Adrenomedullin Inhibits Choroidal Neovascularization via CCL2 in the Retinal Pigment Epithelium

Role of Adrenomedullin 2/Intermedin in the Pathogenesis of Neovascular Age-Related Macular Degeneration

Adrenomedullin 2 (AM2; also known as intermedin) is a member of the adrenomedullin (AM) peptide family. Similarly to AM, AM2 partakes in a variety of physiological activities. AM2 has been reported to exert protective effects on various organ disorders; however, its significance in the eye is unknown. We investigated the role of AM2 in ocular diseases. The receptor system of AM2 was expressed more abundantly in the choroid than in the retina. In an oxygen-induced retinopathy model, physiological and pathologic retinal angiogenesis did not differ between AM2-knockout (AM2-/-) and wild-type mice. In contrast, in laser-induced choroidal neovascularization, a model of neovascular age-related macular degeneration, AM2-/- mice had enlarged and leakier choroidal neovascularization lesions, with exacerbated subretinal fibrosis and macrophage infiltration. Contrary to this, exogenous administration of AM2 ameliorated the laser-induced choroidal neovascularization-associated pathology and suppressed gene expression associated with inflammation, fibrosis, and oxidative stress, including that of VEGF-A, VEGFR-2, CD68, CTGF, and p22-phox. The stimulation of human adult retinal pigment epithelial (ARPE) cell line 19 cells with TGF-β2 and TNF-α induced epithelial-to-mesenchymal transition (EMT), whereas AM2 expression was also elevated. The induction of EMT was suppressed when the ARPE-19 cells were pretreated with AM2. A transcriptome analysis identified 15 genes, including mesenchyme homeobox 2 (Meox2), whose expression was significantly altered in the AM2-treated group compared with that in the control group. The expression of Meox2, a transcription factor that inhibits inflammation and fibrosis, was enhanced by AM2 treatment and attenuated by endogenous AM2 knockout in the early phase after laser irradiation. The AM2 treatment of endothelial cells inhibited endothelial to mesenchymal transition and NF-κB activation; however, this effect tended to be canceled following Meox2 gene knockdown. These results indicate that AM2 suppresses the neovascular age-related macular degeneration-related pathologies partially via the upregulation of Meox2. Thus, AM2 may be a promising therapeutic target for ocular vascular diseases.

Suppression of CCL2 angiocrine function by adrenomedullin promotes tumor growth

Within the tumor microenvironment, tumor cells and endothelial cells regulate each other. While tumor cells induce angiogenic responses in endothelial cells, endothelial cells release angiocrine factors, which act on tumor cells and other stromal cells. We report that tumor cell-derived adrenomedullin has a pro-angiogenic as well as a direct tumor-promoting effect, and that endothelium-derived CC chemokine ligand 2 (CCL2) suppresses adrenomedullin-induced tumor cell proliferation. Loss of the endothelial adrenomedullin receptor CALCRL or of the G-protein Gs reduced endothelial proliferation. Surprisingly, tumor cell proliferation was also reduced after endothelial deletion of CALCRL or Gs. We identified CCL2 as a critical angiocrine factor whose formation is inhibited by adrenomedullin. Furthermore, CCL2 inhibited adrenomedullin formation in tumor cells through its receptor CCR2. Consistently, loss of endothelial CCL2 or tumor cell CCR2 normalized the reduced tumor growth seen in mice lacking endothelial CALCRL or Gs. Our findings show tumor-promoting roles of adrenomedullin and identify CCL2 as an angiocrine factor controlling adrenomedullin formation by tumor cells.

Exploring the choroidal vascular labyrinth and its molecular and structural roles in health and disease

The choroid is a key player in maintaining ocular homeostasis and plays a role in a variety of chorioretinal diseases, many of which are poorly understood. Recent advances in the field of single-cell RNA sequencing have yielded valuable insights into the properties of choroidal endothelial cells (CECs). Here, we review the role of the choroid in various physiological and pathophysiological mechanisms, focusing on the role of CECs. We also discuss new insights regarding the phenotypic properties of CECs, CEC subpopulations, and the value of measuring transcriptomics in primary CEC cultures derived from post-mortem eyes. In addition, we discuss key phenotypic, structural, and functional differences that distinguish CECs from other endothelial cells such as retinal vascular endothelial cells. Understanding the specific clinical and molecular properties of the choroid will shed new light on the pathogenesis of the broad clinical range of chorioretinal diseases such as age-related macular degeneration, central serous chorioretinopathy and other diseases within the pachychoroid spectrum, uveitis, and diabetic choroidopathy. Although our knowledge is still relatively limited with respect to the clinical features and molecular pathways that underlie these chorioretinal diseases, we summarise new approaches and discuss future directions for gaining new insights into these sight-threatening diseases and highlight new therapeutic strategies such as pluripotent stem cell‒based technologies and gene therapy.

