The role of receptor MAS in microglia-driven retinal vascular development.

Objective: The receptor MAS, encoded by Mas1, is expressed in microglia and its activation has been linked to anti-inflammatory actions. However, microglia are involved in several different processes in the central nervous system, including the promotion of angiogenesis. We therefore hypothesized that the receptor MAS also plays a role in angiogenesis via microglia.

Approach And Results: To assess the role of MAS on vascular network development, flat-mounted retinas from 3-day-old wild-type (WT) and Mas1 mice were subjected to Isolectin B4 staining. The progression of the vascular front was reduced (- 24%, p < 0.0001) and vascular density decreased (- 38%, p < 0.001) in Mas1 compared to WT mice with no change in the junction density. The number of filopodia and filopodia bursts were decreased in Mas1 mice at the vascular front (- 21%, p < 0.05; - 29%, p < 0.0001, respectively). This was associated with a decreased number of vascular loops and decreased microglial density at the vascular front in Mas1 mice (-32%, p < 0.001; - 26%, p < 0.05, respectively). As the front of the developing vasculature is characterized by reduced oxygen levels, we determined the expression of Mas1 following hypoxia in primary microglia from 3-day-old WT mice. Hypoxia induced a 14-fold increase of Mas1 mRNA expression (p < 0.01). Moreover, stimulation of primary microglia with a MAS agonist induced expression of Notch1 (+ 57%, p < 0.05), Dll4 (+ 220%, p  < 0.001) and Jag1 (+ 137%, p < 0.001), genes previously described to mediate microglia/endothelial cell interaction during angiogenesis.

Conclusions: Our study demonstrates that the activation of MAS is important for microglia recruitment and vascular growth in the developing retina.