Characterization of a factor that promotes neurite outgrowth: evidence linking activity to a heparan sulfate proteoglycan

Rat sympathetic neurons, plated onto extracellular matrix produced by cultured bovine corneal endothelial cells, rapidly extended neurites in the absence of nerve growth factor (NGF). The response was unaffected by antiserum to NGF. Rapid outgrowth also occurred when sympathetic neurons were plated onto polylysine-coated surfaces that had been exposed to serum-free medium conditioned by corneal endothelial cells (CMSF). A response was seen even when the neurons were cultured without serum. When plated onto a polylysine-coated dish treated with CMSF over half its surface, only the neurons on the treated half extended neurites. The active factor in CMSF was destroyed by trypsin, acid (pH 1.6), base (pH 12.7), or heating to 80 degrees C; it was stable to heating to 60 degrees C, collagenase, deoxyribonuclease, and neuraminidase. The factor elutes just after the void volume of a Sepharose 6B column. In associative cesium chloride gradients, it sediments as a peak centered at a density of 1.36-1.37, corresponding to a peak of material that can be biosynthetically labeled with [35S]sulfate or [3H]leucine. Material from this fraction was inactivated by heparinase, but not chondroitinase ABC, implying that a heparin sulfate proteoglycan is essential for the factor's activity. Inactivation by contaminants in the heparinase preparation was ruled out. Further purification indicated that the active factor may exist as an aggregate containing a heparin sulfate proteoglycan and other molecules. CMSF also promoted neurite outgrowth by other types of neurons. Furthermore, a variety of cell types were shown to produce factors similar to that in CMSF.