inflammatory cytokines mRNA and protein levels, barrier morphology/function, and microglial activation were evaluated using proliferation assays, qRT-PCR, ELISA, immunocytochemistry, trans-epithelial electrical resistance, and permeability assay. Moreover, after vitreous injection into the mouse, outer BRB morphology, glial activation, and cytokine expression were assessed. Each OMD and LTA alone did not affect the viability or cytokines expression of the three types of cells. In ARPE-19 cells, the co-stimulation of OMD and LTA increased the mRNA and protein levels of inflammatory cytokines (IL-6, TNF-α, and VEGF-A) and decreased the barrier function and the junction-related protein (ZO-1 and β-catenin). By contrast in HRMECs, the co-stimulation affected significant differences in the mRNA levels of some cytokine (IL-6 and TNF-α) but enhanced the barrier function. In MG5 cells, the cytokines mRNA and size of Iba1-expressed cell were increased. A non-steroidal anti-inflammatory inhibited the barrier dysfunction and the junction-related protein downregulation in ARPE-19 cells and activation of MG5 cells. Also in vivo, the co-stimulation induced outer BRB disruption, cytokine increase, and retinal glial activation. Therefore, the co-stimulation of EP2 and FP induced the inflammatory cytokine-mediated outer BRB disruption, the enhanced inner BRB function, and the microglial activation. The BRB imbalance and the intrinsic prostaglandin production may be involved in OMD-related inflammation.

iPSC-based model of Vogt-Koyanagi-Harada disease for phenotype recapitulation and drug screening

Vogt-Koyanagi-Harada (VKH) disease, a major blinding eye disease, is characterized by an autoimmune response against melanocytes in multiple organs throughout the body. Currently, the aetiology and pathogenesis of VKH disease are unclear, and the treatment strategy needs to be further optimized. The retinal pigment epithelium (RPE), a monolayer of pigmented cells of the fundus, is essential for maintaining normal visual function and is involved in both the acute and chronic stages of VKH disease. Therefore, the functions of the RPE may play a critical role in the aetiology and treatment of VKH disease. Herein, we established a human induced pluripotent stem cell (hiPSC) RPE model of VKH disease by reprogramming peripheral blood mononuclear cells (PBMCs) into iPSCs and then differentiating them into RPE cells. Patient-derived RPE cells exhibited barrier disruption, impaired phagocytosis, and depigmentation compared with those from normal controls, which was consistent with the features of VKH disease. Furthermore, a small molecular compound targeting EGR2 was found to rescue the barrier and phagocytic functions of the hiPSC-RPE cells through high-throughput virtual screening and functional studies, suggesting a promising strategy for the treatment of VKH disease.

Calcium-dependent cAMP mediates the mechanoresponsive behaviour of endothelial cells to high-frequency nanomechanostimulation

The endothelial junction plays a central role in regulating intravascular and interstitial tissue permeability. The ability to manipulate its integrity therefore not only facilitates an improved understanding of its underlying molecular mechanisms but also provides insight into potential therapeutic solutions. Herein, we explore the effects of short-duration nanometer-amplitude MHz-order mechanostimulation on interendothelial junction stability and hence the barrier capacity of endothelial monolayers. Following an initial transient in which the endothelial barrier is permeabilised due to Rho-ROCK-activated actin stress fibre formation and junction disruption typical of a cell's response to insults, we observe, quite uniquely, the integrity of the endothelial barrier to not only spontaneously recover but also to be enhanced considerably-without the need for additional stimuli or intervention. Central to this peculiar biphasic response, which has not been observed with other stimuli to date, is the role of second messenger calcium and cyclic adenosine monophosphate (cAMP) signalling. We show that intracellular Ca²⁺, modulated by the high frequency excitation, is responsible for activating reorganisation of the actin cytoskeleton in the barrier recovery phase, in which circumferential actin bundles are formed to stabilise the adherens junctions via a cAMP-mediated Epac1-Rap1 pathway. Despite the short-duration stimulation (8 min), the approximate 4-fold enhancement in the transendothelial electrical resistance (TEER) of endothelial cells from different tissue sources, and the corresponding reduction in paracellular permeability, was found to persist over hours. The effect can further be extended through multiple treatments without resulting in hyperpermeabilisation of the barrier, as found with prolonged use of chemical stimuli, through which only 1.1- to 1.2-fold improvement in TEER has been reported. Such an ability to regulate and enhance endothelial barrier capacity is particularly useful in the development of in vitro barrier models that more closely resemble their in vivo counterparts.

