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Blocking the signaling activated by the plasma membrane receptor CD93 has recently been demonstrated a useful tool in antiangiogenic treatment and oncotherapy. In the proliferating endothelium, CD93 regulates cell adhesion, migration, and vascular maturation, yet it is unclear how CD93 interacts with the extracellular matrix activating signaling pathways involved in the vascular remodeling. Here for the first time we show that in endothelial cells CD93 is structured as a dimer and that this oligomeric form is physiologically instrumental for the binding of CD93 to its ligand Multimerin-2. Crystallographic X-ray analysis of recombinant CD93 reveals the crucial role played by the C-type lectin-like and sushi-like domains in arranging as an antiparallel dimer to achieve a functional binding state, providing key information for the future design of new drugs able to hamper CD93 function in neovascular pathologies., Competing Interests: Declaration of competing interest The authors declare that there are no conflicts of interest., (Copyright © 2022. Published by Elsevier B.V.)

During angiogenesis, cell adhesion molecules expressed on the endothelial cell surface promote the growth and survival of newly forming vessels. Hence, elucidation of the signaling pathways activated by cell-to-matrix adhesion may assist in the discovery of new targets to be used in antiangiogenic therapy. In proliferating endothelial cells, the single-pass transmembrane glycoprotein CD93 has recently emerged as an important endothelial cell adhesion molecule regulating vascular maturation. In this study, we unveil a signaling pathway triggered by CD93 that regulates actin cytoskeletal dynamics responsible of endothelial cell adhesion. We show that the Src-dependent phosphorylation of CD93 and the adaptor protein Cbl leads to the recruitment of Crk, which works as a downstream integrator in the CD93-mediated signaling. Moreover, confocal microscopy analysis of FRET-based biosensors shows that CD93 drives the coordinated activation of Rac1 and RhoA at the cell edge of spreading cells, thus promoting the establishment of cell polarity and adhesion required for cell motility.

Purpose: The purpose of this study was to investigate the involvement of CD93 and Multimerin-2 in three choroidal neovascularization (CNV) models and to evaluate their contribution in the neovascular progression of age-related macular degeneration (AMD)., Methods: Choroidal neovascular membranes collected during surgery from AMD patients were analyzed by microscopy methods. Laser-induced CNV mouse models and choroid sprouting assays (CSAs) were carried out using the CD93 knockout mouse model. An original ex vivo CSA of vascular angiogenesis, employing choroid tissues isolated from human donors, was developed., Results: In contrast to healthy choroid endothelium, hyperproliferative choroidal endothelial cells (ECs) of AMD patients expressed high levels of CD93, and Multimerin-2 was abundantly deposited along the choroidal neovasculature. CD93 knockout mice showed a significant reduced neovascularization after laser photocoagulation, and their choroidal ECs displayed a decreased ability to produce sprouts in ex vivo angiogenesis assays. Moreover, the presence of an antibody able to hamper the CD93/Multimerin-2 interaction reduced vascular sprouting in the human CSA., Conclusions: Our results demonstrate that CD93 and its interaction with Multimerin-2 play an important role in pathological vascularization of the choroid, disclosing new possibilities for therapeutic intervention to neovascular AMD.

Background: In the endothelium, the single-pass membrane protein CD93, through its interaction with the extracellular matrix protein Multimerin-2, activates signaling pathways that are critical for vascular development and angiogenesis. Trafficking of adhesion molecules through endosomal compartments modulates their signaling output. However, the mechanistic basis coordinating CD93 recycling and its implications for endothelial cell (EC) function remain elusive., Methods: Human umbilical vein ECs (HUVECs) and human dermal blood ECs (HDBEC) were used in this study. Fluorescence confocal microscopy was employed to follow CD93 retrieval, recycling, and protein colocalization in spreading cells. To better define CD93 trafficking, drug treatments and transfected chimeric wild type and mutant CD93 proteins were used. The scratch assay was used to evaluate cell migration. Gene silencing strategies, flow citometry, and quantification of migratory capability were used to determine the role of Rab5c during CD93 recycling to the cell surface., Results: Here, we identify the recycling pathway of CD93 following EC adhesion and migration. We show that the cytoplasmic domain of CD93, by its interaction with Moesin and F-actin, is instrumental for CD93 retrieval in adhering and migrating cells and that aberrant endosomal trafficking of CD93 prevents its localization at the leading edge of migration. Moreover, the small GTPase Rab5c turns out to be a key component of the molecular machinery that is able to drive CD93 recycling to the EC surface. Finally, in the Rab5c endosomal compartment CD93 forms a complex with Multimerin-2 and active β1 integrin, which is recycled back to the basolaterally-polarized cell surface by clathrin-independent endocytosis., Conclusions: Our findings, focusing on the pro-angiogenic receptor CD93, unveil the mechanisms of its polarized trafficking during EC adhesion and migration, opening novel therapeutic opportunities for angiogenic diseases.

