Molecular targets for retinal vascular diseases

Diabetic macular ischaemia- a new therapeutic target?

Diabetic macular ischaemia (DMI) is traditionally defined and graded based on the angiographic evidence of an enlarged and irregular foveal avascular zone. However, these anatomical changes are not surrogate markers for visual impairment. We postulate that there are vascular phenotypes of DMI based on the relative perfusion deficits of various retinal capillary plexuses and choriocapillaris. This review highlights several mechanistic pathways, including the role of hypoxia and the complex relation between neurons, glia, and microvasculature. The current animal models are reviewed, with shortcomings noted. Therefore, utilising the advancing technology of optical coherence tomography angiography (OCTA) to identify the reversible DMI phenotypes may be the key to successful therapeutic interventions for DMI. However, there is a need to standardise the nomenclature of OCTA perfusion status. Visual acuity is not an ideal endpoint for DMI clinical trials. New trial endpoints that represent disease progression need to be developed before irreversible vision loss in patients with DMI. Natural history studies are required to determine the course of each vascular and neuronal parameter to define the DMI phenotypes. These DMI phenotypes may also partly explain the development and recurrence of diabetic macular oedema. It is also currently unclear where and how DMI fits into the diabetic retinopathy severity scales, further highlighting the need to better define the progression of diabetic retinopathy and DMI based on both multimodal imaging and visual function. Finally, we discuss a complete set of proposed therapeutic pathways for DMI, including cell-based therapies that may provide restorative potential.

Recognition Sites for Cancer-targeting Drug Delivery Systems

BACKGROUND

Target-homing drug delivery systems are now gaining significant attention for use as novel therapeutic approaches in antitumor targeting for cancer therapy. Numerous targeted drug delivery systems have been designed to improve the targeting effects because these systems can display a range of favorable properties, thus, providing suitable characteristics for clinical applicability of anticancer drugs, such as increasing the solubility, and improving the drug distribution at target sites. The majority of these targeting systems are designed with respect to differences between cancerous and normal tissues, for instance, the low pH of tumor tissues or overexpressed receptors on tumor cell membranes. Due to the growing number of targeting possibilities, it is important to know the tumor-specific recognition strategies for designing novel, targeted, drug delivery systems. Herein, we identify and summarize literature pertaining to various recognition sites for optimizing the design of targeted drug delivery systems to augment current chemotherapeutic approaches.

OBJECTIVE

This review focuses on the identification of the recognition sites for developing targeted drug delivery systems for use in cancer therapeutics.

METHODS

We have reviewed and compiled cancer-specific recognition sites and their abnormal characteristics within tumor tissues (low pH, high glutathione, targetable receptors, etc.), tumor cells (receptor overexpression or tumor cell membrane changes) and tumor cell organelles (nuclear and endoplasmic reticular dysregulation) utilizing existing scientific literature. Moreover, we have highlighted the design of some targeted drug delivery systems that can be used as homing tools for these recognition sites.

RESULTS AND CONCLUSION

Targeted drug delivery systems are a promising therapeutic approach for tumor chemotherapy. Additional research focused on finding novel recognition sites, and subsequent development of targeting moieties for use with drug delivery systems will aid in the evaluation and clinical application of new and improved chemotherapeutics.

Microarray Analysis of Long Non-Coding RNAs and Messenger RNAs in a Mouse Model of Oxygen-Induced Retinopathy