Rap1A, Rap1B, and β-Adrenergic Signaling in Autologous HCT: A Randomized Controlled Trial of Propranolol

Successful hematopoietic cell transplantation (HCT) depends on rapid engraftment of the progenitor and stem cells that will reestablish hematopoiesis. Rap1A and Rap1B are two closely related small GTPases that may affect platelet and neutrophil engraftment during HCT through their roles in cell adhesion and migration. β-adrenergic signaling may regulate the participation of Rap1A and Rap1B in engraftment through their inhibition or activation. We conducted a correlative study of a randomized controlled trial evaluating the effects of the nonselective β-antagonist propranolol on expression and prenylation of Rap1A and Rap1B during neutrophil and platelet engraftment in 25 individuals receiving an autologous HCT for multiple myeloma. Propranolol was administered for 1 week prior to and 4 weeks following HCT. Blood was collected 7 days (baseline) and 2 days (Day -2) before HCT, and 28 days after HCT (Day +28). Circulating polymorphonuclear cells (PMNC) were isolated and analyzed via immunoblotting to determine levels of prenylated and total Rap1A versus Rap1B. Twelve participants were randomized to the intervention and 13 to the control. Rap1A expression significantly correlated with Rap1B expression. Rap1B expression significantly correlated with slower platelet engraftment; however, this association was not observed in the propranolol-treated group. There were no significant associations between neutrophil engraftment and Rap1A or Rap1B expression. Post hoc exploratory analyses did not reveal an association between social health variables and Rap1A or Rap1B expression. This study identifies a greater regulatory role for Rap1B than Rap1A in platelet engraftment and suggests a possible role for β-adrenergic signaling in modulating Rap1B function during HCT.

Regulation of Rac1 Activation in Choroidal Endothelial Cells: Insights into Mechanisms in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is one of the leading causes of blindness worldwide. Vision loss from the neovascular form is associated with the invasion of choroidal endothelial cells into the neural retina to form vision-threatening macular neovascularization (MNV). Anti-angiogenic agents are the current standard of care but are effective in only ~50% of AMD cases. The molecular mechanisms involved in invasive MNV point to the importance of regulating signaling pathways that lead to pathologic biologic outcomes. In studies testing the effects of AMD-related stresses, activation of the Rho GTPase, Rac1, was found to be important for the choroidal endothelial cell invasion into the neural retina. However, current approaches to prevent Rac1 activation are inefficient and less effective. We summarize active Rac1-mediated mechanisms that regulate choroidal endothelial cell migration. Specifically, we discuss our work regarding the role of a multidomain protein, IQ motif containing GTPase activating protein 1 (IQGAP1), in sustaining pathologic Rac1 activation and a mechanism by which active Rap1, a Ras-like GTPase, may prevent active Rac1-mediated choroidal endothelial cell migration.

The Role of Retinal Pigment Epithelial Cells in Regulation of Macrophages/Microglial Cells in Retinal Immunobiology

The ocular tissue microenvironment is immune privileged and uses several mechanisms of immunosuppression to prevent the induction of inflammation. Besides being a blood-barrier and source of photoreceptor nutrients, the retinal pigment epithelial cells (RPE) regulate the activity of immune cells within the retina. These mechanisms involve the expression of immunomodulating molecules that make macrophages and microglial cells suppress inflammation and promote immune tolerance. The RPE have an important role in ocular immune privilege to regulate the behavior of immune cells within the retina. Reviewed is the current understanding of how RPE mediate this regulation and the changes seen under pathological conditions.