Adrenomedullin-Receptor Activity-Modifying Protein 2 System Ameliorates Subretinal Fibrosis by Suppressing Epithelial-Mesenchymal Transition in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of visual impairment. Anti-vascular endothelial growth factor drugs used to treat AMD carry the risk of inducing subretinal fibrosis. We investigated the use of adrenomedullin (AM), a vasoactive peptide, and its receptor activity-modifying protein 2, RAMP2, which regulate vascular homeostasis and suppress fibrosis. The therapeutic potential of the AM-RAMP2 system was evaluated after laser-induced choroidal neovascularization (LI-CNV), a mouse model of AMD. Neovascular formation, subretinal fibrosis, and macrophage invasion were all enhanced in both AM and RAMP2 knockout mice compared with those in wild-type mice. These pathologic changes were suppressed by intravitreal injection of AM. Comprehensive gene expression analysis of the choroid after LI-CNV with or without AM administration revealed that fibrosis-related molecules, including Tgfb, Cxcr4, Ccn2, and Thbs1, were all down-regulated by AM. In retinal pigment epithelial cells, co-administration of transforming growth factor-β and tumor necrosis factor-α induced epithelial-mesenchymal transition, which was also prevented by AM. Finally, transforming growth factor-β and C-X-C chemokine receptor type 4 (CXCR4) inhibitors eliminated the difference in subretinal fibrosis between RAMP2 knockout and wild-type mice. These findings suggest the AM-RAMP2 system suppresses subretinal fibrosis in LI-CNV by suppressing epithelial-mesenchymal transition.

Protective Effects and Molecular Signaling of n-3 Fatty Acids on Oxidative Stress and Inflammation in Retinal Diseases

Oxidative stress and inflammation play crucial roles in the development and progression of retinal diseases. Retinal damage by various etiologies can result in retinopathy of prematurity (ROP), diabetic retinopathy (DR), and age-related macular degeneration (AMD). n-3 fatty acids are essential fatty acids and are necessary for homeostasis. They are important retinal membrane components and are involved in energy storage. n-3 fatty acids also have antioxidant and anti-inflammatory properties, and their suppressive effects against ROP, DR, and AMD have been previously evaluated. α-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and their metabolites have been shown to alleviate retinal oxidative stress and inflammation involving various biological signaling pathways. In this review, we summarize the current understanding of the n-3 fatty acids effects on the mechanisms of these retinal diseases and how they exert their therapeutic effects, focusing on ALA, EPA, DHA, and their metabolites. This knowledge may provide new remedial strategies for n-3 fatty acids in the prevention and treatment of retinal diseases associated with oxidative stress and inflammation.

Asian age-related macular degeneration: from basic science research perspective

In Asian populations, polypoidal choroidal vasculopathy (PCV), a distinct phenotype of neovascular age-related macular degeneration (AMD), is more prevalent than Caucasians. Recently, there has been significant focus on how PCV differs from typical AMD. Although typical AMD and PCV share a variety of mechanisms by which abnormal angiogenic process occurs at the retinochoroidal interface, PCV has different clinical characteristics such as aneurysm-like dilation at the terminal of choroidal neovascular membranes, less frequent drusen and inner choroidal degeneration due to the thickened choroid. Recent studies support an important role for inflammation, angiogenesis molecules and lipid metabolism in the pathogenesis of neovascular AMD. Furthermore, although less attention has been paid to the role of the choroid in AMD, accumulating evidence suggests that the choriocapillaris and choroid also play a pivotal role in drusenogenesis, typical AMD and PCV. This review discusses the basic pathogenic mechanisms of AMD and explores the difference between typical AMD and PCV.

