Recruitment of marrow-derived endothelial cells to experimental choroidal neovascularization by local expression of vascular endothelial growth factor

Role of immune inflammation regulated by macrophage in the pathogenesis of age-related macular degeneration

Age-related macular degeneration (AMD) can lead to irreversible impairment of visual function, and the number of patients with AMD has been increasing globally. The immunoinflammatory theory is an important pathogenic mechanism of AMD, with macrophages serving as the primary inflammatory infiltrating cells in AMD lesions. Its powerful immunoinflammatory regulatory function has attracted considerable attention. Herein, we provide an overview of the involvement of macrophage-regulated immunoinflammation in different stages of AMD. Additionally, we summarize novel therapeutic approaches for AMD, focusing on targeting macrophages, such as macrophage/microglia modulators, reduction of macrophage aggregation in the subretinal space, modulation of macrophage effector function, macrophage phenotypic alterations, and novel biomimetic nanocomposites development based on macrophage-associated functional properties. We aimed to provide a basis and reference for the further exploration of AMD pathogenesis, developmental influences, and new therapeutic approaches.

Incomplete response to Anti-VEGF therapy in neovascular AMD: Exploring disease mechanisms and therapeutic opportunities

Intravitreal anti-vascular endothelial growth factor (VEGF) drugs have revolutionized the treatment of neovascular age-related macular degeneration (NVAMD). However, many patients suffer from incomplete response to anti-VEGF therapy (IRT), which is defined as (1) persistent (plasma) fluid exudation; (2) unresolved or new hemorrhage; (3) progressive lesion fibrosis; and/or (4) suboptimal vision recovery. The first three of these collectively comprise the problem of persistent disease activity (PDA) in spite of anti-VEGF therapy. Meanwhile, the problem of suboptimal vision recovery (SVR) is defined as a failure to achieve excellent functional visual acuity of 20/40 or better in spite of sufficient anti-VEGF treatment. Thus, incomplete response to anti-VEGF therapy, and specifically PDA and SVR, represent significant clinical unmet needs. In this review, we will explore PDA and SVR in NVAMD, characterizing the clinical manifestations and exploring the pathobiology of each. We will demonstrate that PDA occurs most frequently in NVAMD patients who develop high-flow CNV lesions with arteriolarization, in contrast to patients with capillary CNV who are highly responsive to anti-VEGF therapy. We will review investigations of experimental CNV and demonstrate that both types of CNV can be modeled in mice. We will present and consider a provocative hypothesis: formation of arteriolar CNV occurs via a distinct pathobiology, termed neovascular remodeling (NVR), wherein blood-derived macrophages infiltrate the incipient CNV lesion, recruit bone marrow-derived mesenchymal precursor cells (MPCs) from the circulation, and activate MPCs to become vascular smooth muscle cells (VSMCs) and myofibroblasts, driving the development of high-flow CNV with arteriolarization and perivascular fibrosis. In considering SVR, we will discuss the concept that limited or poor vision in spite of anti-VEGF may not be caused simply by photoreceptor degeneration but instead may be associated with photoreceptor synaptic dysfunction in the neurosensory retina overlying CNV, triggered by infiltrating blood-derived macrophages and mediated by Müller cell activation Finally, for each of PDA and SVR, we will discuss current approaches to disease management and treatment and consider novel avenues for potential future therapies.

Circulating endothelial and progenitor cells in age-related macular degeneration

PURPOSE

To evaluate circulating endothelial and circulating progenitor cells as biomarkers in age-related macular degeneration patients (both exudative and atrophic forms) in order to establish the possible clinical implication of their assessment.

METHODS

We have enrolled 44 age-related macular degeneration patients: 22 patients with a recently diagnosed exudative (neovascular) form (Group A) and 22 patients with an atrophic (dry) form (Group B). The control group consisted of 22 age and sex-matched healthy subjects (Group C). The number of circulating endothelial progenitor cells (CD34+/KDR+, CD133+/KDR+, and CD34+/KDR+/CD133+), circulating progenitor cells (CD34+, CD133+, and CD34+/CD133+), and circulating endothelial cells were determined in the peripheral venous blood samples by flow cytometry. Neovascular age-related macular degeneration patients were evaluated at baseline and 4 weeks after a loading phase of three consequent intravitreal injections of ranibizumab.

RESULTS

Comparing age-related macular degeneration patients with the control group, endothelial progenitor cell and circulating progenitor cell levels were not significantly different, while age-related macular degeneration patients showed significantly higher levels of circulating endothelial cells (p = 0.001). Anti-vascular endothelial growth factor treatment with intravitreal ranibizumab was associated with a significant reduction of endothelial progenitor cell levels, with no significant influence on circulating progenitor cells and circulating endothelial cells.

CONCLUSION

We reported higher levels of circulating endothelial cells in age-related macular degeneration patients in comparison with the control group, thereby supporting the hypothesis of an involvement of endothelial dysregulation in the age-related macular degeneration and a reduction of the endothelial progenitor cell level in neovascular age-related macular degeneration patients after three intravitreal injections of ranibizumab.

In vivo bioluminescence imaging of hyperglycemia exacerbating stem cells on choroidal neovascularization in mice

AIM

To investigate the influence of hyperglycemia on the severity of choroidal neovascularization (CNV), especially the involvement of bone marrow-derived cells (BMCs) and underlying mechanisms.

METHODS

BMCs from firefly luciferase (Fluc)/green fluorescent protein (GFP) double transgenic mice were transplanted into C57BL/6J wide-type mice. The recipient mice were injected intraperitoneally with streptozotocin (STZ) daily for 5 consecutive days to induce diabetes mellitus (DM), followed by CNV laser photocoagulation. The BMCs recruitment in CNV exposed to hyperglycemia was firstly examined in Fluc/GFP chimeric mice by in vivo optical bioluminescence imaging (BLI) and in vitro Fluc assays. The CNV severity was evaluated by H&E staining and choroidal flatmount. The expression of vascular endothelial growth factor (VEGF) and stromal cell derived factor-1 (SDF-1) was detected by Western Blot.