Objective: Retinal neovascularization is a severe complication of many ocular diseases. To clarify the possible functions and therapeutic potential of long non-coding RNAs (lncRNAs) and messenger RNAs (mRNAs) in retinal neovascularization, we assessed their expression profile in a mouse model of oxygen-induced retinopathy (OIR). Methods: Microarray analysis was performed to identify altered lncRNA and mRNA expressions between OIR and control mice. The microarray results were validated by qRT-PCR. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were conducted to determine biological functions and signaling pathways of the altered or interacted mRNAs. A coding-non-coding gene co-expression (CNC) network was constructed to identify the interaction of lncRNAs and mRNAs. Results: We identified 198 up-regulated and 175 down-regulated lncRNAs (fold change≥2.0, P<0.05), respectively in OIR mice compared to control mice. We also identified 412 up-regulated and 127 down-regulated mRNAs (fold change≥2.0, P<0.05), respectively in OIR mice compared to control mice. GO and KEGG analyses suggested that altered mRNAs were enriched in immune system process, exopeptidase activity, ECM-receptor interaction and protein digestion and absorption. Four validated lncRNAs (ENSMUST00000165968, ENSMUST00000153785, ENSMUST00000134409, and ENSMUST00000154285) and the nearby coding gene pairs were analyzed. A CNC network profile based on those validated altered lncRNAs as well as 410 interacted mRNAs was composed of 509 connections. Moreover, the GO and KEGG analyses demonstrated that these interacted mRNAs mainly enriched in blood vessel development, angiogenesis, cell adhesion molecules and leukocyte transendothelial migration pathways. Conclusion: Our data highlight the utility of altered lncRNA and mRNA profiling in understanding the pathogenesis of ischemia-induced retinal neovascularization and further suggest that therapeutic potential of altered lncRNA for retinal neovascularization.

Streptozotocin induced diabetic retinopathy in rat and the expression of vascular endothelial growth factor and its receptor

AIM

To establish the rat model of streptozotocin (STZ)-induced diabetic retinopathy (DR), which is the most common cause of visual loss and blindness in patients with diabetes, and observe the gene expression of vascular endothelial growth factor (VEGF) and its receptors during the development of DR.

METHODS

A rat model of diabetes was established by intraperitoneal injection of STZ. The diabetic rats were housed for 2, 3 and 4 months after the development of diabetes. Retinal histopathological observation was performed. The retinal vessels were observed by immunofluorescence staining by CD31. The mRNA expression of VEGF, VEGF receptor 1 and 2 (VEGFR1/2) in rat retina was detected by reverse transcription-polymerase chain reaction (RT-PCR) analysis.

RESULTS

Retinal histopathological observation showed the morphological changes of inner nuclear layer (INL) and outer nuclear layer (ONL) at any time-point, and also demonstrated the increased new vessels at both 3, 4 months after the development of diabetes. The CD31 staining results showed that the number of vessels was increased in the retinas of diabetic rats at both 3 and 4 months after the development of diabetes. As compared to the normal rats, the mRNA expression of VEGF was increased in retinas of diabetic rats at 3 months after the development of diabetes, while VEGFR1 and VEGFR2 mRNA expression was increased at 2, 3 and 4 months after the development of diabetes.

CONCLUSION

Taken together, our results demonstrated that DR was occurred at 3 months after the development of diabetes, and the mRNA expression of VEGF, VEGFR1 and VEGFR2 were increased in the process of DR. The present study further evidenced the involvement of VEGF and its receptors in the process of DR.

Novel targets against retinal angiogenesis in diabetic retinopathy

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

Effect of cediranib, an inhibitor of vascular endothelial growth factor receptor tyrosine kinase, in a mouse model of choroidal neovascularization

BACKGROUND

This study was conducted to evaluate the effect of cediranib, an inhibitor of vascular endothelial growth factor receptor tyrosine kinase, in a mouse model of laser-induced choroidal neovascularization.

METHODS

Choroidal neovascularization was induced in C57BL/6 mice by rupturing Bruch's membrane using laser photocoagulation. Following laser injury, the mice were divided into three groups and administered either vehicle, 1 mg/kg or 5 mg/kg of cediranib daily by oral gavage for 2 weeks. Two weeks after laser injury, the area of choroidal neovascularization lesions was measured by choroidal flat mounts using fluorescein-labelled dextran. Immunofluorescence staining with isolectin IB4 was also used to quantify the choroidal neovascularization lesions.

