Cell surface proteoglycans control astrocyte migration and retinal angiogenesis by regulating basement membrane assembly

Abstract

Elaborate vascularization of the retina is crucial for the development and functioning of the eye. The proper patterning of astrocytes is a key event preceding retinal angiogenesis by providing guidance cues for endothelial cells, yet how this is regulated still remains obscure. The dual function of proteoglycans in both extracellular matrix (ECM) composition and cell signal transduction suggests their potential in the regulation of astrocyte migration. The current study demonstrated that non-cell-autonomous regulation by neuroretina cell surface proteoglycan is crucial for PDGF-A regulated astrocyte migration. Ablation of glycosaminoglycan side chains of proteoglycans in neuroretina led to impaired astrocyte migration, incomplete retinal angiogenesis, and hyaloid vessel persistence. This is followed by severe photoreceptor degeneration as a result of reactive gliosis, which cannot be rescued by constitutively activated Kras signaling. Notably, inner limiting membrane (ILM), the basement membrane of the retina, was breached in proteoglycan-deficient retinae prior to the formation of astrocytic network. Herein we propose that cell surface proteoglycans are essential for the initial assembly of ILM, and this cannot be compensated by secreted ECM proteoglycans. In support of this, after removal of ILM in retinal explant by Collagenase digestion, establishment of a new ILM can be achieved by incubation with exogenous laminin-supplemented Matrigel. This basement membrane reconstitution failed, however, in proteoglycan-deficient retinae or in wild type samples digested with a combination of Heparinase and ChABC in addition to Collagenase. Taken together, our study reveals a novel function of neuroretinal cell surface proteoglycans in the initial assembly of basement membrane which subsequently serves as a permissive substratum necessary for astrocyte migration.