Active Rap1-mediated inhibition of choroidal neovascularization requires interactions with IQGAP1 in choroidal endothelial cells

Neovascular age-related macular degeneration (nAMD) is a leading cause of blindness. The pathophysiology involves activation of choroidal endothelial cells (CECs) to transmigrate the retinal pigment epithelial (RPE) monolayer and form choroidal neovascularization (CNV) in the neural retina. The multidomain GTPase binding protein, IQGAP1, binds active Rac1 and sustains activation of CECs, thereby enabling migration associated with vision-threatening CNV. IQGAP1 also binds the GTPase, Rap1, which when activated reduces Rac1 activation in CECs and CNV. In this study, we tested the hypothesis that active Rap1 binding to IQGAP1 is necessary and sufficient to reduce Rac1 activation in CECs, and CNV. We found that pharmacologic activation of Rap1 or adenoviral transduction of constitutively active Rap1a reduced VEGF-mediated Rac1 activation, migration, and tube formation in CECs. Following pharmacologic activation of Rap1, VEGF-mediated Rac1 activation was reduced in CECs transfected with an IQGAP1 construct that increased active Rap1-IQGAP1 binding but not in CECs transfected with an IQGAP1 construct lacking the Rap1 binding domain. Specific knockout of IQGAP1 in endothelial cells reduced laser-induced CNV and Rac1 activation in CNV lesions, but pharmacologic activation of Rap1 did not further reduce CNV compared to littermate controls. Taken together, our findings provide evidence that active Rap1 binding to the IQ domain of IQGAP1 is sufficient to interfere with active Rac1-mediated CEC activation and CNV formation.

The Mystery of Rap1 Suppression of Oncogenic Ras

Decades ago, Rap1, a small GTPase very similar to Ras, was observed to suppress oncogenic Ras phenotype, reverting its transformation. The proposed reason, persisting since, has been competition between Ras and Rap1 for a common target. Yet, none was found. There was also Rap1's puzzling suppression of Raf-1 versus activation of BRAF. Reemerging interest in Rap1 envisages capturing its Ras suppression action by inhibitors. Here, we review the literature and resolve the enigma. In vivo oncogenic Ras exists in isoform-distinct nanoclusters. The presence of Rap1 within the nanoclusters reduces the number of the clustered oncogenic Ras molecules, thus suppressing Raf-1 activation and mitogen-activated protein kinase (MAPK) signaling. Nanoclustering suggests that Rap1 suppression is Ras isoform dependent. Altogether, a potent Rap1-like inhibitor appears unlikely.

Optimal Inhibition of Choroidal Neovascularization by scAAV2 with VMD2 Promoter-driven Active Rap1a in the RPE

Age-related macular degeneration (AMD) is a multifactorial chronic disease that requires long term treatment. Gene therapy is being considered as a promising tool to treat AMD. We found that increased activation of Rap1a in the retinal pigment epithelium (RPE) reduces oxidative signaling to maintain barrier integrity of the RPE and resist neural sensory retinal angiogenesis from choroidal endothelial cell invasion. To optimally deliver constitutively active Rap1a (CARap1a) into the RPE of wild type mice, self-complementary AAV2 (scAAV2) vectors driven by two different promoters, RPE65 or VMD2, were generated and tested for optimal active Rap1a expression and inhibition of choroidal neovascularization (CNV) induced by laser injury. scAAV2-VMD2, but not scAAV2-RPE65, specifically and efficiently transduced the RPE to increase active Rap1a protein in the RPE. Mice with increased Rap1a from the scAAV2-VMD2-CARap1a had a significant reduction in CNV compared to controls. Increased active Rap1a in the RPE in vivo or in vitro inhibited inflammatory and angiogenic signaling determined by decreased activation of NF-κB and expression of VEGF without causing increased cell death or autophagy measured by increased LCA3/B. Our study provides a potential future strategy to deliver active Rap1a to the RPE in order to protect against both atrophic and neovascular AMD.

Protection of tight junction between RPE cells with tissue factor targeting peptide

AIM

To investigate the effect of tissue factor targeting peptide (TF-TP) on retinal pigment epithelium (RPE) cells tight junctions.

METHODS

Cell counting kit-8 (CCK-8) was used to measure the proliferation of ARPE-19 cells. Expression of tight junction, ZO-1 in ARPE-19 cells was measured by Western blot and immunofluorescent staining. Western blot was also used to detect the expression of tissue factor (TF). CEC Transmigration Assay was used to measure the migration of ARPE-19 cells. The transport of fluorescent markers [fluorescein isothiocyanate dextrans of 4, 10, 20 (FD4, FD10, FD20)] and the transepithelial electrical resistance (TEER) were used to measure in ARPE-19 cell.

