Retinal Proteoglycans Act as Cellular Receptors for Basement Membrane Assembly to Control Astrocyte Migration and Angiogenesis

Abstract

SUMMARY The basement membrane is crucial for cell polarity, adhesion and motility, but how it is assembled on cell surface remains unclear. Here, we find that ablation of glycosaminoglycan (GAG) side chains of proteoglycans in the neuroretina disrupts the retinal basement membrane, leading to arrested astrocyte migration and reduced angiogenesis. Using genetic deletion and time-lapse imaging, we show that retinal astrocytes require neuronal-derived PDGF as a chemoattractive cue and the retinal basement membrane as a migratory substrate. Genetic ablation of heparan sulfates does not produce the same defects as GAG null mutants. In contrast, enzymatic removal of heparan sulfates and chondroitin sulfates together inhibits de novo laminin network assembly. These results indicate that both heparan and chondroitin sulfate proteoglycans participate in retinal basement membrane assembly, thus promoting astrocyte migration and angiogenesis.

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

“…Two recent studies demonstrate that different specific manipulations of astrocytes, conditional knockout of HIF‐2α (Duan et al, ) or PDGFRα (Tao & Zhang, ), disrupt vascular development. These reports provide the first direct experimental evidence that astrocytes can modulate developmental angiogenesis in a manner reminiscent of their role in pathological neovascularization (Dorrell et al, ; Weidemann et al, ).…”

“…Because the glial network is present first, and because it resembles the vascular plexus that will later form (Figure A), it has been proposed that astrocytes provide a template that guides angiogenesis. Several lines of evidence support this notion (Duan, Takeda, & Fong, ; Fruttiger et al, ; Gnanaguru et al, ; Uemura, Kusuhara, Wiegand, Yu, & Nishikawa, ), including a recent study demonstrating that astrocytes are critical for onset of retinal angiogenesis (Tao & Zhang, ). However, in this study, the complete removal of astrocytes led to a complete failure of vascular development, leaving open the question of how vessel pattern might develop in the absence of an astrocyte template.…”

“…Owing to the precision with which astrocytes conform to axons, we favor the hypothesis that axons provide either a contact cue, or a secreted cue that acts at short range, perhaps by becoming affixed to the ILM basement membrane directly beneath axon fascicles. An intriguing candidate is PDGF-A, which is expressed by RGCs (Mudhar, Pollock, Wang, Stiles, & Richardson, 1993) and stimulates proliferation of retinal astrocytes (Fruttiger et al, 1996;Fruttiger, Calver, & Richardson, 2000) but has recently been shown to act as an astrocyte chemoattractant in ex vivo assays (Tao & Zhang, 2016). It was proposed that PDGF-A could be an RGC-derived guidance cue for centrifugal migration (Tao & Zhang, 2016), but as RGCs are at higher density in central than peripheral retina (Rodriguez et al, 2014;Wang et al, 2016), PDGF-A may not be well positioned to attract astrocytes to the periphery.…”

“…Because the glial network is present first, and because it resembles the vascular plexus that will later form ( Figure 1A), it has been proposed that astrocytes provide a template that guides angiogenesis. Several lines of evidence support this notion (Duan, Takeda, & Fong, 2014;Fruttiger et al, 1996;Gnanaguru et al, 2013;Uemura, Kusuhara, Wiegand, Yu, & Nishikawa, 2006), including a recent study demonstrating that astrocytes are critical for onset of retinal angiogenesis (Tao & Zhang, 2016).…”

Retinal Ultrastructural and Microvascular Defects in Decorin Deficient (Dcn −/−) Mice

“…Furthermore, evidence shows that Dcn abrogates angiogenesis by direct inhibition of vascular endothelial receptor 2 (VEGFR2) [4]. In the retina, Dcn plays a key role as a neurotrophic factor during retinal differentiation [5], and localizes to nerve fiber layer and ganglion cell layer in the mature retina [6]. Upon retinal injury in mice, Dcn was shown to be highly upregulated in the inner retinal layers [7], while treatment with decorin was shown to improve vitrectomy outcome in rabbits with proliferative vitreoretinopathy [8], and prevent loss of tight junctions in human retinal pigmented epithelial cells under hyperglycemic and hypoxia stress [9].…”