RESULTS

BLI showed that the BMCs exerted dynamic effects in CNV model in Fluc/GFP chimeric mice exposed to hyperglycemia. The signal intensity of transplanted Fluc(+)GFP(+) BMCs in the DM chimeric mice was significantly higher than that in the control chimeric mice with CNV induction at days 5, 7, 14 and 21 (121861.67±9948.81 vs 144998.33±13787.13 photons/second/cm(2)/sr for control and DM mice, P 5d<0.05; 178791.67±30350.8 vs 240166.67±22605.3, P 7d<0.05; 124176.67±16253.52 vs 196376.67±18556.79, P 14d<0.05; 97951.60±10343.09 vs 119510.00±14383.76, P 21d<0.05), which was consistent with in vitro Fluc assay at day 7 [relative light units of Fluc (RLU1)], 215.00±52.05 vs 707.33±88.65, P<0.05; RLU1/ relative light units of renilla luciferase (RLU2), 0.90±0.17 vs 1.83±0.17, P<0.05]. The CNVs in the DM mice were wider than those in the control group at days 5, 7, 14 and 21 (147.83±17.36 vs 220.33±20.17 µm, P 5d<0.05; 212.17±24.63 vs 326.83±19.49, P 7d<0.05; 163.17±18.24 vs 265.17±20.55, P 14d<0.05; 132.00±10.88 vs 205.33±12.98, P 21d<0.05). The average area of CNV in the DM group was larger at 7d (20688.67±3644.96 vs 32218.00±4132.69 µm(2), P<0.05). The expression of VEGF and SDF-1 was enhanced in the DM mice.

CONCLUSION

Hyperglycemia promots the vasculogenesis of CNV, especially the contribution of BMCs, which might be triggered by VEGF and SDF-1 production.

Glycobiology of ocular angiogenesis

Ocular neovascularization can affect almost all the tissues of the eye: the cornea, the iris, the retina, and the choroid. Pathological neovascularization is the underlying cause of vision loss in common ocular conditions such as diabetic retinopathy, retinopathy of prematurity and age-related macular neovascularization. Glycosylation is the most common covalent posttranslational modification of proteins in mammalian cells. A growing body of evidence demonstrates that glycosylation influences the process of angiogenesis and impacts activation, proliferation, and migration of endothelial cells as well as the interaction of angiogenic endothelial cells with other cell types necessary to form blood vessels. Recent studies have provided evidence that members of the galectin class of β-galactoside-binding proteins modulate angiogenesis by novel carbohydrate-based recognition systems involving interactions between glycans of angiogenic cell surface receptors and galectins. This review discusses the significance of glycosylation and the role of galectins in the pathogenesis of ocular neovascularization.

Hyperglycemia promotes vasculogenesis in choroidal neovascularization in diabetic mice by stimulating VEGF and SDF-1 expression in retinal pigment epithelial cells

To investigate the influence of hyperglycemia on the severity of choroidal neovascularization (CNV) in diabetic mice, especially the involvement of bone marrow-derived cells (BMCs) and underlying molecular mechanisms. The mice were randomly divided into control group, diabetes group and diabetes treated with insulin group, which were laser treated to induce CNV. The CNV severity was evaluated by fundus fluorescein angiography, HE staining and choroidal flatmount. The BMCs recruitment and differentiation in CNV were examined in GFP chimeric mice by choroidal flatmount and immunofluorescence. The bone marrow-derived mesenchymal stem cells (BMSCs) recruitment and migration were tested in vivo and in vitro. VEGF and SDF-1 production in vivo and in vitro were tested by realtime PCR and ELISA. The CNV severity and expression of VEGF and SDF-1 were enhanced in DM mice compared with control mice and that insulin treatment decreased CNV severity in DM mice. The DM mice demonstrated more BMCs and bone marrow-derived mesenchymal stem cells (BMSCs) recruited and incorporated into CNV, increased ratio of BMCs expressing endothelial cell marker or macrophage marker, and up-regulated expression of VEGF and SDF-1 in CNV. Human BMSCs migration and expression of VEGF and SDF-1 in retinal pigment epithelial (RPE) cells increased when cultured under high glucose. This study suggested that hyperglycemia enhanced the expression of VEGF and SDF-1 in RPE cells, and promoted recruitment and incorporation of BMCs and affected differentiation of BMCs in CNV, which led to more severe CNV in diabetic mice.

Bone marrow transplantation transfers age-related susceptibility to neovascular remodeling in murine laser-induced choroidal neovascularization

PURPOSE

Neovascular remodeling (NVR), the progression of small capillaries into large-caliber arterioles with perivascular fibrosis, represents a major therapeutic challenge in neovascular age-related macular degeneration (AMD). Neovascular remodeling occurs after laser-induced choroidal neovascularization (CNV) in aged but not young mice. Additionally, bone marrow-derived cells, including macrophages, endothelial precursor cells, and mesenchymal precursor cells, contribute to CNV severity. In this study, we investigated the impact of aged bone marrow transplantation (BMT) on the degree of fibrosis, size, and vascular morphology of CNV lesions in a mouse model of laser-induced CNV.

METHODS

Young (2 months) and old (16 months) mice were transplanted with green fluorescent protein (GFP)-labeled bone marrow isolated from either young or old donors. Laser CNV was induced 1 month following transplant, and eyes were analyzed via choroidal flat mounts and immunohistochemistry 1 month postlaser. The identity of cells infiltrating CNV lesions was determined using specific markers for the labeled transplanted cells (GFP+), macrophages (F4/80+), perivascular mesenchymal-derived cells (smooth muscle actin, SMA+), and endothelial cells (CD31+).

RESULTS

Bone marrow transplantation from aged mice transferred susceptibility to NVR into young recipients. Inversely, transplantation of young marrow into old mice prevented NVR, preserving small size and minimal fibrosis. Mice with NVR demonstrated a greater relative contribution of marrow-derived SMA+ perivascular mesenchymal cells as compared to other cells.