RESULTS

Choroidal flat mount analysis revealed that orally administered cediranib reduced the extent of choroidal neovascularization. The groups treated with 1 and 5 mg/kg/day showed 57.2 and 66.0% reduction of choroidal neovascularization lesions, respectively, compared with the control group treated with vehicle alone (P = 0.012). The size of the fluorescently labelled choroidal neovascularization complex in cediranib-treated groups was much smaller than that from vehicle-treated group (P = 0.035).

CONCLUSIONS

Cediranib inhibited laser-induced choroidal neovascularization in mice and may have therapeutic potential for patients with neovascular age-related macular degeneration.

Misfolded proteins and retinal dystrophies

Many mutations associated with retinal degeneration lead to the production of misfolded proteins by cells of the retina. Emerging evidence suggests that these abnormal proteins cause cell death by activating the Unfolded Protein Response, a set of conserved intracellular signaling pathways that detect protein misfolding within the endoplasmic reticulum and control protective and proapoptotic signal transduction pathways. Here, we review the misfolded proteins associated with select types of retinitis pigmentosa, Stargadt-like macular degeneration, and Doyne Honeycomb Retinal Dystrophy and discuss the role that endoplasmic reticulum stress and UPR signaling play in their pathogenesis. Last, we review new therapies for these diseases based on preventing protein misfolding in the retina.

Microvascular modifications in diabetic retinopathy

Patients struggling with diabetes are at elevated risks for several sight-threatening diseases, including proliferative diabetic retinopathy (DR). DR manifests in two stages: first, the retinal microvasculature is compromised and capillary degeneration occurs; subsequently, an over-compensatory angiogenic response is initiated. Early changes in the retinal microcirculation include disruptions in blood flow, thickening of basement membrane, eventual loss of mural cells, and the genesis of acellular capillaries. Endothelial apoptosis and capillary dropout lead to a hypoxic inner retina, alterations in growth factors, and upregulation of inflammatory mediators. With disease progression, pathologic angiogenesis generates abnormal preretinal microvessels. Current therapies, which include panretinal photocoagulation and vitrectomy, have remained unaltered for several decades. With several exciting preclinical advances, emergent technologies and innovative cellular targets may offer newfound hope for developing "next-generation" interventional or preventive clinical approaches that will significantly advance current standards of care and clinical outcomes.

Regulation of ocular angiogenesis by Notch signaling: implications in neovascular age-related macular degeneration

PURPOSE

Wet age-related macular degeneration (AMD), which accounts for most AMD-related vision loss, is characterized by choroidal neovascularization (CNV). The underlying mechanism of CNV is poorly understood, but evidence indicates pathologic recruitment of normal angiogenic signaling pathways such as the VEGF pathway. Recent evidence suggests that the VEGF pathway regulates angiogenesis in concert with Notch signaling. Here, the authors examined the role of Notch signaling in CNV in the backdrop of Notch signaling-mediated regulation of retinal angiogenesis.

METHODS

Choroid sclera complexes, after laser-induced CNV, were examined for changes in CNV lesion volume and in proangiogenic and antiangiogenic gene expression after perturbation in Notch signaling. Retinal vessels and angiogenic gene expression in retinal endothelial cells were analyzed in postnatal rats after perturbations in Notch signaling. Notch signaling was activated and inhibited by intravitreal or systemic injection of Jagged1 peptide and gamma secretase inhibitor DAPT, respectively.

RESULTS

The authors demonstrated that activation of the canonical Notch pathway reduced the volume of CNV lesions as it attenuated the development of postnatal retinal vasculature. In contrast, inhibition of the Notch pathway exacerbated CNV lesions as it led to the development of hyperdense retinal vasculature. The authors also identified genes associated with proangiogenesis (Vegfr2, Ccr3, and Pdgfb) and antiangiogenesis (Vegfr1 and Unc5b) as targets of Notch signaling-mediated vascular homeostasis, the disruption of which might underlie CNV.

CONCLUSIONS

This study suggests that Notch signaling is a key regulator of CNV and thus a molecular target for therapeutic intervention in wet AMD.

Identification of genes and pathways involved in retinal neovascularization by microarray analysis of two animal models of retinal angiogenesis

PURPOSE

Comparative retinal gene expression analysis in two rodent models of oxygen-induced retinopathy (OIR) was performed to identify the genes and pathways involved in retinal neovascularization.

