Amyloid β interferes with the capacity of NGF to activate protein tyrosine phosphatase 1B in hippocampal neurons. Cultured 7 DIV neurons (about 250,000 cells per experimental point) were treated as indicated and PTP1B activity was assessed. (A) Whereas NGF (100 ng/ml) increased PTP1B activity several fold, Aβ (5 μM) did not alter the activity of the phosphatase, although Aβ did prevent the increase of PTP1B activity induced by NGF (B). Another activator of RhoA activity, CNFy (200 ng/ml), also prevented the activation of PTP1B caused by NGF. Both Aβ and CNFy were applied to cultures 18 h before stimulating with NGF for 4 h. (C) The inhibition of RhoA activity after incubating the cells with either C3 (1 μM) or TAT-pep5 (1 μM) for 18 h increased the activity of PTP1B. By contrast to NGF, such increases were not counteracted by prior application of Aβ.