CONCLUSIONS

Our findings indicate that the status of bone marrow is an important determining factor of neovascular severity. Furthermore, we find that perivascular mesenchymal cells, rather than endothelial cells, derived from aged bone marrow may contribute to increased CNV severity in this murine model of experimental neovascularization.

Circulating hematopoietic progenitor cells are decreased in COPD

RATIONALE

Bone marrow derived progenitor cells participate in the repair of injured vessels. The lungs of individuals with emphysema have reduced alveolar capillary density and increased endothelial apoptosis. We hypothesized that circulating levels of endothelial and hematopoietic progenitor cells would be reduced in this group of patients.

OBJECTIVES

The goal of this study was to measure circulating levels of endothelial progenitor cells (EPCs) and hematopoietic progenitor cells (HPCs) in subjects with COPD and to determine if progenitor levels correlated with disease severity and the presence of emphysema.

METHODS

Peripheral blood mononuclear cells were isolated from 61 patients with COPD and 32 control subjects. Levels of EPCs (CD45(dim) CD34+) and HPCs (CD45(+) CD34(+) VEGF-R2(+)) were quantified using multi-parameter flow cytometry. Progenitor cell function was assessed using cell culture assays. All subjects were evaluated with spirometry and CT scanning.

MEASUREMENTS AND MAIN RESULTS

HPC levels were reduced in subjects with COPD compared to controls, whereas circulating EPC levels were similar between the two groups. HPC levels correlated with severity of obstruction and were lowest in subjects with severe emphysema. These associations remained after correction for factors known to affect progenitor cell levels including age, smoking status, the use of statin medications and the presence of coronary artery disease. The ability of mononuclear cells to form endothelial cell colony forming units (EC-CFU) was also reduced in subjects with COPD.

CONCLUSIONS

HPC levels are reduced in subjects with COPD and correlate with emphysema phenotype and severity of obstruction. Reduction of HPCs may disrupt maintenance of the capillary endothelium, thereby contributing to the pathogenesis of COPD.

Sudden visual loss in the untreated eye of a patient with neovascular glaucoma following an intravitreal bevacizumab injection: A case report

The current study presents the case of a patient with a rare adverse event characterized by sudden vision loss in the untreated eye following an intravitreal injection of bevacizumab for neovascular glaucoma (NVG). The patient was diagnosed with NVG refractory to Ahmed glaucoma valve implantation and a vitreous hemorrhage in the right eye, which was treated with 1.25 mg intravitreal bevacizumab. Ten days after the bevacizumab injection, the left eye exhibited sudden visual loss. The patient's best-corrected visual acuity (BCVA) decreased from 80 to 25 letters [Early Treatment Diabetic Retinopathy Study (ETDRS) chart]. A fundus examination revealed a swollen optic disk with unclear boundaries, retinal hemorrhages and thinning retinal vessels. Fundus fluorescein angiography (FA) identified hyperfluorescence in the optic disk and an enlarged foveal avascular zone. The visual field revealed quadrantal defects that confirmed the diagnosis of anterior ischemic optic neuropathy associated with ischemic maculopathy. Six months later, following medical treatment, the patient's BCVA was increased to 44 letters. However, a clinical examination found neovessels with one papilla disk (PD) above the disk. Laser photocoagulation treatment was administered immediately. The area of neovessels above the disk was reduced to 1/4 PD at the last follow-up. In conclusion, although intravitreal anti-vascular endothelial growth factor (Bevacizumab) is an effective treatment for neovascular ocular diseases, its adverse effects must be taken into consideration for the treatment of NVG. Photocoagulation remains an effective treatment for proliferative diabetic retinopathy.

Myeloid cells expressing VEGF and arginase-1 following uptake of damaged retinal pigment epithelium suggests potential mechanism that drives the onset of choroidal angiogenesis in mice

Whilst data recognise both myeloid cell accumulation during choroidal neovascularisation (CNV) as well as complement activation, none of the data has presented a clear explanation for the angiogenic drive that promotes pathological angiogenesis. One possibility that is a pre-eminent drive is a specific and early conditioning and activation of the myeloid cell infiltrate. Using a laser-induced CNV murine model, we have identified that disruption of retinal pigment epithelium (RPE) and Bruch's membrane resulted in an early recruitment of macrophages derived from monocytes and microglia, prior to angiogenesis and contemporaneous with lesional complement activation. Early recruited CD11b(+) cells expressed a definitive gene signature of selective inflammatory mediators particularly a pronounced Arg-1 expression. Accumulating macrophages from retina and peripheral blood were activated at the site of injury, displaying enhanced VEGF expression, and notably prior to exaggerated VEGF expression from RPE, or earliest stages of angiogenesis. All of these initial events, including distinct VEGF (+) Arg-1(+) myeloid cells, subsided when CNV was established and at the time RPE-VEGF expression was maximal. Depletion of inflammatory CCR2-positive monocytes confirmed origin of infiltrating monocyte Arg-1 expression, as following depletion Arg-1 signal was lost and CNV suppressed. Furthermore, our in vitro data supported a myeloid cell uptake of damaged RPE or its derivatives as a mechanism generating VEGF (+) Arg-1(+) phenotype in vivo. Our results reveal a potential early driver initiating angiogenesis via myeloid-derived VEGF drive following uptake of damaged RPE and deliver an explanation of why CNV develops during any of the stages of macular degeneration and can be explored further for therapeutic gain.