METHODS

Three independent experimental runs were conducted for each species, according to standard protocols for induction of OIR. Total retinal RNA was isolated at two time points, corresponding to the early response to relative hypoxia (P13 in mouse, P15 in rat) and to the later phase of maximum retinal neovascularization (P18 in mouse, P20 in rat) and was used to prepare labeled probes for hybridization. Gene expression was compared between normal and experimental conditions for each species at each time point. Probesets with a false-discovery rate of <or=0.05 were considered significantly different and were classified as cellular functions or biological pathways. Changes in expression of selected genes were confirmed by quantitative rtPCR.

RESULTS

At the early time point, there were changes in 43 genes in each species, with two in common. Increased expression of members of the VEGF and ephrin receptor signaling pathways were identified in both models. At the later time point, there were changes in 26 genes in the rat and in 1622 in the mouse, with 13 in common. Four pathways were identified in both models.

CONCLUSIONS

Genes and pathways known to be involved in angiogenesis, as well as other biologically plausible genes and pathways, were identified. This work serves as a comprehensive resource for the study of retinal neovascularization and identification of potential rational targets for antiangiogenic therapy.

In vivo gene regulation using tetracycline-regulatable systems

Numerous preclinical studies have demonstrated the efficacy of viral gene delivery vectors, and recent clinical trials have shown promising results. However, the tight control of transgene expression is likely to be required for therapeutic applications and in some instances, for safety reasons. For this purpose, several ligand-dependent transcription regulatory systems have been developed. Among these, the tetracycline-regulatable system is by far the most frequently used and the most advanced towards gene therapy trials. This review will focus on this system and will describe the most recent progress in the regulation of transgene expression in various organs, including the muscle, the retina and the brain. Since the development of an immune response to the transactivator was observed following gene transfer in the muscle of nonhuman primate, focus will be therefore, given on the immune response to transgene products of the tetracycline inducible promoter.

Role and therapeutic potential of VEGF in the nervous system

The development of the nervous and vascular systems constitutes primary events in the evolution of the animal kingdom; the former provides electrical stimuli and coordination, while the latter supplies oxygen and nutrients. Both systems have more in common than originally anticipated. Perhaps the most striking observation is that angiogenic factors, when deregulated, contribute to various neurological disorders, such as neurodegeneration, and might be useful for the treatment of some of these pathologies. The prototypic example of this cross-talk between nerves and vessels is the vascular endothelial growth factor or VEGF. Although originally described as a key angiogenic factor, it is now well established that VEGF also plays a crucial role in the nervous system. We describe the molecular properties of VEGF and its receptors and review the current knowledge of its different functions and therapeutic potential in the nervous system during development, health, disease and in medicine.

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

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

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

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

Prolonged blockade of VEGF family members does not cause identifiable damage to retinal neurons or vessels

Several ocular diseases complicated by neovascularization are being treated by repeated intraocular injections of vascular endothelial growth factor (VEGF) antagonists. While substantial benefits have been documented, there is concern that unrecognized damage may be occurring, because blockade of VEGF may damage the fenestrated vessels of the choroicapillaris and deprive retinal neurons of input from a survival factor. One report has suggested that even temporary blockade of all isoforms of VEGF-A results in significant loss of retinal ganglion cells. In this study, we utilized double transgenic mice with doxycycline-inducible expression of soluble VEGF receptor 1 coupled to an Fc fragment (sVEGFR1Fc), a potent antagonist of several VEGF family members, including VEGF-A, to test the effects of VEGF blockade in the retina. Expression of sVEGFR1Fc completely blocked VEGF-induced retinal vascular permeability and significantly suppressed the development of choroidal neovascularization at rupture sites in Bruch's membrane, but did not cause regression of established choroidal neovascularization. Mice with constant expression of sVEGFR1Fc in the retina for 7 months had normal electroretinograms and normal retinal and choroidal ultrastructure including normal fenestrations in the choroicapillaris. They also showed no significant difference from control mice in the number of ganglion cell axons in optic nerve cross sections and the retinal level of mRNA for 3 ganglion cell-specific genes. These data indicate that constant blockade of VEGF for up to 7 months has no identifiable deleterious effects on the retina or choroid and support the use of VEGF antagonists in the treatment of retinal diseases.

