Astrocytes follow ganglion cell axons to establish an angiogenic template during retinal development

Abstract

Immature astrocytes and blood vessels enter the developing mammalian retina at the optic nerve head, and migrate peripherally to colonize the entire retinal nerve fiber layer (RNFL). Retinal vascularization is arrested in retinopathy of prematurity (ROP), a major cause of bilateral blindness in children. Despite their importance in normal development and ROP, the factors that control vascularization of the retina remain poorly understood. Because astrocytes form a reticular network that appears to provide a substrate for migrating endothelial cells, they have long been proposed to guide angiogenesis. However, whether astrocytes do in fact impose a spatial pattern on developing vessels remains unclear, and how astrocytes themselves are guided is unknown. Here we explore the cellular mechanisms that ensure complete retinal coverage by astrocytes and blood vessels in mouse. We find that migrating astrocytes associate closely with the axons of retinal ganglion cells (RGCs), their neighbors in the RNFL. Analysis of Robo1; Robo2 mutants, in which RGC axon guidance is disrupted, and Math5 (Atoh7) mutants, which lack RGCs, reveals that RGCs provide directional information to migrating astrocytes that sets them on a centrifugal trajectory. Without this guidance, astrocytes exhibit polarization defects, fail to colonize the peripheral retina, and display abnormal fine-scale spatial patterning. Further, using cell type-specific chemical-genetic tools to selectively ablate astrocytes, we show that the astrocyte template is required for angiogenesis and vessel patterning. Our results are consistent with a model whereby RGC axons guide formation of an astrocytic network that subsequently directs vessel development.

Sox2 regulates astrocytic and vascular development in the retina

“…Immature astrocytes expressing Sox2 migrate into the retina from the ONH shortly before the day of birth, by responding to environmental cues such as platelet-derived growth factor A-chain (PDGF-A) (Fruttiger et al, 1996), laminins β2 and γ3 (Gnanaguru et al, 2013), and the axons of retinal ganglion cells (Distler, Dreher, & Stone, 1991; O’Sullivan et al, 2017). These immature astrocytes, visualized by expression of Pax2, migrate radially across the retinal surface, reaching the peripheral edge of the retina by postnatal (P) day 5 (Chu, Hughes, & Chan-Ling, 2001; Chan-Ling, Chu, Baxter, Weible Li, & Hughes, 2009; Ling, Mitrofanis, & Stone, 1989; Sandercoe, Madigan, Billson, Penfold, & Provis, 1999; Stone & Dreher, 1987; Watanabe & Raff, 1988).…”

“…Within the retina, the stellate morphology of the astrocytes forms a web-like network of overlapping processes that use gap-junctions to propagate intracellular signals and establish metabolic homeostasis (Hollander et al, 1991; Ramirez, Trivino, Ramirez, Salazar, & Garcia-Sanchez, 1996). Astrocytes also play an important role in the vascularization of the retina (Dorrell, Aguilar, & Friedlander, 2002; Dorrell et al, 2010; O’Sullivan et al, 2017), providing a scaffold for blood vessels to follow as they migrate from the optic nerve head (ONH) across the inner surface of the retina via the secretion of growth factors such as vascular endothelial growth factor (VEGF).…”

Retinal blood vessel‐origin yes‐associated protein (YAP) governs astrocytic maturation via leukaemia inhibitory factor (LIF)

“…Astrocytes and blood vessels play important role not only in retinal development but also in retinopathy. Since astrocytes form a reticular network that appears to provide a substrate for EC migration, they have long been proposed to guide angiogenesis . Despite their important relationship in normal development, the factors that control vascularization and astrocytogenesis of the retina remain underdetermined.…”

Fat1 regulates astrocyte maturation and angiogenesis in the retina

“…Thus, retinal astrocytes represent a novel model of planar and polarized migration, the modulation of which is functionally linked with the 498 dynamics of angiogenesis. This preferential direction is instructed, at least in part, by radially 499 oriented RGC axons (O'Sullivan et al, 2017). The polarization is best seen at the migration front, 500…”