RESULTS

CCK-8 assay showed that 5 µmol/L TF-TP can inhibit ARPE-19 cells abnormally proliferation stimulated by lipopolysaccharide (LPS; P<0.05). LPS increased the transport of fluorescent markers (FD4, FD10, FD20) and decreased TEER levels in ARPE-19 cells, respectively, which were prevented by 5 µmol/L TF-TP pretreatment (P<0.05). Furthermore, LPS significantly up-regulated the expression of TF and downregulated the expression of ZO-1 (P<0.05) in ARPE-19 cell which was inhibited by the TF-TP (P<0.05). In addition, TF-TP inhibited the abnormal migration induced by LPS in ARPE-19 cell (P<0.05).

CONCLUSION

Our findings suggest that TF-TP suppressed proliferation and migration of ARPE-19 cells induced by LPS, and maintained the RPE tight junctions through inhibition of TF expression and increased expression of ZO-1.

Ras and Rap1: A tale of two GTPases

Ras oncoproteins play pivotal roles in both the development and maintenance of many tumor types. Unfortunately, these proteins are difficult to directly target using traditional pharmacological strategies, in part due to their lack of obvious binding pockets or allosteric sites. This obstacle has driven a considerable amount of research into pursuing alternative ways to effectively inhibit Ras, examples of which include inducing mislocalization to prevent Ras maturation and inactivating downstream proteins in Ras-driven signaling pathways. Ras proteins are archetypes of a superfamily of small GTPases that play specific roles in the regulation of many cellular processes, including vesicle trafficking, nuclear transport, cytoskeletal rearrangement, and cell cycle progression. Several other superfamily members have also been linked to the control of normal and cancer cell growth and survival. For example, Rap1 has high sequence similarity to Ras, has overlapping binding partners, and has been demonstrated to both oppose and mimic Ras-driven cancer phenotypes. Rap1 plays an important role in cell adhesion and integrin function in a variety of cell types. Mechanistically, Ras and Rap1 cooperate to initiate and sustain ERK signaling, which is activated in many malignancies and is the target of successful therapeutics. Here we review the role activated Rap1 in ERK signaling and other downstream pathways to promote invasion and cell migration and metastasis in various cancer types.

Activation of the Small GTPase Rap1 Inhibits Choroidal Neovascularization by Regulating Cell Junctions and ROS Generation in Rats

PURPOSE

Choroidal neovascularization (CNV) is a common vision-threatening complication associated with many  fundus diseases. The retinal pigment epithelial (RPE) cell junction barrier has critical functions in preventing CNV, and oxidative stress can cause compromise of barrier integrity and induce angiogenesis. Rap1, a small guanosine triphosphatase (GTPase), is involved in regulating endothelial and epithelial cell junctions. In this work, we explored the function and mechanism of Rap1 in CNV in vivo.

METHODS

A laser-induced rat CNV model was developed. Rap1 was activated through intravitreal injection of the Rap1 activator 8CPT-2'-O-Me-cAMP (8CPT). At 14 days after laser treatment, CNV size in RPE/choroid flat mounts was measured by fluorescein isothiocyanate-dextran staining. Expression of vascular endothelial growth factor (VEGF) and cell junction proteins in RPE/choroid tissues were analyzed by western blots and quantitative real-time PCR assays. Reactive oxygen species (ROS) in RPE cells were detectedbydichloro-dihydro-fluorescein diacetate assays. The antioxidant apocynin was intraperitoneally injected into rats.

RESULTS

Activating Rap1 by 8CPT significantly reduced CNV size and VEGF expression in the rat CNV model. Rap1 activation enhanced protein and mRNA levels of ZO-1 and occludin, two tight junction proteins in the RPE barrier. In addition, reducing ROS generation by injection of apocynin, a NADPH oxidase inhibitor, inhibited CNV formation. Rap1 activation reduced ROS generation and expression of NADPH oxidase 4.

CONCLUSIONS

Rap1 activation inhibits CNV through regulating barrier integrity and ROS generation of RPE in vivo, and selectively activating Rap1 may be a way to reduce vision loss from CNV.

