Brief Report VASCULAR BIOLOGY Macrophage Wnt-Calcineurin-Flt1 signaling regulates mouse wound angiogenesis and repair

Recently a great deal of research has been devoted to understanding the underlying mechanisms behind wound repair. Many studies have carefully documented the cells involved in wound repair, their function, and the mediators by which they perform such functions. One process critical to wound repair is angiogenesis, the development of new blood vessels. One of the most potent proangiogenic molecules is vascular endothelial growth factor A (VEGF-A). Produced by a variety of cells including macrophages, VEGF-A can bind to Flk1 (VEGF-R2) on endothelial cells to induce migration and proliferation. Mice deficient in VEGF-A or Flk1 die early in development from a lack of blood vessel formation. Importantly, another VEGF-A receptor Flt1 (VEGFR1) binds VEGF-A, but has severely deficient signaling capacity. Therefore, Flt1, which can be spliced into a soluble Flt1 or membrane-tethered Flt1, can act as a suppressor of angiogenesis. Consistent with this, mice deficient in Flt1 die in utero of excessive angioblast proliferation. In uninjured skin, VEGF-A is expressed at basal levels, but in response to injury, VEGF-A levels rise. When VEGF-A is neutralized during wound repair, granulation tissue is diminished and wound repair is perturbed. Many of the known proangiogenic factors are products of cells that infiltrate during the inflammatory phase. Macrophages, for instance, are sources of both proangiogenic and antiangiogenic factors. Such factors can affect blood vessel development by modifying the extracellular matrix, affecting the growth factor milieu, and guiding vessel anastomosis. During wound repair, mice deficient in macrophages have abnormally overgrown vasculature that surprisingly results in enhanced repair rate. One of the mechanisms by which macrophages regulate vessel branching in development involves a noncanonical Wnt-Flt1 signaling pathway. Wnt signaling is one of the core pathways regulating developmental patterning. Wnt ligands are transported to the cell surface for secretion by Wntless/GPR177 (Wls). In canonical Wnt signaling, Wnt ligands (eg, Wnt3a) bind to a Frizzled-Lrp5 receptor complex that results in the stabilization and nuclear translocation of b-catenin. Noncanonical Wnt signaling (eg, Wnt5a) involves the many downstream Wnt signaling responses independent of b-catenin. One important noncanonical response is the enhancement of intracellular calcium, activation of calcineurin, and dephosporylation of nuclear factor of activated T cells (NFAT). Because macrophages are known to regulate developmental angiogenesis via noncanonical Wnt signaling, and because wound angiogenesis has been linked to macrophage presence,