Hif-1α regulates macrophage-endothelial interactions during blood vessel development in zebrafish

Abstract

Macrophages are known to interact with endothelial cells during developmental and pathological angiogenesis but the molecular mechanisms modulating these interactions remain unclear. Here, we show a role for the Hif-1α transcription factor in this cellular communication. We generated hif-1aa;hif-1ab double mutants in zebrafish, hereafter referred to as hif-1α mutants, and find that they exhibit impaired macrophage mobilization from the aorta-gonad-mesonephros (AGM) region as well as angiogenic defects and defective vascular repair. Importantly, macrophage ablation is sufficient to recapitulate the vascular phenotypes observed in hif-1α mutants, revealing for the first time a macrophage-dependent angiogenic process during development. Further substantiating our observations of vascular repair, we find that most macrophages closely associated with ruptured blood vessels are Tnfα-positive, a key feature of classically activated macrophages. Altogether, our data provide genetic evidence that Hif-1α regulates interactions between macrophages and endothelial cells starting with the mobilization of macrophages from the AGM.

Live imaging of wound angiogenesis reveals macrophage orchestrated vessel sprouting and regression

“…Cancer growth and progression towards malignancy in mouse and man is generally associated with an increased local vasculature that is also heavily influenced by macrophage presence and phenotypic state (Lin et al , ; Williams et al , ), with some studies indicating a direct physical interaction between pro‐angiogenic macrophages and cancer blood vessels (De Palma et al , ; Ojalvo et al , ). These direct, physical macrophage–vessel interactions have also been reported in other contexts, such as vessel damage observed in zebrafish models of stroke (Liu et al , ) and hypoxic stress (Gerri et al , ). However, macrophages are remarkably heterogeneous, and there is controversy as to whether pro‐ or anti‐inflammatory phenotypes of these cells are responsible for the observed increases in vasculature (Squadrito & De Palma, ; Gerri et al , ).…”

“…These direct, physical macrophage–vessel interactions have also been reported in other contexts, such as vessel damage observed in zebrafish models of stroke (Liu et al , ) and hypoxic stress (Gerri et al , ). However, macrophages are remarkably heterogeneous, and there is controversy as to whether pro‐ or anti‐inflammatory phenotypes of these cells are responsible for the observed increases in vasculature (Squadrito & De Palma, ; Gerri et al , ). A likely key role for macrophages in wound neoangiogenesis is strongly indicated by their ablation at early timepoints following murine wounding, which results in reduced vessel sprouting and aberrant wound angiogenesis (Lucas et al , ; Willenborg et al , ).…”

“…A recent study in which zebrafish vessels were damaged by hypoxia also reports that the responding macrophages are largely tnfα positive (Gerri et al , ). Our live‐imaging studies in fish combined with in vitro studies go beyond this and demonstrate that this inflammatory phenotype is needed for sprouting in wounds and that the pro‐inflammatory macrophage‐mediated sprouting is facilitated by vegf .…”

Specialized functions of resident macrophages in brain and heart

“…Indeed, PU.1‐ or CSF1‐mutant mice that lack microglia displayed reduced complexity of the brain vascular network early in development . This has been additionally shown to be dependent on hypoxia‐induced factor 1 (HIF‐1α) signalling during vessel injury because HIF‐α‐deficient macrophages are not able to assist repair of blood vessels . An elegant study of vascular injury in zebrafish visualized a process by which macrophages mediated repair of cerebrovascular rupture through direct physical interaction with endothelial cells.…”

“…124 This has been additionally shown to be dependent on hypoxia-induced factor 1 (HIF-1 ) signalling during vessel injury because HIF--deficient macrophages are not able to assist repair of blood vessels. 125 An elegant study of vascular injury in zebrafish visualized a process by which macrophages mediated repair of cerebrovascular rupture through direct physical interaction with endothelial cells. Brain macrophages were proposed to generate mechanical traction forces that brought together endothelial ends and facilitated their ligation, because depolymerization of microfilaments and inhibition of phosphatidylinositide 3-kinase or Rac1 activity in macrophages impaired cerebrovascular repair.…”