Heparan sulfate in the inner limiting membrane of embryonic chicken retinabinds basic fibroblast growth factor to promote axonal outgrowth

During neural development retinal ganglion cell axons migrate over the reti nal basal lamina (inner limiting membrane, ILM) in directed growth toward t he optic nerve. We found that both growth rate and distribution density of the ganglion cell axons on isolated cell-free ILM was greatly inhibited by pretreatment with heparitinase but not with chondroitinase ABC. The persist ence of radioactively labeled proteoglycans added to the culture medium eli minated residual heparitinase as an explanation for the inhibition. A cell binding assay showed that heparitinase acted on the ILM to influence axonal behavior without apparent inhibition of cell adhesion. These results indic ated that the neurite outgrowth promoting activity of the ILM depended on t he heparan sulfate (:HS) side chains of its proteoglycans. Basic fibroblast growth factor (bFGF) stimulated additional neuronal sprouting and neurite elongation on the ILM. This neurotropic activity of bFGF was inhibited by h eparitinase pretreatment of the ILM, suggesting that bFGF bound to HS on th e ILM. The activity of bFGF was enhanced by exogenous heparin added to the culture medium;; although heparin alone failed to stimulate either neurite extension or neuronal cell sprouting. These results demonstrate that HS in the ILM possesses neurotropic activity for axons of the ganglion cells by b inding bFGF for presentation to cell-surface receptors and may, therefore, play a significant role in stimulating axonal outgrowth during development. (C) 1999 Academic Press.