Adrenomedullin Suppresses Vascular Endothelial Growth Factor-Induced Vascular Hyperpermeability and Inflammation in Retinopathy

Diabetic macular edema (DME) is caused by blood-retinal barrier breakdown associated with retinal vascular hyperpermeability and inflammation, and it is the major cause of visual dysfunction in diabetic retinopathy. Adrenomedullin (ADM) is an endogenous peptide first identified as a strong vasodilator. ADM is expressed in the eyes and is up-regulated in various eye diseases, although the pathophysiological significance is largely unknown. We investigated the effect of ADM on DME. In Kimba mice, which overexpress human vascular endothelial growth factor in their retinas, the capillary dropout, vascular leakage, and vascular fragility characteristic of diabetic retinopathy were observed. Intravitreal or systemic administration of ADM to Kimba mice ameliorated both the capillary dropout and vascular leakage. Evaluation of the transendothelial electrical resistance and fluorescein isothiocyanate-dextran permeability of an endothelial cell monolayer using TR-iBRB retinal capillary endothelial cells revealed that vascular endothelial growth factor enhanced vascular permeability but that co-administration of ADM suppressed the effect, in part by enhancing tight junction formation between endothelial cells. In addition, a comprehensive PCR array analysis showed that ADM administration suppressed various molecules related to inflammation and NF-κB signaling within retinas. From these results, we suggest that by exerting inhibitory effects on retinal inflammation, vascular permeability, and blood-retinal barrier breakdown, ADM could serve as a novel therapeutic agent for the treatment of DME.

Hypoxia-Inducible Factor-1α Target Genes Contribute to Retinal Neuroprotection

Hypoxia-inducible factor (HIF) is a transcription factor that facilitates cellular adaptation to hypoxia and ischemia. Long-standing evidence suggests that one isotype of HIF, HIF-1α, is involved in the pathogenesis of various solid tumors and cardiac diseases. However, the role of HIF-1α in retina remains poorly understood. HIF-1α has been recognized as neuroprotective in cerebral ischemia in the past two decades. Additionally, an increasing number of studies has shown that HIF-1α and its target genes contribute to retinal neuroprotection. This review will focus on recent advances in the studies of HIF-1α and its target genes that contribute to retinal neuroprotection. A thorough understanding of the function of HIF-1α and its target genes may lead to identification of novel therapeutic targets for treating degenerative retinal diseases including glaucoma, age-related macular degeneration, diabetic retinopathy, and retinal vein occlusions.

Choroidal Neovascularization Is Inhibited in Splenic-Denervated or Splenectomized Mice with a Concomitant Decrease in Intraocular Macrophage

PURPOSE

To determine the involvement of sympathetic activity in choroidal neovascularization (CNV) using laser-induced CNV in a mouse model.

METHODS

We investigated changes in the proportions of intraocular lymphocytes, granulocytes, and three macrophage subtypes (Ly6Chi, Ly6Cint, and Ly6Clo) after laser injury in mice using flow cytometry, and evaluated CNV lesion size in mice lacking inflammatory cells. Further, we evaluated the lesion size in mice administered the β3 receptor antagonist, splenic-denervated and splenectomized mice. We also assessed changes in the proportions of intraocular macrophages and peripheral blood monocytes in splenic-denervated and splenectomized mice. Lastly, lesion size was compared between splenic-denervated mice with or without adoptive transfer of macrophages following laser injury. After Ly5.1 mice spleen-derived Ly6Chi cells were transferred into Ly5.2 mice, the proportions of intraocular Ly5.1+Ly6Chi cells were compared.

RESULTS

In WT mice, the proportion of CD4+ T cells recruited into the eye increased progressively from day 3 to day 7 after laser injury, whereas, intraocular CD8+ T cells did not change significantly. Proportions of B220+ cells, granulocytes, and two subtypes of intraocular macrophages (Ly6Chi and Ly6Clo) peaked at day 3 following laser injury. In contrast, Ly6Cint/loCD64+ subtype showed a significantly higher percentage at day 7 after laser injury. There were no differences in lesion size between CD4-/-or Rag2-/-mice and controls, whereas lesion size was significantly reduced in CCR2-/- mice and clodronate liposome-treated mice. CNV lesion area was significantly reduced in mice with β3 blocker treatment, splenic-denervated and splenectomized mice compared with controls. Intraocular Ly6Chi macrophages were also reduced by splenic denervation or splenectomy. Adoptive transfer of spleen-derived Ly6Chi cells increased the lesion size in splenic-denervated mice. Compared with controls, intraocular donor-derived Ly6Chi cells recruited into the eye were reduced in splenic-denervated and splenectomized mice.

CONCLUSIONS

Although lymphocytes had little effect on CNV formation, Ly6Chi macrophages/monocytes exacerbated CNV in mice. Sympathetic activity might contribute to CNV via the recruitment of macrophages to the eye.

