Abeta oligomer-mediated long-term potentiation impairment involves protein phosphatase 1-dependent mechanisms

Abstract

Amyloid beta (Abeta) oligomers are derived from proteolytic cleavage of amyloid precursor protein (APP) and can impair memory and hippocampal long-term potentiation (LTP) in vivo and in vitro. They are recognized as the primary neurotoxic agents in Alzheimer's disease. The mechanisms underlying such toxicity on synaptic functions are complex and not fully understood. Here, we provide the first evidence that these mechanisms involve protein phosphatase 1 (PP1). Using a novel transgenic mouse model expressing human APP with the Swedish and Arctic mutations that render Abeta more prone to form oligomers (arcAbeta mice), we show that the LTP impairment induced by Abeta oligomers can be fully reversed by PP1 inhibition in vitro. We further demonstrate that the genetic inhibition of endogenous PP1 in vivo confers resistance to Abeta oligomer-mediated toxicity and preserves LTP. Overall, these results reveal that PP1 is a key player in the mechanisms of AD pathology.

Figure 1.

CA1 hippocampal LTP is impaired in arcAβ mice. A, B, CA1 hippocampal LTP is severely impaired in slices from 3.5- and 7.5-month-old arcAβ mice (n = 5 tg, 5 wt mice) but basal transmission is normal (right inset). C, LTP and basal transmission (right inset) are normal in slices from 1-month-old arcAβ mice (n = 5 tg, 5 wt mice). Left insets, Individual fEPSP traces for tg and wt mice before (black) and after (red) LTP induction. Calibration: A, B, 0.4 mV, 5 ms; C, 0.6 mV, 5 ms. D, Real-time RT-PCR: normal mRNA expression of synaptophysin, NMDA and AMPA receptors, and CaMKII, but reduced arg3.1 (67%; range, 58–78%; n = 5 tg and 5 wt mice) and zif268 (52%; range, 45–61%) expression in the hippocampus of 6-month-old arcAβ mice. Error bars represent SEM. *p < 0.05. norm., Normalized; Synapto., synaptophysin; Tg, transgenic.

Figure 2.

In vitro inhibition of PP1 reverses Aβ oligomer-mediated LTP impairment. A, B, PP1 is inhibited by bath application of 1 nm tautomycin (tauto) in slices from 3-month-old arcAβ mice and wild-type littermates. Tautomycin fully reverses the LTP deficit in slices from tg mice (n = 5) but has no effect in wt (n = 5 mice). It also does not change basal transmission (right insets). Left insets, Individual fEPSP traces for tg and wt mice before (black) and after (red) LTP induction. Calibration: 0.3 mV, 5 ms. C, Phosphatase activity assays showing that 1 nm tautomycin specifically inhibits the activity of recombinant PP1 but not of recombinant calcineurin. D, Frequency–response curve showing mean fEPSP slopes over 10 min starting 35 min after stimulation with 1, 2, 5, 10, and 100 Hz in slices from tg and wt mice treated or not with tautomycin (n = 3–5 slices per condition and frequency). E, PP1 inhibition with 1 nm tautomycin also reverses the LTP deficit seen in 3-month-old APPSwe/PS1 mice, another mouse model of AD (n = 5 tg). Error bars represent SEM. CN, Calcineurin; Tg, transgenic; w/o, without.

Figure 3.

Endogenous PP1 inhibition in vivo confers resistance to Aβ oligomer-mediated toxicity. A, Electron microscopy of Aβ oligomer preparation produced from synthetic Aβ_1–42 (according to Klein, 2002) showing globular Aβ assemblies of 7–12 nm after 24 h incubation at 4°C. Scale bar, 200 nm. B, Confirmation of Aβ oligomer-mediated toxicity in wt mice. Aβ oligomers were bath applied to wt slices for at least 1 h. Aβ oligomers impair LTP (n = 3–4; 3 slices with Aβ oligomers and 4 control slices). Right inset, Basal transmission was not affected. Left insets, Individual fEPSP traces with and without Aβ oligomer application before (black) and after (red) LTP induction. Calibration: 0.6 mV, 5 ms. C, D, Aβ oligomers are bath applied for at least 1 h to slices from I-1* transgenic mice and control littermates. Aβ oligomers impair LTP in I-1* control mice (n = 5) but not in I-1* transgenic mice (n = 5). Right inset, Basal transmission is not affected. Left insets, Individual fEPSP traces with and without application of Aβ oligomers before (black) and after (red) LTP induction. Calibration: 0.6 mV, 5 ms. Error bars represent SEM. oligo., Oligomer.