Effective mobilisation of bone marrow-derived cells through proteolytic activity: a new treatment strategy for age-related macular degeneration

Selective targeting of bone marrow-derived cells (BMCs) has been heralded as a promising avenue for age-related macular degeneration (AMD) therapeutics. Many researchers have demonstrated that the function of circulating BMCs is related to disease severity in patients with AMD. Transplanted BMCs are able to transdifferentiate into retina-specific cells to replace those lost due to damage or degeneration in the pathologic process of experimental models of AMD, which may provide beneficial effects in patients with AMD. However, a major barrier to transferring the use of BMCs into clinical practice is the limited quantity of BMCs in the peripheral circulation. Technology has not yet reached a stage where ex vivo-expanded BMCs can be routinely used for cell therapy. A feasible strategy to circumvent this issue of BMC scarcity is to increase the mobilisation of autologous BMCs from the patient's bone marrow into the blood circulation. Extensive studies have demonstrated that the SDF-1/CXCR4 axis is a key regulator for BMC mobilisation. Moreover, abrogation of the SDF-1/CXCR4 axis by proteolytic modification can efficiently increase BMC mobilisation. We speculate that BMC mobilisation by proteolytic enzymes may supply a sufficient amount of autologous cells to repair and regenerate injured and degenerated the retinal pigment epithelium (RPE), photoreceptors, or other retina-specific cells, which could prevent AMD progression. If the BMC mobilisation strategy is used to treat AMD, it may overcome the existing problems of transferring BMC-based therapy into the clinic and become a particularly feasible therapeutic approach for AMD.

Amyloid beta enhances migration of endothelial progenitor cells by upregulating CX3CR1 in response to fractalkine, which may be associated with development of choroidal neovascularization

OBJECTIVE

Deposits that accumulate beneath retinal pigment epithelium, called drusen, are early signs of age-related macular degeneration (AMD). We have shown that amyloid β (Aβ) is present in drusen, and Aβ may be involved in AMD development. We have also shown that endothelial progenitor cells (EPCs) may contribute to the development of choroidal neovascularization (CNV). Thus, the purpose of this study was to investigate the role played by CX3CR1, a chemokine receptor, in EPC migration and CNV formation.

METHODS AND RESULTS

EPCs collected from human umbilical cords were found to express higher levels of CX3CR1 than human umbilical vein endothelial cells, and exposure of EPCs to Aβ caused further upregulation of CX3CR1. This upregulation was decreased by blocking fractalkine, a ligand of CX3CR1. Exposure of EPCs to fractalkine increased their migration, but pretreatment with Aβ enhanced the migration. The fractalkine-induced EPC migration was more inhibited by EPCs derived from CX3CR1(-/-) mice than wild-type mice. The area of laser-induced CNV was significantly smaller in wild-type mice that received bone marrow transplantation from CX3CR1(-/-) mice than in those that received transplantation from wild-type mice.

CONCLUSIONS

These data suggest that Aβ enhances EPC migration through the upregulation of CX3CR1. This upregulation might play a role in development of CNV.

Evaluation of variants in the selectin genes in age-related macular degeneration

Background

Age-related macular degeneration (AMD) is a common disease of the elderly that leads to loss of the central visual field due to atrophic or neovascular events. Evidence from human eyes and animal models suggests an important role for macrophages and endothelial cell activation in the pathogenesis of AMD. We sought to determine whether common ancestral variants in genes encoding the selectin family of proteins are associated with AMD.

Methods

Expression of E-selectin, L-selectin and P-selectin was examined in choroid and retina by quantitative PCR and immunofluorescence. Samples from patients with AMD (n = 341) and controls (n = 400) were genotyped at a total of 34 SNPs in the SELE, SELL and SELP genes. Allele and genotype frequencies at these SNPs were compared between AMD patients and controls as well as between subtypes of AMD (dry, geographic atrophy, and wet) and controls.

Results

High expression of all three selectin genes was observed in the choroid as compared to the retina. Some selectin labeling of retinal microglia, drusen cores and the choroidal vasculature was observed. In the genetic screen of AMD versus controls, no positive associations were observed for SELE or SELL. One SNP in SELP (rs3917751) produced p-values < 0.05 (uncorrected for multiple measures). In the subtype analyses, 6 SNPs (one in SELE, two in SELL, and three in SELP) produced p-values < 0.05. However, when adjusted for multiple measures with a Bonferroni correction, only one SNP in SELP (rs3917751) produced a statistically significant p-value (p = 0.0029).

Conclusions

This genetic screen did not detect any SNPs that were highly associated with AMD affection status overall. However, subtype analysis showed that a single SNP located within an intron of SELP (rs3917751) is statistically associated with dry AMD in our cohort. Future studies with additional cohorts and functional assays will clarify the biological significance of this discovery. Based on our findings, it is unlikely that common ancestral variants in the other selectin genes (SELE and SELL) are risk factors for AMD. Finally, it remains possible that sporadic or rare mutations in SELE, SELL, or SELP have a role in the pathogenesis of AMD.

Bone marrow-derived mesenchymal cells and MMP13 contribute to experimental choroidal neovascularization

In this study, we evaluate the potential involvement of collagenase-3 (MMP13), a matrix metalloproteinase (MMP) family member, in the exudative form of age-related macular degeneration characterized by a neovascularisation into the choroid. RT-PCR analysis revealed that human neovascular membranes issued from patients with AMD expressed high levels of Mmp13. The contribution of MMP13 in choroidal neovascularization (CNV) formation was explored by using a murine model of laser-induced CNV and applying it to wild-type mice (WT) and Mmp13-deficient mice (Mmp13 ( -/- ) mice). Angiogenic and inflammatory reactions were explored by immunohistochemistry. The implication of bone marrow (BM)-derived cells was determined by BM engraftment into irradiated mice and by injecting mesenchymal stem cells (MSC) isolated from WT BM. The deficiency of Mmp13 impaired CNV formation which was fully restored by WT BM engraftment and partially rescued by several injections of WT MSC. The present study sheds light on a novel function of MMP13 during BM-dependent choroidal vascularization and provides evidence for a role for MSC in the pathogenesis of CNV.

