Obligatory participation of macrophages in an angiopoietin 2-mediated cell death switch

Abstract

Macrophages have a critical function in the recognition and engulfment of dead cells. In some settings, macrophages also actively signal programmed cell death. Here we show that during developmentally scheduled vascular regression, resident macrophages are an obligatory participant in a signaling switch that favors death over survival. This switch occurs when the signaling ligand angiopoietin 2 has the dual effect of suppressing survival signaling in vascular endothelial cells (VECs) and stimulating Wnt ligand production by macrophages. In response to the Wnt ligand, VECs enter the cell cycle and in the absence of survival signals, die from G1 phase of the cell cycle. We propose that this mechanism represents an adaptation to ensure that the macrophage and its disposal capability are on hand when cell death occurs.

Pericytes and Perivascular Fibroblasts Are the Primary Source of Collagen-Producing Cells in Obstructive Fibrosis of the Kidney

“…Recent studies in the eye suggest that pericytes and macrophages may cooperate to delete endothelial cells during scheduled developmental vascular regression. 23 In these studies pericyte release of angiopoietin-2 is necessary for withdrawal of a survival signal to the endothelial cell. Fibrotic injury has been associated with loss of capillaries in various settings following in some cases by aberrant revascularization.…”

Pro-apoptotic BIM is an essential initiator of physiological endothelial cell death independent of regulation by FOXO3

“…Using Bim lacZ/ þ mice, 32 we found that in addition to endothelium, Bim was expressed in pericytes and ocular macrophages-cell types with active roles in hyaloid regression (Figure 2b). 30 Quantification of hyaloid vessels at P8 showed that Bim À / À pups contained significantly more hyaloid vessels than control littermates, demonstrating that BIM was indeed required for hyaloid vessel regression (Figure 3a). BIM deficiency afforded less protection than that reported in the absence of Ang2, WNT7b or ocular macrophages however, 30,31 and examples of apoptotic vessels were still observed in BIM-deficient mice (n ¼ 5) (Figure 3b).…”

“…30 Quantification of hyaloid vessels at P8 showed that Bim À / À pups contained significantly more hyaloid vessels than control littermates, demonstrating that BIM was indeed required for hyaloid vessel regression (Figure 3a). BIM deficiency afforded less protection than that reported in the absence of Ang2, WNT7b or ocular macrophages however, 30,31 and examples of apoptotic vessels were still observed in BIM-deficient mice (n ¼ 5) (Figure 3b). Nonetheless, loss of BIM provided longterm protection from regression as those vessels that failed to regress during the neonatal period persisted into adulthood ( Figure 3c).…”

“…Both these cell types influence endothelial cell viability, and depending on the context may act to promote 30,31,35 or suppress apoptosis 40,41 This reinforces the need to address the extent to which BIM acts cell-autonomously to promote endothelial cell apoptosis, independently of effects on other cell types that influence the vasculature. As BIM is essential for apoptosis in many hematopoietic cell types 42 and many endothelial cell-specific Cre lines are highly active in hematopoietic cells, 43,44 we established an in vitro viability assay for primary mouse endothelial cells to formally assess the cell-intrinsic role of BIM in endothelial cell death in the absence of other cell types.…”

“…Hyaloid vessel regression requires the intrinsic apoptosis pathway as it is prevented in the combined absence of BAK and BAX. 16 Hyaloid endothelial apoptosis is initiated by pericyte-derived Ang2, which blocks Ang1-dependent activation of Akt 30 and requires macrophage-derived WNT7b. 30,31 BIM has been implicated in hyaloid vessel regression, 17 but its expression, the extent of its requirement, and its regulation in this system have not been determined.…”