The sphingosine-1-phosphate receptor S1PR1 restricts sprouting angiogenesis by regulating the interplay between VE-cadherin and VEGFR2.

Angiogenesis, the process by which new blood vessels arise from preexisting ones, is critical for embryonic development and is an integral part of many disease processes. Recent studies have provided detailed information on how angiogenic sprouts initiate, elongate, and branch, but less is known about how these processes cease. Here, we show that S1PR1, a receptor for the blood-borne bioactive lipid sphingosine-1-phosphate (S1P), is critical for inhibition of angiogenesis and acquisition of vascular stability. Loss of S1PR1 leads to increased endothelial cell sprouting and the formation of ectopic vessel branches. Conversely, S1PR1 signaling inhibits angiogenic sprouting and enhances cell-to-cell adhesion. This correlates with inhibition of vascular endothelial growth factor-A (VEGF-A)-induced signaling and stabilization of vascular endothelial (VE)-cadherin localization at endothelial junctions. Our data suggest that S1PR1 signaling acts as a vascular-intrinsic stabilization mechanism, protecting developing blood vessels against aberrant angiogenic responses.

A two-way communication between microglial cells and angiogenic sprouts regulates angiogenesis in aortic ring cultures.

BACKGROUND: Myeloid cells have been associated with physiological and pathological angiogenesis, but their exact functions in these processes remain poorly defined. Monocyte-derived tissue macrophages of the CNS, or microglial cells, invade the mammalian retina before it becomes vascularized. Recent studies correlate the presence of microglia in the developing CNS with vascular network formation, but it is not clear whether the effect is directly caused by microglia and their contact with the endothelium. METHODOLOGY/PRINCIPAL FINDINGS: We combined in vivo studies of the developing mouse retina with in vitro studies using the aortic ring model to address the role of microglia in developmental angiogenesis. Our in vivo analyses are consistent with previous findings that microglia are present at sites of endothelial tip-cell anastomosis, and genetic ablation of microglia caused a sparser vascular network associated with reduced number of filopodia-bearing sprouts. Addition of microglia in the aortic ring model was sufficient to stimulate vessel sprouting. The effect was independent of physical contact between microglia and endothelial cells, and could be partly mimicked using microglial cell-conditioned medium. Addition of VEGF-A promoted angiogenic sprouts of different morphology in comparison with the microglial cells, and inhibition of VEGF-A did not affect the microglia-induced angiogenic response, arguing that the proangiogenic factor(s) released by microglia is distinct from VEGF-A. Finally, microglia exhibited oriented migration towards the vessels in the aortic ring cultures. CONCLUSIONS/SIGNIFICANCE: Microglia stimulate vessel sprouting in the aortic ring cultures via a soluble microglial-derived product(s), rather than direct contact with endothelial cells. The observed migration of microglia towards the growing sprouts suggests that their position near endothelial tip-cells could result from attractive cues secreted by the vessels. Our data reveals a two-way communication between microglia and vessels that depends on soluble factors and should extend the understanding of how microglia promote vascular network formation.

Bone marrow transplantation stimulates pancreatic ß-cell replication after tissue damage.

Bone marrow transplantation has been shown to normalize hyperglycemia but the mechanisms underlying pancreatic ß-cell regeneration remain elusive. Here, we investigate the capacity of transplanted bone marrow cells to engraft into the pancreas, to adopt an endothelial cell phenotype and to stimulate ß-cell regeneration after islet damage. Genetically marked whole bone marrow from Tie2-Cre/ZEG mice was transplanted into lethally irradiated wild-type mice. The fate of the transplanted cells, as well as blood glucose levels and ß-cell mass dynamics, was investigated in normal and hyperglycemic recipient mice. Bone marrow transplantation significantly increased ß-cell mass and reduced the hyperglycemia of mice subjected to ß-cell damage by streptozotocin (STZ). This was associated with enhanced replication of pre-existing ß-cells, proportional to the degree of ß-cell damage, whereas no evidence was obtained for islet neogenesis. The engrafted bone marrow-derived cells in the pancreas showed little capacity to differentiate into blood vessel endothelium but retained a myeloid cell fate. By contrast, the transplantation evoked pronounced proliferation of recipient endothelial cells. These findings illuminate an important adjuvant function of transplanted bone marrow cells in both angiogenesis and ß-cell regeneration. This may have interesting clinical implications, not least for human islet transplantation endeavours, where co-transplantation of islets with bone marrow cells might represent a simple means to improve islet survival and function.