Inflammatory mediators and angiogenic factors in choroidal neovascularization: pathogenetic interactions and therapeutic implications

Choroidal neovascularization (CNV) is a common and severe complication in heterogeneous diseases affecting the posterior segment of the eye, the most frequent being represented by age-related macular degeneration. Although the term may suggest just a vascular pathological condition, CNV is more properly definable as an aberrant tissue invasion of endothelial and inflammatory cells, in which both angiogenesis and inflammation are involved. Experimental and clinical evidences show that vascular endothelial growth factor is a key signal in promoting angiogenesis. However, many other molecules, distinctive of the inflammatory response, act as neovascular activators in CNV. These include fibroblast growth factor, transforming growth factor, tumor necrosis factor, interleukins, and complement. This paper reviews the role of inflammatory mediators and angiogenic factors in the development of CNV, proposing pathogenetic assumptions of mutual interaction. As an extension of this concept, new therapeutic approaches geared to have an effect on both the vascular and the extravascular components of CNV are discussed.

A therapeutic strategy for choroidal neovascularization based on recruitment of mesenchymal stem cells to the sites of lesions

Choroidal neovascularization (CNV) is a common cause of severe and irreversible visual loss; however, the treatment of CNV has been hindered by its complex and poorly understood pathogenesis. It has been postulated that bone marrow (BM)-derived cells (BMCs) contribute to CNV, but little is known about the role of mesenchymal stem cells (MSCs) in CNV and their therapeutic potential for CNV treatment. We found that BM-derived MSCs transplanted by intravenous injection into laser-induced CNV mouse models were specifically recruited into CNV lesions, where they differentiated into multiple cell types and participated in the development of neovascularization, without stagnation in other organs. By taking advantage of this recruitment potential, engineered MSCs were used to produce the antiangiogenic pigment epithelial-derived factor (PEDF) at the CNV sites, thereby inhibiting the growth of CNVs and stimulating regressive features. Further studies indicated that the effect may be mediated, at least partly, by retinal pigment epithelial (RPE) cells, which function as important regulators for CNV development. These results suggest that MSCs contribute to CNV and could serve as delivery vehicles of antiangiogenic agents for the treatment of a range of CNV-associated diseases.

Cathepsin L in bone marrow-derived cells is required for retinal and choroidal neovascularization

Many vision-threatening diseases are characterized by intraocular neovascularization, (e.g., proliferative diabetic retinopathy and age-related macular degeneration). Although a new therapy with anti-VEGF antibodies is being used to treat these intraocular neovascular disorders, the visual recovery is limited, mainly because of the remnants of fibrovascular tissues. The ideal goal of the treatment is to prevent the invasion of new vessels into the avascular tissue through a matrix barrier. The purpose of this study was to determine the role played by cathepsin L, a matrix degrading enzyme, on intraocular angiogenesis. Used established animal models of retinal and choroidal neovascularization, we demonstrated that an inhibition of cathepsin L by specific inhibitors resulted in a significant decrease of intraocular neovascularization. A similar decrease of neovascularization was found in cathepsin L-deficient mice. Transplantation of bone marrow from cathepsin L-deficient mice into wild-type mice significantly reduced the degree of intraocular neovascularization. In addition, immunocytochemical analyses demonstrated that VE cadherin-positive endothelial progenitor cells, but not CD43-positive or Iba-1-positive cells, were the major cells contributing to the production of cathepsin L. These data indicate that cathepsin L expressed in endothelial progenitor cells plays a critical role in intraocular angiogenesis and suggest a potential therapeutic approach of targeting cathepsin L for neovascular ocular diseases.

Aptamers: from bench side research towards patented molecules with therapeutic applications

BACKGROUND

RNA and DNA aptamers recognize their targets with high specificity and affinity. These aptamers can be developed against almost any target protein through iterative cycles of in vitro screening of a combinatorial oligonucleotide library for target binding. Aptamer sequences from the final pool of in vitro selection are screened for pharmacological activity and possible medical applications.

METHODS

Chemical modifications and improvements of the identification of aptamer selection procedures made aptamers rival antibodies in diagnostic and therapeutic applications. This article reviews recent literature and patents and discusses the properties of aptamers as high-affinity and specificity target binders as well as their stability in biological fluids that turns them into therapeutic agents.

CONCLUSION

The development of aptamers into compounds with therapeutic and diagnostic compounds has resulted in patents protecting the sequences and the use of these oligonucleotides. Several of these patented aptamers are currently being tested in Phase I or II clinical trials. Moreover, an anti-VEGF aptamer has already been approved by the FDA for treatment of age-related macular degeneration in humans.

Role of endothelial progenitor cells in diabetes mellitus

Endothelial progenitor cells (EPCs) are bone marrow-derived cells involved in endothelial healing and angiogenesis. EPCs are considered an integrated component of the cardiovascular system, which promotes vascular health. Derangement of EPC biology in diabetes has been hailed as a novel concept in the pathogenesis of micro- and macro-vascular complications. Additionally, EPCs are considered to be disease biomarkers, as they provide an index of cardiovascular risk. The mechanisms leading to EPC dysfunction in diabetes may include defective mobilization from bone marrow to peripheral blood and reduced half-life. Hyperglycemia is considered the major determinant of microvascular complications, while other mechanisms concur to increase the risk of cardiovascular disease in diabetic patients. EPCs may represent a novel pathophysiological connection to understand development and progression of diabetic complications.

17-AAG, a Hsp90 inhibitor, attenuates the hypoxia-induced expression of SDF-1alpha and ILK in mouse RPE cells

The aim of this study was to investigate the changes of SDF-1alpha and ILK expression in mouse retinal pigment epithelium (RPE) cells in response to hypoxia, and the effect of 17-Allylamino-17-demethoxygeldanamycin (17-AAG), a heat shock protein 90 (Hsp90) inhibitor, on the hypoxia-induced expression of SDF-1alpha and ILK. RPE cells were cultured with 200 micromol/L cobalt chloride (CoCl(2)) for different times (1, 3, 6, 12, 24, 72 h) to imitate chemical hypoxia. Pretreatment of 17-AAG was 1 h prior to hypoxic insult. Cellular viability after 17-AAG treatment was assessed by MTT assay, and the changes of SDF-1alpha and ILK expression were examined by RT-PCR and Western blot. Up-regulation of SDF-1alpha and ILK expression in response to hypoxia was observed. One hour pretreatment of 17-AAG could remarkably decreased the hypoxia-induced SDF-1alpha and ILK expression in vitro. Our results indicated that SDF-1alpha and ILK involved in the hypoxic response of RPE cells, and 1 h pretreatment of 17-AAG had an inhibitive effect on the hypoxia-induced SDF-1alpha and ILK expression.

