Tissue macrophages act as cellular chaperones for vascular anastomosis downstream of VEGF-mediated endothelial tip cell induction

Abstract

Blood vessel networks expand in a 2-step process that begins with vessel sprouting and is followed by vessel anastomosis. Vessel sprouting is induced by chemotactic gradients of the vascular endothelial growth factor (VEGF), which stimulates tip cell protrusion. Yet it is not known which factors promote the fusion of neighboring tip cells to add new circuits to the existing vessel network. By combining the analysis of mouse mutants defective in macrophage development or VEGF signaling with live imaging in zebrafish, we now show that macrophages promote tip cell fusion downstream of VEGF-mediated tip cell induction. Macrophages therefore play a hitherto unidentified and unexpected role as vascular fusion cells. Moreover, we show that there are striking molecular similarities between the pro-angiogenic tissue macrophages essential for vascular development and those that promote the angiogenic switch in cancer, including the expression of the cell-surface proteins TIE2 and NRP1. Our findings suggest that tissue macrophages are a target for antiangiogenic therapies, but that they could equally well be exploited to stimulate tissue vascularization in ischemic disease. (Blood. 2010;116(5): 829-840) IntroductionBlood vessels are essential for tissue homeostasis in all vertebrates, and new vessel growth, termed neo-angiogenesis, is therefore a critical process in wound repair to counter tissue ischemia. Undesirably, neo-angiogenesis also promotes the expansion of tumors. Moreover, nonproductive neo-angiogenesis, which fails to restore oxygenation of ischemic tissues, promotes disease progression in, for example, diabetic retinopathy. Much current research is therefore focused on the identification of molecular and cellular targets for either pro-or antiangiogenic therapies. We previously elucidated the mechanism by which alternative splice forms of the vascular endothelial growth factor (VEGF) cooperate to promote blood vessel growth. 1,2 This work led to the current model of angiogenesis, in which blood vessel endothelium specializes into tip and stalk cells to promote vascular network expansion by sprouting growth. While the stalk cells form a lumen to transport blood, the tip cells extend filopodia to detect chemotactic growth factor gradients, which are formed by a combination of VEGF isoforms with a differential affinity for the extracellular matrix. Cooperating with VEGF, notch-delta signaling controls the balance of tip versus stalk cell specialization. 3 Even though much progress has been made in elucidating the mechanism of vascular sprout induction and guidance, a fundamental yet unanswered problem is which mechanism promotes the fusion of nascent vessel sprouts to add new circuits to the existing plexus.Macrophages promote pathologic angiogenesis in several diseases. Thus, circulating bone marrow-derived cells differentiate into proangiogenic cells with macrophage characteristics at adult sites of VEGF expression 4 and are recruited to growing tumors to promote tumor vascularization and therefore progression....

Up‐regulated basigin‐2 in microglia induced by hypoxia promotes retinal angiogenesis

“…Fantin's study elaborately revealed that microglia was in contact with neighbouring endothelial cells and concluded microglia bridging vascular sprouts 21. We also notice microglia BV2 cells residing at the branch point of the tubular‐like structures in vitro and in close proximity to endothelial tip cells of the growing vascular tree in vivo .…”

Macrophage Wnt-Calcineurin-Flt1 signaling regulates mouse wound angiogenesis and repair

“…Such factors can affect blood vessel development by modifying the extracellular matrix, 15 affecting the growth factor milieu, 16,17 and guiding vessel anastomosis. 18 During wound repair, mice deficient in macrophages have abnormally overgrown vasculature that surprisingly results in enhanced repair rate. 19 One of the mechanisms by which macrophages regulate vessel branching in development involves a noncanonical Wnt-Flt1 signaling pathway.…”

Yolk-sac–derived macrophages regulate fetal testis vascularization and morphogenesis

“…S5 E and F). All these results suggest that gonadal macrophages are M2-like, a state that is associated with vascular and tissue remodeling (11,36).…”

“…In contrast to postnatal contexts and disease states, there are few documented examples of the role of macrophages in fetal organogenesis. In the fetal brain, macrophages are involved in anastomosis during vascular remodeling, because they associate with endothelial tip cells to promote the fusion of adjacent vascular branches (36). One previous study reported that latestage E18.5 colony stimulating factor 1 (Csf1)-deficient fetuses, which showed reduced numbers of macrophages, had decreased insulin cell mass and abnormal pancreatic islet morphogenesis (12).…”

“…Testis macrophages expressed a number of endothelial markers including Neuropilin 1 (NRP1), which is a vascular endothelial growth factor (VEGF) coreceptor that normally is expressed in endothelial cells but also has been implicated as a marker of macrophages involved in neovascularization of tumors (36)(37)(38) (Fig. S6A); endothelial-specific receptor tyrosine kinase (TEK; also called "TIE2") (Fig.…”