Choroidal neovascularization is inhibited via an intraocular decrease of inflammatory cells in mice lacking complement component C3

In early age-related macular degeneration (AMD), complement component C3 can be observed in drusen, which is the accumulation of material beneath the retinal pigment epithelium. The complement pathways, via the activation of C3, can upregulate the expression of cytokines and their receptors and the recruitment of inflammatory leukocytes, both of which play an important role in the development of choroidal neovascularization (CNV) in exudative AMD. Laser-induced CNV lesions were found to be significantly smaller in C3^−/− mice than in wild-type mice. By using flow cytometry, we demonstrated that the proportions of intraocular granulocytes, CD11b⁺F4/80⁺Ly6C^hi and CD11b⁺F4/80⁺Ly6C^lo cells, were lower in C3^−/− mice than in wild-type mice as early as day 1 after laser injury, and the proportions of granulocytes and three macrophage/monocyte subsets were significantly lower on day 3. In contrast, C3^−/− mice had more granulocytes and CD11b⁺F4/80⁺Ly6C^hi cells in peripheral blood than wild-type mice after injury. Further, the expression levels of Vegfa164 were upregulated in intraocular Ly6C^hi macrophages/monocytes of C3^−/− mice, but not as much as in wild-type mice. Collectively, our data demonstrate that despite a more pronounced induction of systemic inflammation, inhibition of complement factor C3 suppresses CNV by decreasing the recruitment of inflammatory cells to the lesion.

Inhibition of autophagy induces IL-1β release from ARPE-19 cells via ROS mediated NLRP3 inflammasome activation under high glucose stress

Autophagy plays an important role in the development of diabetic retinopathy (DR). Retinal pigment epithelial (RPE) cells are the main cells involved in DR, a process in which hyperglycemia plays a crucial role. This study was conducted to investigate the protective effect of autophagy against high glucose-induced inflammatory response in ARPE-19 cells and its underlying mechanism. In the present study we subjected ARPE-19 cells to high glucose stress and showed that ARPE-19 cells respond to high glucose with an increase in autophagy. 3-methyladenine (3-MA) inhibited occurrence of autophagy and it leaded to the accumulation of damaged-mitochondria-producing-ROS, and the activation of NLRP3 inflammasome, and subsequently, caused IL-1β secretion.

Excessive retinol intake exacerbates choroidal neovascularization through upregulated vascular endothelial growth factor in retinal pigment epithelium in mice

As a part of the visual cycle, all-trans-retinol (all-trans-ROL), the major form of vitamin A in circulating blood, is transported to the retinal pigment epithelium (RPE). All-trans-ROL is essential for normal retina function. However, recent researches have shown that excessive retinol intake can cause increase of all-trans-retinal. This can lead to the accumulation of lipofuscin, which is important in the pathogenesis of retina degeneration disease, such as dry type age-related macular degeneration (AMD). Since there are few reports regarding the involvement of all-trans-ROL in exudative AMD, we investigated the effects of all-trans-ROL in vitro and in vivo. We evaluated vascular endothelial growth factor (VEGF) expression in ARPE-19 cells and THP-1 cells after all-trans-ROL treatment using ELISA and real-time RT-PCR. In-vitro tube formation assay was performed with HUVEC cells using the conditioned medium (CM) obtained from ARPE-19 cells treated with all-trans-ROL. Transcriptional activity of retinoic acid receptor (RAR) was evaluated using luciferase assay. In mice, VEGF expressions were investigated in the retina and RPE/choroid after three weeks of excessive oral retinol intake. Laser-induced choroidal neovascularization (CNV) models were evaluated after they were fed with various doses of retinol. VEGF mRNA expression and VEGF production were significantly increased in all-trans-ROL treated ARPE-19 cells, which were inhibited by an RAR antagonist LE540. In contrast, there were no significant changes in VEGF production in THP-1 cells. Transcriptional activity of RAR was upregulated by all-trans-ROL treatment in ARPE-19 cells. The CM, obtained from ARPE-19 cells treated with all-trans-ROL, induced more capillary-like tube formation than cells treated with control vehicles. In vivo, the high retinol diet group has increased VEGF expression in the RPE/choroid and larger lesion size was induced. Our results suggest that all-trans-ROL is a pro-angiogenic factor. Excessive retinoid intake may be a potential risk factor for exudative AMD.

