Neuropilin-1-Expressing Microglia Are Associated With Nascent Retinal Vasculature Yet Dispensable For Developmental Angiogenesis

PURPOSE. Neuropilin-1 (NRP-1) is a transmembrane receptor that is critical for vascular development within the central nervous system (CNS). It binds and influences signaling of several key angiogenic factors, such as VEGF-165, semaphorin 3A, platelet derived growth factor, and more. Neuropilin-1 is expressed by neurons and endothelial cells as well as a subpopulation of proangiogenic macrophages/microglia that are thought to interact with endothelial tip cells to promote vascular anastomosis during brain vascularization. We previously demonstrated a significant role for NRP-1 in macrophage chemotaxis and showed that NRP-1-expressing microglia are major contributors to pathologic retinal angiogenesis. Given this influence on CNS angiogenesis, we now investigated the involvement of microglia-resident NRP-1 in developmental retinal vascularization.METHODS. We followed NRP-1 expressing microglia during retinal development. We used LysM-cre myeloid lineage-driver cre mice to reduce expression of NRP-1 in retinal myeloid-derived cells and performed a comprehensive morphometric analysis of retinal vasculature during development.RESULTS. We provide evidence that NRP-1_ microglia are present throughout the retina during vascular development with a preference for the non-vascularized retina. Using LysM-Cre/Nrp1(fl/fl) mice, we reduced NRP-1 expression by similar to 65% in retinal microglia and demonstrate that deficiency in NRP-1 in these microglia does not impair retinal angiogenesis.CONCLUSIONS. Our data draw a dichotomous role for NRP-1 in cells of myeloid lineage where it is dispensable for adequate retinal developmental vascularization yet obligate for pathologic retinal angiogenesis.