Primary angle closure glaucoma (PACG) susceptibility gene PLEKHA7 encodes a novel Rac1/Cdc42 GAP that modulates cell migration and blood-aqueous barrier function

PLEKHA7, a gene recently associated with primary angle closure glaucoma (PACG), encodes an apical junctional protein expressed in components of the blood aqueous barrier (BAB). We found that PLEKHA7 is down-regulated in lens epithelial cells and in iris tissue of PACG patients. PLEKHA7 expression also correlated with the C risk allele of the sentinel SNP rs11024102 with the risk allele carrier groups having significantly reduced PLEKHA7 levels compared to non-risk allele carriers. Silencing of PLEKHA7 in human immortalized non-pigmented ciliary epithelium (h-iNPCE) and primary trabecular meshwork cells, which are intimately linked to BAB and aqueous humor outflow respectively, affected actin cytoskeleton organization. PLEKHA7 specifically interacts with GTP-bound Rac1 and Cdc42, but not RhoA, and the activation status of the two small GTPases is linked to PLEKHA7 expression levels. PLEKHA7 stimulates Rac1 and Cdc42 GTP hydrolysis, without affecting nucleotide exchange, identifying PLEKHA7 as a novel Rac1/Cdc42 GAP. Consistent with the regulatory role of Rac1 and Cdc42 in maintaining the tight junction permeability, silencing of PLEKHA7 compromises the paracellular barrier between h-iNPCE cells. Thus, downregulation of PLEKHA7 in PACG may affect BAB integrity and aqueous humor outflow via its Rac1/Cdc42 GAP activity, thereby contributing to disease etiology.

The EPAC-Rap1 pathway prevents and reverses cytokine-induced retinal vascular permeability

Increased retinal vascular permeability contributes to macular edema, a leading cause of vision loss in eye pathologies such as diabetic retinopathy, age-related macular degeneration, and central retinal vein occlusions. Pathological changes in vascular permeability are driven by growth factors such as VEGF and pro-inflammatory cytokines such as TNF-α. Identifying the pro-barrier mechanisms that block vascular permeability and restore the blood-retinal barrier (BRB) may lead to new therapies. The cAMP-dependent guanine nucleotide exchange factor (EPAC) exchange-protein directly activated by cAMP promotes exchange of GTP in the small GTPase Rap1. Rap1 enhances barrier properties in human umbilical endothelial cells by promoting adherens junction assembly. We hypothesized that the EPAC-Rap1 signaling pathway may regulate the tight junction complex of the BRB and may restore barrier properties after cytokine-induced permeability. Here, we show that stimulating EPAC or Rap1 activation can prevent or reverse VEGF- or TNF-α-induced permeability in cell culture and in vivo Moreover, EPAC activation inhibited VEGF receptor (VEGFR) signaling through the Ras/MEK/ERK pathway. We also found that Rap1B knockdown or an EPAC antagonist increases endothelial permeability and that VEGF has no additive effect, suggesting a common pathway. Furthermore, GTP-bound Rap1 promoted tight junction assembly, and loss of Rap1B led to loss of junctional border organization. Collectively, our results indicate that the EPAC-Rap1 pathway helps maintain basal barrier properties in the retinal vascular endothelium and activation of the EPAC-Rap1 pathway may therefore represent a potential therapeutic strategy to restore the BRB.

Roles of Nicotinamide Adenine Dinucleotide Phosphate (NADPH) Oxidase in Angiogenesis: Isoform-Specific Effects

Angiogenesis is the formation of new blood vessels from preexisting ones and is implicated in physiologic vascular development, pathologic blood vessel growth, and vascular restoration. This is in contrast to vasculogenesis, which is de novo growth of vessels from vascular precursors, or from vascular repair that occurs when circulating endothelial progenitor cells home into an area and develop into blood vessels. The objective of this review is to discuss the isoform-specific role of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) in physiologic and pathologic angiogenesis and vascular repair, but will not specifically address vasculogenesis. As the major source of reactive oxygen species (ROS) in vascular endothelial cells (ECs), NOX has gained increasing attention in angiogenesis. Activation of NOX leads to events necessary for physiologic and pathologic angiogenesis, including EC migration, proliferation and tube formation. However, activation of different NOX isoforms has different effects in angiogenesis. Activation of NOX2 promotes pathologic angiogenesis and vascular inflammation, but may be beneficial in revascularization in the hindlimb ischemic model. In contrast, activation of NOX4 appears to promote physiologic angiogenesis mainly by protecting the vasculature during ischemia, hypoxia and inflammation and by restoring vascularization, except in models of oxygen-induced retinopathy and diabetes where NOX4 activation leads to pathologic angiogenesis.