Differences in the temporal expression of regulatory growth factors during choroidal neovascular development

Although the roles of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and hepatocyte growth factor (HGF) in pathologic neovascularization have been well characterized in certain tissues, their particular functions and expression patterns in choroidal neovascularization (CNV) have not been clearly established. After localized laser trauma to Bruch's membrane to induce CNV development, the temporal changes in mRNA and protein expression of these 3 cytokines were documented and compared histologically to areas of immunofluorescence, the proliferation of endothelial cells, neovascular development, and temporal changes in vascular permeability. Changes in mRNA and protein levels of bFGF and HGF occurred quickly and reached peak expression within hours. This activity corresponded in time to intense and localized immunofluorescence for these cytokines within the choriocapillaris within laser lesion sites. During this same initial time period, mRNA upregulation of VEGF occurred, primarily within the neural retina and this expression corresponded to intense immunolabeling of Müller cells immediately adjacent to the lesion sites. By 3 days after lasering, increased VEGF(164) protein expression was measurable, whereas early neovascular development histologically corresponded to HGF and bFGF mRNA expansion into the developing choroidal neovascular membrane (CNVM). At 7 days, CNV expansion, maturation, and increased vascular permeability corresponded to peak VEGF mRNA and protein expression and to immunofluorescence of the CNVM. Differences also occurred in the expression of precursor and activated isoforms of these cytokines in the retinal pigment epithelium/choroid as compared to those in the retina. These molecular and immunocytochemical results suggest that bFGF and HGF may be important as initial regulators neovascularization in this CNV model; whereas VEGF may be important during later phases of angiogenesis and neovascular hyperpermeability.

Vascular endothelial growth factor in eye disease

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

Tumoral and choroidal vascularization: differential cellular mechanisms involving plasminogen activator inhibitor type I

An adequate balance between serine proteases and their plasminogen activator inhibitor-1 (PAI-1) is critical for pathological angiogenesis. PAI-1 deficiency in mice is associated with impaired choroidal neovascularization (CNV) and tumoral angiogenesis. In the present work, we demonstrate unexpected differences in the contribution of bone marrow (BM)-derived cells in these two processes regulated by PAI-1. PAI-1(-/-) mice grafted with BM-derived from wild-type mice were able to support laser-induced CNV formation but not skin carcinoma vascularization. Engraftment of irradiated wild-type mice with PAI-1(-/-) BM prevented CNV formation, demonstrating the crucial role of PAI-1 delivered by BM-derived cells. In contrast, the transient infiltration of tumor transplants by local PAI-1-producing host cells rather than by BM cells was sufficient to rescue tumor growth and angiogenesis in PAI-1-deficient mice. These data identify PAI-1 as a molecular determinant of a local permissive soil for tumor angiogenesis. Altogether, the present study demonstrates that different cellular mechanisms contribute to PAI-1-regulated tumoral and CNV. PAI-1 contributes to BM-dependent choroidal vascularization and to BM-independent tumor growth and angiogenesis.

Ocular neovascularization: basic mechanisms and therapeutic advances

The vast majority of diseases that cause catastrophic loss of vision do so as a result of ocular neovascularization. During normal retinal vascular development, vascular endothelial cells proliferate and migrate through the extracellular matrix in response to a variety of cytokines, leading to the formation of new blood vessels in a highly ordered fashion. During abnormal neovascularization of the iris, retina, or choroid, angiogenesis is unregulated and usually results in the formation of dysfunctional blood vessels. When these newly formed vessels leak fluid, hemorrhage, or are associated with fibrous proliferation, retinal edema, retinal/vitreous hemorrhage, or traction retinal detachments may occur resulting in potentially catastrophic loss of vision. In this review, we will briefly discuss the scope of the clinical problem and the general underlying principles of angiogenesis. We will focus on recent laboratory advances that have led to the development of therapeutics useful in the treatment of neovascular eye diseases. We will describe compounds currently in pre-clinical development stages as well as the results of clinical trials involving the use of these drugs as treatments for ocular neovascularization.

Progenitor cells and retinal angiogenesis

Nothing more dramatically captures the imagination of the visually impaired patient or the ophthalmologist treating them than the possibility of rebuilding a damaged retina or vasculature with "stem cells." Stem cells (SC) have been isolated from adult tissues and represent a pool of cells that may serve to facilitate rescue/repair of damaged tissue following injury or stress. We propose a new paradigm to "mature" otherwise immature neovasculature or, better yet, stabilize existing vasculature to hypoxic damage. This may be possible through the use of autologous bone marrow (BM) or cord blood derived hematopoietic SC that selectively target sites of neovascularization and gliosis where they provide vasculo- and neurotrophic effects. We have demonstrated that adult BM contains a population of endothelial and myeloid progenitor cells that can target activated astrocytes, a hallmark of many ocular diseases, and participate in normal developmental, or injury-induced, angiogenesis in the adult. Intravitreal injection of these cells from mice and humans can prevent retinal vascular degeneration ordinarily observed in mouse models of retinal degeneration; this vascular rescue correlates with functional neuronal rescue as well. The use of autologous adult BM derived SC grafts for the treatment of retinal vascular and degenerative diseases represents a novel conceptual approach that may make it possible to "mature" otherwise immature neovasculature, stabilize existing vasculature to hypoxic damage and/or rescue and protect retinal neurons from undergoing apoptosis. Such a therapeutic approach would obviate the need to employ destructive treatment modalities and would facilitate vascularization of ischemic and otherwise damaged retinal tissue.

