Mara E. Pitulescu

Bio: Mara E. Pitulescu is an academic researcher from University of Münster. The author has contributed to research in topics: Angiogenesis & Endothelial stem cell. The author has an hindex of 17, co-authored 19 publications receiving 3100 citations. Previous affiliations of Mara E. Pitulescu include Max Planck Society.

Ephrin-B2 controls VEGF-induced angiogenesis and lymphangiogenesis

Abstract: In development, tissue regeneration or certain diseases, angiogenic growth leads to the expansion of blood vessels and the lymphatic vasculature. This involves endothelial cell proliferation as well as angiogenic sprouting, in which a subset of cells, termed tip cells, acquires motile, invasive behaviour and extends filopodial protrusions. Although it is already appreciated that angiogenesis is triggered by tissue-derived signals, such as vascular endothelial growth factor (VEGF) family growth factors, the resulting signalling processes in endothelial cells are only partly understood. Here we show with genetic experiments in mouse and zebrafish that ephrin-B2, a transmembrane ligand for Eph receptor tyrosine kinases, promotes sprouting behaviour and motility in the angiogenic endothelium. We link this pro-angiogenic function to a crucial role of ephrin-B2 in the VEGF signalling pathway, which we have studied in detail for VEGFR3, the receptor for VEGF-C. In the absence of ephrin-B2, the internalization of VEGFR3 in cultured cells and mutant mice is defective, which compromises downstream signal transduction by the small GTPase Rac1, Akt and the mitogen-activated protein kinase Erk. Our results show that full VEGFR3 signalling is coupled to receptor internalization. Ephrin-B2 is a key regulator of this process and thereby controls angiogenic and lymphangiogenic growth.

Ephrin-B2 regulates VEGFR2 function in developmental and tumour angiogenesis

Abstract: The formation and guidance of specialized endothelial tip cells is essential for both developmental and pathological angiogenesis. Notch-1 signalling regulates the generation of tip cells, which respond to gradients of vascular endothelial growth factor (VEGF-A). The molecular cues and signalling pathways that control the guidance of tip cells are poorly understood. Bidirectional signalling by Eph receptors and ephrin ligands represents one of the most important guidance cues involved in axon path finding. Here we show that ephrin-B2 reverse signalling involving PDZ interactions regulates endothelial tip cell guidance to control angiogenic sprouting and branching in physiological and pathological angiogenesis. In vivo, ephrin-B2 PDZ-signalling-deficient mice (ephrin-B2DeltaV) exhibit a reduced number of tip cells with fewer filopodial extensions at the vascular front in the mouse retina. In pathological settings, impaired PDZ signalling decreases tumour vascularization and growth. Mechanistically, we show that ephrin-B2 controls VEGF receptor (VEGFR)-2 internalization and signalling. Importantly, internalization of VEGFR2 is necessary for activation and downstream signalling of the receptor and is required for VEGF-induced tip cell filopodial extension. Together, our results suggest that ephrin-B2 at the tip cell filopodia regulates the proper spatial activation of VEGFR2 endocytosis and signalling to direct filopodial extension. Blocking ephrin-B2 reverse signalling may be an attractive alternative or combinatorial anti-angiogenic therapy strategy to disrupt VEGFR2 function in tumour angiogenesis.

Inducible gene targeting in the neonatal vasculature and analysis of retinal angiogenesis in mice

Abstract: The retina is a powerful experimental system for the analysis of angiogenic blood vessel growth in the postnatal organisms. The three-dimensional architecture of the vessel network and processes as diverse as endothelial cell (EC) proliferation, sprouting, perivascular cell recruitment, vessel remodeling or maturation can be investigated at high resolution. The characterization of physiological and pathological angiogenic processes in mice has been greatly facilitated by inducible and cell type-specific loss-of-function and gain-of-function genetics. In this paper, we provide a detailed protocol for tamoxifen-inducible gene deletion in neonatal mice, as well as for retina dissection, whole-mount immunostaining and the quantitation of EC sprouting and proliferation. These methods have been optimized by our laboratory and yield reliable results. The entire protocol takes approximately 10 d to complete.

Eph/ephrin molecules—a hub for signaling and endocytosis

Abstract: The development, homeostasis, and regeneration of complex organ systems require extensive cell–cell communication to ensure that different cells proliferate, migrate, differentiate, assemble, and function in a coordinated and timely fashion. Eph receptor tyrosine kinases and their ephrin ligands are critical regulators of cell contact-dependent signaling and patterning. Eph/ephrin binding can lead to very diverse biological readouts such as adhesion versus repulsion, or increased versus decreased motility. Accordingly, depending on cell type and context, a limited and conserved set of receptor–ligand interactions is translated into a large variety of downstream signaling processes. Recent evidence indicates that the endocytosis of Eph/ephrin molecules, together with the internalization of various associated tissue-specific effectors, might be one of the key principles responsible for such highly diverse and adaptable biological roles. Here, we summarize recent insights into Eph/ephrin signaling and endocytosis in three biological systems; i.e., the brain, intestine, and vasculature.

Direct cell–cell contact with the vascular niche maintains quiescent neural stem cells

Abstract: The vasculature is a prominent component of the subventricular zone neural stem cell niche. Although quiescent neural stem cells physically contact blood vessels at specialized endfeet, the significance of this interaction is not understood. In contrast, it is well established that vasculature-secreted soluble factors promote lineage progression of committed progenitors. Here we specifically investigated the role of cell-cell contact-dependent signalling in the vascular niche. Unexpectedly, we find that direct cell-cell interactions with endothelial cells enforce quiescence and promote stem cell identity. Mechanistically, endothelial ephrinB2 and Jagged1 mediate these effects by suppressing cell-cycle entry downstream of mitogens and inducing stemness genes to jointly inhibit differentiation. In vivo, endothelial-specific ablation of either of the genes which encode these proteins, Efnb2 and Jag1 respectively, aberrantly activates quiescent stem cells, resulting in depletion. Thus, we identify the vasculature as a critical niche compartment for stem cell maintenance, furthering our understanding of how anchorage to the niche maintains stem cells within a pro-differentiative microenvironment.

Molecular mechanisms and clinical applications of angiogenesis

Abstract: Blood vessels deliver oxygen and nutrients to every part of the body, but also nourish diseases such as cancer. Over the past decade, our understanding of the molecular mechanisms of angiogenesis (blood vessel growth) has increased at an explosive rate and has led to the approval of anti-angiogenic drugs for cancer and eye diseases. So far, hundreds of thousands of patients have benefited from blockers of the angiogenic protein vascular endothelial growth factor, but limited efficacy and resistance remain outstanding problems. Recent preclinical and clinical studies have shown new molecular targets and principles, which may provide avenues for improving the therapeutic benefit from anti-angiogenic strategies.

Tumor angiogenesis: molecular pathways and therapeutic targets

Abstract: As angiogenesis is essential for tumor growth and metastasis, controlling tumor-associated angiogenesis is a promising tactic in limiting cancer progression. The tumor microenvironment comprises numerous signaling molecules and pathways that influence the angiogenic response. Understanding how these components functionally interact as angiogenic stimuli or as repressors and how mechanisms of resistance arise is required for the identification of new therapeutic strategies. Achieving a durable and efficient antiangiogenic response will require approaches to simultaneously or sequentially target multiple aspects of the tumor microenvironment.