Topical administration of a multi-targeted kinase inhibitor suppresses choroidal neovascularization and retinal edema

Age-related macular degeneration, diabetic retinopathy, and retinal vein occlusions are complicated by neovascularization and macular edema. Multi-targeted kinase inhibitors that inhibit select growth factor receptor tyrosine kinases and/or components of their down-stream signaling cascades (such as Src kinases) are rationale treatment strategies for these disease processes. We describe the discovery and characterization of two such agents. TG100572, which inhibits Src kinases and selected receptor tyrosine kinases, induced apoptosis of proliferating endothelial cells in vitro. Systemic delivery of TG100572 in a murine model of laser-induced choroidal neovascularization (CNV) caused significant suppression of CNV, but with an associated weight loss suggestive of systemic toxicity. To minimize systemic exposure, topical delivery of TG100572 to the cornea was explored, and while substantial levels of TG100572 were achieved in the retina and choroid, superior exposure levels were achieved using TG100801, an inactive prodrug that generates TG100572 by de-esterification. Neither TG100801 nor TG100572 were detectable in plasma following topical delivery of TG100801, and adverse safety signals (such as weight loss) were not observed even with prolonged dosing schedules. Topical TG100801 significantly suppressed laser-induced CNV in mice, and reduced fluorescein leakage from the vasculature and retinal thickening measured by optical coherence tomography in a rat model of retinal vein occlusion. These data suggest that TG100801 may provide a new topically applied treatment approach for ocular neovascularization and retinal edema.

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.

Targeting the retinal microcirculation to treat diabetic sight problems

Diabetic retinopathy is a secondary complication of hyperglycemia caused by diabetes mellitus. The damage to the retina can ultimately cause vision loss as a result of increased capillary permeability and angiogenesis. Recent progress in the understanding of the mediators that drive angiogenesis, as well as the phenotypes of cells that are involved in this process, has provided a multitude of targets for pharmacologic intervention. This review presents the inhibitors of the biochemical processes that are at the root of diabetic retinopathy (i.e., non-enzymatic glycosylation of biomolecules, oxidative stress, activation of aldose reductase and activation of protein kinase C by formation of diacylglycerol) in addition to the inhibitors of the mechanical damage (i.e., increased vascular permeability, capillary occlusion and neovascularization).

Seeing the light: New insights into the molecular pathogenesis of retinal diseases

In the past, most treatments for retinal diseases have been empirical. Steroids and/or laser photocoagulation and/or surgery have been tried for almost every condition with little or no understanding of the underlying disease. Over the past several years vision researchers have uncovered molecular components of processes, such as visual transduction and the visual cycle, that are critical for visual function, and identified other molecules that lead to dysfunction and disease processes such as neovascularization and macular edema. It is becoming clear that dysregulation of certain molecules can have major effects on retinal structure and function. Studies in animal models have suggested that inhibiting or augmenting levels of a single molecule can have major effects in complex disease processes. Although several molecules probably contribute to neovascularization and excessive vascular permeability in the eye, blockade of vascular endothelial growth factor (VEGF) has remarkable beneficial effects in animal models that have now been proven to apply to human diseases in clinical trials. Intraocular injection of VEGF antagonists has revolutionized the treatment of choroidal neovascularization (CNV) and macular edema and serves as a model of targeted ocular pharmacotherapy. Significant progress elucidating the molecular pathogenesis of several disease processes in the eye may soon lead to new treatments following the lead of VEGF antagonists. Initial treatments that provide benefit from frequent intraocular injections are likely to be followed by sustained delivery of drugs and/or prolonged protein delivery by gene transfer. The eye has entered the era of molecular therapy.