Differences in the Phosphorylation-Dependent Regulation of Prenylation of Rap1A and Rap1B

Two isoforms of the small GTPase Rap1, Rap1A and Rap1B, participate in cell adhesion; Rap1A promotes steady state adhesion, while Rap1B regulates dynamic changes in cell adhesion. These events depend on the prenylation of Rap1, which promotes its membrane localization. Here, we identify previously unsuspected differences in the regulation of prenylation of Rap1A versus Rap1B, due in part to their different phosphorylation-dependent interactions with the chaperone protein SmgGDS-607. Previous studies indicate that the activation of Gα_s protein-coupled receptors (GPCRs) phosphorylates S-179 and S-180 in the polybasic region (PBR) of Rap1B, which inhibits Rap1B binding to SmgGDS-607 and diminishes Rap1B prenylation and membrane localization. In this study, we investigate how phosphorylation in the PBR of multiple small GTPases, including K-Ras4B, RhoA, Rap1A, and Rap1B, affects their binding to SmgGDS, with emphasis on differences between Rap1A and Rap1B. We identify the amino acids in SmgGDS-607 necessary for binding of Rap1A and Rap1B, and present homology models examining the binding between Rap1A or Rap1B and SmgGDS-607. Unlike Rap1B, phosphorylation in the PBR of Rap1A does not detectably inhibit its prenylation or its binding to SmgGDS-607. Activation of GPCRs suppresses Rap1A prenylation, but unlike this effect on Rap1B, the GPCR-mediated suppression of Rap1A prenylation can occur independently of Rap1A phosphorylation and does not detectably diminish Rap1A membrane localization. These data demonstrate unexpected evolutionarily conserved differences in the ability of GPCRs to regulate the prenylation of Rap1B compared to Rap1A.

Retinal pigment epithelial cell expression of active Rap 1a by scAAV2 inhibits choroidal neovascularization

To test the hypothesis that increased Rap1a activity specifically in retinal pigment epithelial cells resists choroidal neovascularization (CNV), self-complementary adeno-associated virus 2 (scAAV2) with RPE65-promoter-driven GFP vectors were generated and introduced subretinally into Rap1b-deficient mice. Six-week-old mice that received subretinal control (scAAV2-Con) or constitutively active Rap1a (scAAV2-CARap1a) showed strong GFP at the 5 × 10(8) viral particle/µl dose 5 weeks later without altering retinal morphology or function. Compared to scAAV2-Con- or phosphate-buffered saline (PBS)-injected, eyes injected with scAAV2-CARap1a had increased Rap1 in retinal pigment epithelial (RPE)/choroidal lysates and a significant reduction in CNV volume 7 days after laser, comparable to eyes that received intravitreal anti-VEGF versus IgG control. scAAV2-CARap1a-, but not anti-VEGF-, injected eyes had increased pan-cadherin in RPE/choroids. In cultured RPE cells, increased active Rap1a inhibited TNFα-induced disassociation of junctional pan-cadherin/β-catenin complexes, increased transepithelial electrical resistance through an interaction of β-catenin with phosphorylated scaffold protein, IQGAP1, and inhibited choroidal endothelial cell (CEC) transmigration of an RPE monolayer. This evidence shows that increased Rap1a activity specifically in RPE cells is sufficient to reduce CEC transmigration and CNV and involves IQGAP1-mediated protection of RPE junctional complexes.

