No abstract

The amyloid precursor protein (APP) of Alzheimer's disease has been shown to stimulate neurite outgrowth in vitro. The effect of APP on neurite outgrowth can be enhanced if APP is presented to neurons in substratebound form, in the presence of heparan sulfate proteoglycans. To identify specific heparan sulfate proteoglycans that bind to APP, conditioned medium from neonatal mouse brain cells was subjected to affinity chromatography with recombinant APP 695 as a ligand. Glypican bound strongly to the APP affinity column. Purified glypican bound to APP with an equilibrium dissociation constant of 2.8 nM and inhibited APP-induced neurite outgrowth from chick sympathetic neurons. The effect of glypican was specific for APP, as glypican did not inhibit laminin-induced neurite outgrowth. Furthermore, treatment of cultures with 4-methylumbelliferyl-␤-D-xyloside, a competitive inhibitor of proteoglycan glycanation, inhibited APP-induced neurite outgrowth but did not inhibit laminin-induced neurite outgrowth. This result suggests that endogenous proteoglycans are required for substrate-bound APP to stimulate neurite outgrowth. Secreted glypican may act to inhibit APP-induced neurite outgrowth in vivo by competing with endogenous proteoglycans for binding to APP.Alzheimer's disease is a progressive dementia that is characterized by neuronal degeneration, synaptic loss, and the deposition of amyloid fibrils in the brain. The major constituent of the amyloid is the A␤ protein (1, 2), which is derived from the amyloid precursor protein (APP).

reported. Binding of HSPGs to an N-terminal heparin-binding domain (HBD-1) stimulates the effect of substrate-bound APP on neurite outgrowth. In the mature nervous system, APP may play an important role in the regulation of wound repair. It is highly likely that studies on the normal functions of APP will shed further light on aspects of the pathogenesis of AD.

No abstract

The binding of the amyloid protein precursor (APP) to heparan sulfate proteoglycans has been shown to stimulate the neurite‐promoting activity of APP. In this study, proteoglycans that bind with high affinity to APP were characterized. Conditioned medium from cultures of postnatal day 3 mouse brain cells was applied to an affinity column containing a peptide homologous to a heparin‐binding domain of APP. A fraction 17‐fold enriched in proteoglycans was recovered by elution with a salt gradient. APP bound saturably and with high affinity to the affinity‐purified proteoglycan fraction. Scatchard analysis of the binding showed that APP bound to high‐ and low‐affinity sites with equilibrium dissociation constants of 1.4 × 10−11 and 6.5 × 10−10M, respectively. APP, in conjunction with the affinity‐purified proteoglycan fraction, promoted neurite outgrowth. The affinity‐purified proteoglycan fraction contained a heparan sulfate proteoglycan and a chondroitin sulfate proteoglycan. Digestion of the affinity‐purified fraction with heparitinase I revealed a core protein of 63–69‐kDa molecular mass, whereas digestion with chondroitinase ABC revealed a core protein of 100–110 kDa. The results suggest that expression of specific APP‐binding proteoglycans may be an important step in the regulation of the neurite outgrowth‐promoting activity of APP.