Anti-VEGF aptamer (pegaptanib) therapy for ocular vascular diseases

Vascular endothelial growth factor (VEGF) is a central regulator of both physiological and pathological angiogenesis. Pegaptanib, a 28-nucleotide RNA aptamer specific for the VEGF(165) isoform, binds to it in the extracellular space, leaving other isoforms unaffected, and inhibits such key VEGF actions as promotion of endothelial cell proliferation and survival, and vascular permeability. Pegaptanib already has been examined as a treatment for two diseases associated with ocular neovascularization, age-related macular degeneration (AMD) and diabetic macular edema (DME). Preclinical studies have shown that VEGF(165) alone mediates pathological ocular neovascularization and that its inactivation by pegaptanib inhibits the choroidal neovascularization observed in patients with neovascular AMD. In contrast, physiological vascularization, which is supported by the VEGF(121) isoform, is unaffected by this inactivation of VEGF(165). In addition, animal model studies have shown that intravitreous injection of pegaptanib can inhibit the breakdown of the blood-retinal barrier characteristic of diabetes and even can reverse this damage to some degree. These preclinical findings formed the basis for randomized controlled trials examining the efficacy of pegaptanib as a therapy for AMD and DME. The VEGF Inhibition Study in Ocular Neovascularization (VISION) trial comprising two replicate, pivotal phase 3 studies, demonstrated that intravitreous injection of pegaptanib resulted in significant clinical benefit, compared with sham injection, for all prespecified clinical end points, irrespective of patient demographics or angiographic subtype, and led to pegaptanib's approval as a treatment for AMD. A phase 2 trial has provided support for the efficacy of intravitreous pegaptanib in the treatment of DME.

Fibrosis and diseases of the eye

Most diseases that cause catastrophic loss of vision do so as a result of abnormal angiogenesis and wound healing, often in response to tissue ischemia or inflammation. Disruption of the highly ordered tissue architecture in the eye caused by vascular leakage, hemorrhage, and concomitant fibrosis can lead to mechanical disruption of the visual axis and/or biological malfunctioning. An increased understanding of inflammation, wound healing, and angiogenesis has led to the development of drugs effective in modulating these biological processes and, in certain circumstances, the preservation of vision. Unfortunately, such pharmacological interventions often are too little, too late, and progression of vision loss frequently occurs. The recent development of progenitor and/or stem cell technologies holds promise for the treatment of currently incurable ocular diseases.

The dynamic conduct of bone marrow-derived cells in the choroidal neovascularization microenvironment

PURPOSE

Choroidal neovascularization (CNV) is one of the most frequent causes of severe and progressive vision loss. Prior studies have shown that bone marrow-derived cells (BMCs) play an important role in CNV, indicating that BMCs can be a potential target for inhibiting the development of CNV. It could be helpful for our understanding of CNV to study the dynamic conduct of BMCs in the CNV microenvironment.

METHODS

Green fluorescent protein (gfp) chimeric mice were developed by transplanting bone marrow cells from gfp+/+ transgenic mice to adult C57BL/6J mice. The chimeric mice underwent laser rupture of Bruch's membrane to induce CNV and were killed at 1, 2, 3, and 4 weeks after laser injury. The eyes were enucleated and processed for immunofluorescence to detect markers for vascular smooth muscle cells (alpha smooth muscle actin, alpha SMA), endothelial cells (CD31), or macrophages (F4/80) on gfp+ cells. All sections were qualitatively and quantitatively assessed by confocal microscopy.

RESULTS

Large number of gfp-labeled cells appeared in the lesions and integrated into CNV. Gfp+ cells, which were immunoreactive for alpha SMA, CD31, or F4/80, can be detected through the whole study. The constituent ratio of those three cell-types in total gfp+ cells in CNV altered as CNV developed. The maximal ratios of CD31-labeled cells and F4/80-labeled cells presented at 2 week, while the ratio of alpha SMA-labeled cells upgraded continuously.

CONCLUSIONS

BMCs underwent a serial of changes in position and expression during the progression of CNV. Those changes may result from the interaction between BMCs and the CNV microenvironment.

Targeting angiogenesis, the underlying disorder in neovascular age-related macular degeneration

Angiogenesis has a causal role in many diseases, including neovascular age-related macular degeneration (AMD). Identification of key regulators of angiogenesis, including vascular endothelial growth factor (VEGF), fibroblast growth factor 2, pigment epithelium-derived growth factor, angiopoietins and extracellular matrix molecules, has facilitated the development of novel therapeutic agents that target the underlying pathological angiogenic process. Among these, VEGF serves as a "master switch" for many ocular neovascular conditions through its promotion of endothelial cell proliferation and survival, vascular permeability and ocular inflammation. Two anti-VEGF agents are now clinically available: bevacizumab, an antibody for metastatic colorectal cancer, and pegaptanib sodium, an aptamer for neovascular AMD. Unlike bevacizumab, which binds all VEGF isoforms, pegaptanib targets only VEGF165, the isoform responsible for pathological ocular neovascularization and thus an ideal target for treatment of AMD. Although other therapies targeting angiogenesis in AMD are in clinical development, to date, pegaptanib is the only therapy approved by the Food and Drug Administration of the United States for the treatment of all neovascular AMD and represents a valuable addition to the hitherto limited options available for patients.

Hematopoietic stem cells provide repair functions after laser-induced Bruch's membrane rupture model of choroidal neovascularization

Vascular repair by adult hematopoietic stem cells (HSCs) is well-appreciated because these cells are known for their plasticity. We have shown that adult HSCs differentiate into endothelial cells and participate in both retinal and choroidal neovascularization. We asked whether HSCs participated in the wounding response by forming astrocytes, retinal pigment epithelia (RPE), macrophages, and pericytes. Lethally irradiated C57BL6/J mice were reconstituted with HSCs from mice homozygous for green fluorescent protein (GFP) and then subjected to laser-induced rupture of Bruch's membrane. After immunohistochemical examination of ocular tissue, GFP(+) astrocytes were observed concentrated along the edge of the laser wound, where they and mural cells closely ensheathed the neovasculature. GFP(+) vascular endothelial cells and macrophages/microglia were also evident. Large irregularly shaped GFP(+) RPE cells constituted approximately 93% of RPE cells adjacent to the edge of the denuded RPE area. In regions farther away from the wound, GFP(+) RPE cells were integrated among the GFP(-) host RPE. Thus, postnatal HSCs can differentiate into cells expressing markers specific to astrocytes, macrophages/microglia, mural cells, or RPE. These studies suggest that HSCs could serve as a therapeutic source for long-term regeneration of injured retina and choroid in diseases such as age-related macular degeneration and retinitis pigmentosa.

