p75 and TrkA receptor signaling independently regulate amyloid precursor protein mRNA expression, isoform composition, and protein secretion in PC12 cells

The pheochromocytoma PC12 cell line was used as a model system to characterize the role of the p75 neurotrophin receptor (p75(NTR)) and tyrosine kinase (Trk) A nerve growth factor (NGF) receptors on amyloid precursor protein (APP) expression and processing. NGF increased in a dose- dependent fashion neurite out-growth, APP mRNA expression, and APP secretion with maximal effects at concentrations known to saturate TrkA receptor binding. Displacement of NGF binding to p75(NTR) by addition of an excess of brain-derived neurotrophic factor abolished NGF's effects on neurite outgrowth and APP metabolism, whereas addition of brain-derived neurotrophic factor alone did not induce neurite outgrowth or affect APP mRNA or protein processing. However, treatment of PC12 cells with C2-ceramide, an analogue of ceramide, the endogenous product produced by the activity of p75(NTR)- activated sphingomyelinase, mimicked the effects of NGF on cell morphology and stimulation of both APP mRNA levels and APP secretion. Specific stimulation of TrkA receptors by receptor cross-linking, on the other hand, selectively stimulated neurite out-growth and APP secretion but not APP mRNA levels, which were decreased. These findings demonstrate that in PC12 cells expressing p75(NTR) and TrkA receptors, binding of NGF to the p75(NTR) is required to mediate NGF effects on cell morphology and APP metabolism. Furthermore, our data are consistent with NGF having specific effects on p75(NTR) not shared with other neurotrophins. Lastly, we have shown that specific activation of TrkA receptors - in contrast to p75(NTR)-associated signaling stimulates neurite outgrowth and increases nonamyloidogenic secretory APP processing without increases in APP mRNA levels.