Regulation of signaling events involved in the pathophysiology of neovascular AMD

Neovascular age-related macular degeneration (AMD) is a complex disease in which an individual's genetic predisposition is affected by aging and environmental stresses, which trigger signaling pathways involving inflammation, oxidation, and/or angiogenesis in the RPE cells and choroidal endothelial cells (CECs), to lead to vision loss from choroidal neovascularization. Antiangiogenic therapies have greatly improved clinical outcomes in the last decade; however, vision improves in less than half of patients treated for neovascular AMD, and treatments remain inadequate for atrophic AMD. Many studies focus on genetic predisposition or the association of outcomes in trials of human neovascular AMD but are unable to evaluate the effects between different cell types involved in AMD and the signaling events that take place to cause pathologic biologic events. This manuscript complements other reviews in that it describes what is known generally in human AMD studies and clinical trials testing methods to inhibit vascular endothelial growth factor (VEGF inhibitors) and presents pathologic signaling events that develop in two important cell types, the RPE cells and the CECs, when stimulated by stresses or placed into conditions similar to what is currently understood to occur in neovascular AMD. This manuscript complements other reviews by discussing signaling events that are activated by cell-cell or cell-matrix interactions. These considerations are particularly important when considering growth factors, such as VEGF, which are important in physiologic and pathologic processes, or GTPases that are present but active only if GTP bound. In either case, it is essential to understand the role of signaling activation to distinguish what is pathologic from what is physiologic. Particularly important is the essential role of activated Rac1 in CEC transmigration of the RPE monolayer, an important step in blindness associated with neovascular AMD. Other concepts discussed include the importance of feed-forward loops that overwhelm mechanisms that seek to restore homeostasis in cells and the importance of regulating, instead of abolishing, signaling events in a chronic, complex disease, such as neovascular AMD. These concepts are important as we move to the next stages in developing treatments for neovascular AMD. A novel therapeutic strategy that will be discussed is activating an isoform of the GTPase, Rap1, which can regulate downstream signaling and a pathologic feed-forward loop leading to Rac1 activation and migration of CECs.

Rap1 GTPase Inhibits Tumor Necrosis Factor-α-Induced Choroidal Endothelial Migration via NADPH Oxidase- and NF-κB-Dependent Activation of Rac1

Macrophage-derived tumor necrosis factor (TNF)-α has been found in choroidal neovascularization (CNV) surgically removed from patients with age-related macular degeneration. However, the role of TNF-α in CNV development remains unclear. In a murine laser-induced CNV model, compared with un-lasered controls, TNF-α mRNA was increased in retinal pigment epithelial and choroidal tissue, and TNF-α colocalized with lectin-stained migrating choroidal endothelial cells (CECs). Inhibition of TNF-α with a neutralizing antibody reduced CNV volume and reactive oxygen species (ROS) level around CNV. In CECs, pretreatment with the antioxidant apocynin or knockdown of p22phox, a subunit of NADPH oxidase, inhibited TNF-α-induced ROS generation. Apocynin reduced TNF-α-induced NF-κB and Rac1 activation, and inhibited TNF-α-induced CEC migration. TNF-α-induced Rac1 activation and CEC migration were inhibited by NF-κB inhibitor Bay11-7082. Overexpression of Rap1a prevented TNF-α-induced ROS generation and reduced NF-κB and Rac1 activation. Activation of Rap1 by 8-(4-chlorophenylthio)adenosine-2'-O-Me-cAMP prevented TNF-α-induced CEC migration and reduced laser-induced CNV volume, ROS generation, and activation of NF-κB and Rac1. These findings provide evidence that active Rap1a inhibits TNF-α-induced CEC migration by inhibiting NADPH oxidase-dependent NF-κB and Rac1 activation and suggests that Rap1a de-escalates CNV development by interfering with ROS-dependent signaling in several steps of the pathogenic process.

Exchange protein directly activated by cyclic AMP (EPAC) activation reverses neutrophil dysfunction induced by β2-agonists, corticosteroids, and critical illness

BACKGROUND

Neutrophils play a role in the pathogenesis of asthma, chronic obstructive pulmonary disease, and pulmonary infection. Impaired neutrophil phagocytosis predicts hospital-acquired infection. Despite this, remarkably few neutrophil-specific treatments exist.

OBJECTIVES

We sought to identify novel pathways for the restoration of effective neutrophil phagocytosis and to activate such pathways effectively in neutrophils from patients with impaired neutrophil phagocytosis.