Endothelial cell proliferation in the choriocapillaris during human retinal differentiation

BACKGROUND

Differentiation patterns of the neural retina and its retinal vasculature are not well matched. The foveal region differentiates first, however the central retina is not vascularised until late in gestation. The authors explored the hypothesis that higher rates of endothelial cell proliferation in the choriocapillaris of the central retina might compensate for the slow growth of central retinal vessels, providing supplementary nutrients to the region during the early stages of neuronal maturation.

METHODS

Frozen sections of five human fetal eyes (14-18.5 weeks' gestation), were examined for Ki-67 and CD34 immunoreactivity using confocal microscopy. Measurements of choriocapillaris area and the number of proliferating choroidal endothelial cells were used to calculate the rate of choroidal endothelial proliferation at five different chorioretinal locations.

RESULTS

The choriocapillaris area is consistently greater in the foveal region than at other locations and increases progressively with age. A higher rate of endothelial cell proliferation was found in parts of the choriocapillaris associated with the undifferentiated (proliferating) neural retina, compared with the differentiated, central region.

CONCLUSION

The findings suggest that mechanisms regulating proliferation and growth of the choroidal vasculature are independent of differentiation in the neural retina, and are thus profoundly different from mechanisms that regulate formation of the retinal vasculature.

Pegaptanib, a targeted anti-VEGF aptamer for ocular vascular disease

Aptamers are oligonucleotide ligands that are selected for high-affinity binding to molecular targets. Pegaptanib sodium (Macugen; Eyetech Pharmaceuticals/Pfizer) is an RNA aptamer directed against vascular endothelial growth factor (VEGF)-165, the VEGF isoform primarily responsible for pathological ocular neovascularization and vascular permeability. After nearly a decade of preclinical development to optimize and characterize its biological effects, pegaptanib was shown in clinical trials to be effective in treating choroidal neovascularization associated with age-related macular degeneration. Pegaptanib therefore has the notable distinction of being the first aptamer therapeutic approved for use in humans, paving the way for future aptamer applications.

A regenerative role for bone marrow following experimental colitis: contribution to neovasculogenesis and myofibroblasts

BACKGROUND & AIMS

Bone marrow (BM) cells form differentiated adult lineages within nonhematopoietic tissues, with a heightened propensity with increasing regenerative pressure dictated by disease. We have previously shown that BM cells engraft into the gut and contribute substantially to the subepithelial intestinal myofibroblast population in the lamina propria. To investigate the reparative capacity of BM in inflammatory bowel disease (IBD), a well-established model of experimental colitis was used.

METHODS

Lethally irradiated female mice were rescued by a BM transplant from male donors. Colitis was induced 6 weeks posttransplantation by injection of trinitrobenzene sulfonic acid (TNBS), and tissues were analyzed 1-14 days later. Donor-derived cells were detected by in situ hybridization using a Y chromosome-specific probe, and their phenotype was determined by immunohistochemistry.

RESULTS

TNBS-induced colitis was manifest as patchy lesions that increased in severity between days 1 and 8, and the mucosa gradually regenerated between days 8 and 14. The contribution of BM to intestinal myofibroblasts was significantly increased in regions of colitis compared with noninflamed regions. Furthermore, BM-derived endothelial cells, pericytes, and vascular smooth muscle cells were frequently interspersed throughout blood vessels, suggesting that these cells facilitate angiogenesis in tissue repair, substantiated by a significant increase in the incidence of BM-derived vascular smooth muscle cells in colitic compared with noninflamed regions. Blood vessels formed entirely from BM-derived cells were also seen, suggesting a role for BM in neovasculogenesis.

CONCLUSIONS

Our data show that BM contributes to multiple intestinal cell lineages in colitis, with an important function in tissue regeneration and vasculogenesis after injury.

Quantitative enumeration of vascular smooth muscle cells and endothelial cells derived from bone marrow precursors in experimental choroidal neovascularization

Choroidal neovascularization (CNV) is characterized by the subretinal invasion of a pathologic new vessel complex from the choriocapillaris. Although CNV is traditionally considered to consist of endothelial cells, the cellular population of CNV is likely more complex in nature, comprising several different cell types. In addition, recent studies suggest that the CNV cell population has a dual origin (circulating versus resident populations). In this study we sought to determine the contribution and origin of different cell types in experimental CNV. Laser-induced CNV was performed on chimeric mice generated by reconstituting C57BL/6 mice with bone marrow from green fluorescent protein (GFP)-transgenic mice. In these mice, bone marrow-derived cells are GFP-labeled. Immunofluorescence staining was used to examine both flatmount preparations of the choroid and cross sections of the posterior pole for macrophages, endothelial cells, vascular smooth muscle cells, retinal pigment epithelial (RPE) cells, lymphocytes, or neutrophils at day 3, 7, 14 and 28 post-laser (n=5 per group). Cell types present in CNV included macrophages (20% of the cells in CNV), endothelial cells (25%), vascular smooth muscle cells (11%), RPE cells (12%) and non-labeled cells (32%). The macrophage population was mostly derived from circulating monocytes at all timepoints studied (70% were GFP labeled), while endothelial and vascular smooth muscle cells were partly bone marrow derived (50-60% were GFP labeled), and RPE cells appeared to be entirely derived from preexisting tissue resident cells. These results demonstrate that bone marrow-derived progenitor cells contribute significantly to the vascular and inflammatory components of CNV. Knowledge of the cellular composition and origin might help understand the pathogenic mechanisms controlling CNV severity as well as indicate potential targets for therapeutic intervention.