The glycoprotein CD93 has recently been recognized to play an important role in the regulation of the angiogenic process. Moreover, CD93 is highly expressed in the endothelial cells of tumor blood vessel and faintly expressed in the non-proliferating endothelium. Much evidence suggests that CD93 mediates adhesion in the endothelium. Here we identify Multimerin 2 (MMRN2), a pan-endothelial extracellular matrix protein, as a specific ligand for CD93. We found that CD93 and MMRN2 are co-expressed in the blood vessels of various human tumors. Moreover, disruption of the CD93-MMRN2 interaction reduced endothelial cell adhesion and migration, making the interaction of CD93 with MMRN2 an ideal target to block pathological angiogenesis. Model structures and docking studies served to envisage the region of CD93 and MMRN2 involved in the interaction. Site-directed mutagenesis identified different residue hotspots either directly or indirectly involved in the binding. We propose a molecular model in which the coiled-coil domain of MMRN2 is engaged by F238 of CD93. Altogether, these studies identify the key interaction surfaces of the CD93-MMRN2 complex and provide a framework for exploring how to inhibit angiogenesis by hindering the CD93-MMRN2 interaction., (Copyright © 2017 Elsevier B.V. All rights reserved.)

In patients with age-related macular degeneration (AMD), choroidal neovascularization is the major cause of severe visual loss. In these patients, the persistence of neovascular growth despite vascular endothelial growth factor-A blockage needs the discovery of new endothelial cell targets. The glycoprotein CD93, highly expressed in activated endothelial cells, has been recently involved in the regulation of the angiogenic process both as transmembrane and soluble protein. Choroidal neovascular membranes from patients affected by AMD were examined by immunofluorescence using anti-CD93 and anti-von Willebrand factor antibodies. Blood vessels within intraocular and extraocular neoplasias were used as controls for CD93 expression. All choroidal neovascular membranes displayed strong CD93 staining in the von Willebrand factor-positive endothelial cells, consistently with the analyses showing a high colocalization coefficient in the blood vessels. Intraocular and extraocular tumor vessels showed similar results, whereas the normal choroid displayed blood vessels with only faint CD93 staining. Additionally, the concentration of soluble CD93 was determined in the aqueous humor of patients affected by naïve neovascular AMD by enzyme-linked immunosorbent assays. Age-matched cataract patients served as controls. Soluble CD93 was significantly increased in the aqueous humor of naïve neovascular AMD patients and tended to decrease after treatment with an antiangiogenic drug. In conclusion, both transmembrane and soluble CD93 are overexpressed in patients with neovascular AMD, indicating that CD93 may represent a potential new antiangiogenic target in the treatment of choroidal neovascularization. J. Cell. Physiol. 232: 1767-1773, 2017. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

CD93 is a transmembrane glycoprotein predominantly expressed in endothelial cells. Although CD93 displays proangiogenic activity, its molecular function in angiogenesis still needs to be clarified. To get molecular insight into the biological role of CD93 in the endothelium, we performed proteomic analyses to examine changes in the protein profile of endothelial cells after CD93 silencing. Among differentially expressed proteins, we identified dystroglycan, a laminin-binding protein involved in angiogenesis, whose expression is increased in vascular endothelial cells within malignant tumors. Using immunofluorescence, FRET, and proximity ligation analyses, we observed a close interaction between CD93 and β-dystroglycan. Moreover, silencing experiments showed that CD93 and dystroglycan promoted endothelial cell migration and organization into capillary-like structures. CD93 proved to be phosphorylated on tyrosine 628 and 644 following cell adhesion on laminin through dystroglycan. This phosphorylation was shown to be necessary for a proper endothelial migratory phenotype. Moreover, we showed that during cell spreading phosphorylated CD93 recruited the signaling protein Cbl, which in turn was phosphorylated on tyrosine 774. Altogether, our results identify a new signaling pathway which is activated by the cooperation between CD93 and dystroglycan and involved in the control of endothelial cell function.

The inhibition of tumor angiogenesis is one of the main challenges in cancer therapy. With the aim of developing monoclonal antibodies able to inhibit angiogenesis, we immunized mice with proliferating human umbilical vein endothelial cells. We generated a library of monoclonal antibodies able to recognize antigens expressed on endothelial cells and screened the antibodies for their ability to inhibit endothelial cell proliferation, migration, and sprouting in vitro. Here, we show that the antibody, designated as 4E1, is able to neutralize the formation of new vessels both in vitro and in vivo without affecting endothelial cell survival. By mass spectrometry we identified CD93 as the antigen bound by 4E1 and mapped the recognized epitope. CD93 is a transmembrane protein heavily glycosylated preferentially expressed in the vascular endothelium. CD93 silencing by lentiviral-mediated small hairpin RNA expression impairs human endothelial cell proliferation, migration, and sprouting. Altogether these findings reveal 4E1 as a novel antiangiogenic antibody and identify CD93 as a new target suitable for antiangiogenic therapy.