METHODS

Blood neutrophils were isolated from healthy volunteers and patients with impaired neutrophil function. In healthy neutrophils phagocytic impairment was induced experimentally by using β2-agonists. Inhibitors and activators of cyclic AMP (cAMP)-dependent pathways were used to assess the influence on neutrophil phagocytosis in vitro.

RESULTS

β2-Agonists and corticosteroids inhibited neutrophil phagocytosis. Impairment of neutrophil phagocytosis by β2-agonists was associated with significantly reduced RhoA activity. Inhibition of protein kinase A (PKA) restored phagocytosis and RhoA activity, suggesting that cAMP signals through PKA to drive phagocytic impairment. However, cAMP can signal through effectors other than PKA, such as exchange protein directly activated by cyclic AMP (EPAC). An EPAC-activating analog of cAMP (8CPT-2Me-cAMP) reversed neutrophil dysfunction induced by β2-agonists or corticosteroids but did not increase RhoA activity. 8CPT-2Me-cAMP reversed phagocytic impairment induced by Rho kinase inhibition but was ineffective in the presence of Rap-1 GTPase inhibitors. 8CPT-2Me-cAMP restored function to neutrophils from patients with known acquired impairment of neutrophil phagocytosis.

CONCLUSIONS

EPAC activation consistently reverses clinical and experimental impairment of neutrophil phagocytosis. EPAC signals through Rap-1 and bypasses RhoA. EPAC activation represents a novel potential means by which to reverse impaired neutrophil phagocytosis.

Leukocyte transcellular diapedesis: Rap1b is in control

The neutrophil transmigration across the blood endothelial cell barrier represents the prerequisite step of innate inflammation. It is well known that neutrophils cross the endothelial barrier by transmigrating at the endothelial cell junction ('paracellular'). However, in vivo and in vitro evidence have clearly demonstrated occurrence of an alternate mode of migration directly through the endothelial cell body ('transcellular'). Despite our knowledge on mechanisms of transendothelial migration, it remains unclear which factors determine distinct modes of migration. We recently found that the Ras-like Rap1b GTPase limits neutrophil transcellular migration. Rap1b restrains transcellular migration by suppressing Akt-driven invasive protrusions while leaving the paracellular route unaffected. Furthermore, Rap1b limits neutrophil tissue infiltration in mice and prevents hyper susceptibility to endotoxin shock. These findings uncover a novel role for Rap1b in neutrophil migration and inflammation. Importantly, they offer emerging evidences that paracellular and transcellular migration of neutrophils are regulated by separate mechanisms. Here, we discuss the mechanisms of neutrophil transmigration and their clinical importance for vascular integrity and innate inflammation.

Activation of Rap1 inhibits NADPH oxidase-dependent ROS generation in retinal pigment epithelium and reduces choroidal neovascularization

Activation of Rap1 GTPase can improve the integrity of the barrier of the retina pigment epithelium (RPE) and reduce choroidal neovascularization (CNV). Inhibition of NADPH oxidase activation also reduces CNV. We hypothesize that Rap1 inhibits NADPH oxidase-generated ROS and thereby reduces CNV formation. Using a murine model of laser-induced CNV, we determined that reduced Rap1 activity in RPE/choroid occurred with CNV formation and that activation of Rap1 by 2'-O-Me-cAMP (8CPT)-reduced laser-induced CNV via inhibiting NADPH oxidase-generated ROS. In RPE, inhibition of Rap1 by Rap1 GTPase-activating protein (Rap1GAP) increased ROS generation, whereas activation of Rap1 by 8CPT reduced ROS by interfering with the assembly of NADPH oxidase membrane subunit p22phox with NOX4 or cytoplasmic subunit p47phox. Activation of NADPH oxidase with Rap1GAP reduced RPE barrier integrity via cadherin phosphorylation and facilitated choroidal EC migration across the RPE monolayer. Rap1GAP-induced ROS generation was inhibited by active Rap1a, but not Rap1b, and activation of Rap1a by 8CPT in Rap1b(-/-) mice reduced laser-induced CNV, in correlation with decreased ROS generation in RPE/choroid. These findings provide evidence that active Rap1 reduces CNV by interfering with the assembly of NADPH oxidase subunits and increasing the integrity of the RPE barrier.