Inhibition of autophagy induces retinal pigment epithelial cell damage by the lipofuscin fluorophore A2E

In this study, we show augmented autophagy in the retinal pigment epithelial cell line ARPE-19 when cultured in the presence of the lipofuscin pigment A2E. A2E alone does not induce RPE cell death, but cell death was induced in the presence of A2E with the autophagy inhibitor 3-methyladenine (3MA), with a concomitant increase in the generation of mitochondrial reactive oxygen species. On the other hand, the ATP production capacity of mitochondria was decreased in the presence of A2E, and pharmacological inhibition of autophagy had no additional effects. The altered mRNA expression level of mitochondrial function markers was confirmed by real-time polymerase chain reaction, which showed that the antioxidant enzymes SOD1 and SOD2 were not reduced in the presence of A2E alone, but significantly suppressed with the addition of 3MA. Furthermore, transmission electron micrography revealed autophagic vacuole formation in the presence of A2E, and inhibition of autophagy resulted in the accumulation of abnormal mitochondria with loss of cristae. Spheroid culture of human RPE cells demonstrated debris accumulation in the presence of A2E, and this accumulation was accelerated in the presence of 3MA. These results indicate that autophagy in RPE cells is a vital cytoprotective process that prevents the accumulation of damaged cellular molecules.

Metabolic adaptation and neuroprotection differ in the retina and choroid in a piglet model of acute postnatal hypoxia

BACKGROUND

Hypoxic-ischemic insults to the neonatal brain may cause neurodevelopmental disorders. Vulnerability of different areas of the neural tissue to hypoxic-ischemic stress might be explained by either heterogeneous sensitivity to oxygen or neuroprotective capability. Our understanding of regional heterogeneity is still incomplete in terms of metabolic reconfiguration and/or activation of neuroprotective mechanisms.

METHODS

We studied, by western blotting, reverse-transcriptase PCR, and tandem mass spectrometry, the response of retina and choroid at protein, gene, and metabolic levels during hypoxia in a piglet model of acute postnatal hypoxia.

RESULTS

We evidenced a metabolic shift towards glycolysis in choroid after hypoxia while retina experienced a dramatic energy stress with decreased mitochondrial metabolites. Hypoxia-inducible transcription factor-1α (HIF-1α) was not stabilized in retina during hypoxia, supported by a deficient signaling from v-akt murine thymoma viral oncogene (AKT) and ERK1/2, and unchanged glutathione redox status. In retina, but not in choroid, phosphorylation of p65 (NF-κB) and increased transcription of target genes may have a major role during hypoxic stress.

CONCLUSION

We showed that the retina engages a distinct pattern of signaling and transcriptional events than observed in the choroid. Retina and choroid may reflect regional sensitivity to hypoxia. While prolonged and intense hypoxia may jeopardize retinal cell survival, choroid sets up a different pattern of response, which promotes adaptation to these adverse conditions.

Adrenomedullin-RAMP2 system is crucially involved in retinal angiogenesis

Adrenomedullin (ADM) is an endogenous peptide first identified as a strong vasodilating molecule. We previously showed that in mice, homozygous knockout of ADM (ADM(-/-)) or its receptor regulating protein, RAMP2 (RAMP2(-/-)), is embryonically lethal due to abnormal vascular development, thereby demonstrating the importance of ADM and its receptor signaling to vascular development. ADM expression in the retina is strongly induced by ischemia; however, its role in retinal pathophysiology remains unknown. Here, we analyzed oxygen-induced retinopathy (OIR) using heterozygous ADM and RAMP2 knockout mice models (ADM(+/-) or RAMP2(+/-), respectively). In addition, we analyzed the role of the ADM-RAMP2 system during earlier stages of retinal angiogenesis using an inducible endothelial cell-specific RAMP2 knockout mouse line (DI-E-RAMP2(-/-)). Finally, we assessed the ability of antibody-induced ADM blockade to control pathological retinal angiogenesis in OIR. In OIR, neovascular tufts, avascular zones, and hypoxic areas were all smaller in ADM(+/-) retinas compared with wild-type mice. ADM(+/-) retinas also exhibited reduced levels of VEGF and eNOS expression. DI-E-RAMP2(-/-) showed abnormal retinal vascular patterns in the early stages of development. However, ADM enhanced the proliferation and migration of retinal endothelial cells. Finally, we found intravitreal injection of anti-ADM antibody reduced pathological retinal angiogenesis. In conclusion, the ADM-RAMP2 system is crucially involved in retinal angiogenesis. ADM and its receptor system are potential therapeutic targets for controlling pathological retinal angiogenesis.