AFFINITY PURIFICATION OF PROTEOGLYCANS THAT BIND TO THE AMYLOID PROTEIN-PRECURSOR OF ALZHEIMERS-DISEASE

The binding of the amyloid protein precursor (APP) to heparan sulfate proteoglycans has been shown to stimulate the neurite-promoting activi ty of APP. In this study, proteoglycans that bind with high affinity t o APP were characterized. Conditioned medium from cultures of postnata l day 3 mouse brain cells was applied to an affinity column containing a peptide homologous to a heparin-binding domain of APP. A fraction 1 7-fold enriched in proteoglycans was recovered by elution with a salt gradient. APP bound saturably and with high affinity to the affinity-p urified proteoglycan fraction. Scatchard analysis of the binding showe d that APP bound to high- and low-affinity sites with equilibrium diss ociation constants of 1.4 x 10-(11) and 6.5 x 10(-10) M, respectively, APP, in conjunction with the affinity-purified proteoglycan fraction, promoted neurite outgrowth. The affinity-purified proteoglycan fracti on contained a heparan sulfate proteoglycan and a chondroitin sulfate proteoglycan. Digestion of the affinity-purified fraction with heparit inase I revealed a core protein of 63-69-kDa molecular mass, whereas d igestion with chondroitinase ABC revealed a core protein of 100-110 kD a. The results suggest that expression of specific APP-binding proteog lycans may be an important step in the regulation of the neurite outgr owth-promoting activity of APP.