Amyloid β-induced impairments in hippocampal synaptic plasticity are rescued by decreasing mitochondrial superoxide

Abstract

Generation of reactive oxygen species (ROS) causes cellular oxidative damage and has been implicated in the etiology of Alzheimer's disease (AD). In contrast, multiple lines of evidence indicate that ROS can normally modulate long-term potentiation (LTP), a cellular model for memory formation. We recently showed that decreasing the level of superoxide through the overexpression of mitochondrial superoxide dismutase (SOD-2) prevents memory deficits in the Tg2576 mouse model of AD. In the current study, we explored whether AD-related LTP impairments could be prevented when ROS generation from mitochondria was diminished either pharmacologically or via genetic manipulation. In wild-type hippocampal slices treated with exogenous amyloid β peptide (Aβ1-42) and in slices from APP/PS1 mutant mice that model AD, LTP was impaired. The LTP impairments were prevented by MitoQ, a mitochondria-targeted antioxidant, and EUK134, an SOD and catalase mimetic. In contrast, inhibition of NADPH oxidase either by diphenyliodonium (DPI) or by genetically deleting gp91(phox), the key enzymatic component of NADPH oxidase, had no effect on Aβ-induced LTP blockade. Moreover, live staining with MitoSOX Red, a mitochondrial superoxide indicator, combined with confocal microscopy, revealed that Aβ-induced superoxide production could be blunted by MitoQ, but not DPI, in agreement with our electrophysiological findings. Finally, in transgenic mice overexpressing SOD-2, Aβ-induced LTP impairments and superoxide generation were prevented. Our data suggest a causal relationship between mitochondrial ROS imbalance and Aβ-induced impairments in hippocampal synaptic plasticity.

Figure 1.

Aβ-induced LTP impairments are rescued by EUK134 and MitoQ. A, HFS induced LTP in vehicle-treated slices (black triangles, n = 7), but not in slices pretreated with 500 nm Aβ1-42 (gray triangles, n = 9). In contrast, Aβ1-42 treatment did not inhibit LTP in the presence of either EUK134 (250 nm, dark gray circles, n = 6) or MitoQ (500 nm, light gray hexagons, n = 8). The dashed line represents the normalized baseline value of 100%. B, Compared with vehicle-treated slices (gray diamonds, n = 10), either 500 nm MitoQ (black triangles, n = 7), or 250 nm EUK134 (gray circles, n = 7) alone did not have an effect on LTP. In addition, treatment of slices with either EUK134 (gray hexagons, n = 10) or MitoQ (black hexagons, n = 9) did not affect baseline fEPSPs. C, Cumulative data showing mean fEPSP slopes 80 min post-HFS based on the LTP experiments in A and B. D, HFS-induced LTP was expressed normally in slices treated with scrambled Aβ1-42 peptide (500 nm, n = 5). sc, Scrambled.

Figure 2.

Inhibition of NADPH oxidase activity with either DPI or by genetic deletion of gp91^phox does not prevent Aβ-induced LTP impairments. A, LTP persisted in slices treated with DPI alone (10 nm; open circles, n = 5) and Aβ1-42 (500 nm; filled circles, n = 5) treatment resulted in impaired LTP in the presence of DPI. DPI alone (filled triangles, n = 7) did not alter baseline fEPSPs. B, In slices from gp91^phox knock-out (KO) mice, HFS elicited persistent LTP (open circles, n = 6) but LTP was impaired in the presence of Aβ1-42 (500 nm; filled circles, n = 7).

Figure 3.

MitoQ, but not DPI, prevents Aβ-induced increases in mitochondrial superoxide. A, Treatment of hippocampal slices with Aβ1-42 (500 nm) for 60 min increased the MitoSOX fluorescent signal (red) in area CA1 compared with control slices. DAPI staining is shown as blue. Pretreatment of slices with MitoQ (500 nm) blunted the Aβ-induced elevation in the MitoSOX fluorescent signal. In slices pretreated with DPI (10 nm), Aβ still caused increases in the MitoSOX fluorescent signal. B, Treatment of slices with a scrambled Aβ1-42 peptide (500 nm) for 60 min did not alter the MitoSOX fluorescent signal. Results are representative of three independent experiments. Scale bar, 50 μm. CTL, Control; sc, scrambled.

Figure 4.

MitoQ and EUK134 prevent LTP impairments in APP/PS1 mutant mice. A, HFS-induced LTP was impaired in slices from 10 to 12 month old APP/PS1 transgenic mice (gray circles, n = 8), compared with slices from wild-type (WT) mice in which HFS induced normal LTP (black circles, n = 6). In APP/PS1 slices treated with MitoQ (500 nm), HFS induced LTP similar to that observed in wild-type mice (triangles, n = 7). B, Cumulative data showing mean fEPSP slopes 80 min post-HFS. C, Impairment of hippocampal LTP in APP/PS1 mice (gray circles, n = 10) was prevented with EUK134 (250 nm) treatment (open circles, n = 4).

Figure 5.

Aβ failed to block hippocampal LTP in SOD-2 transgenic mice. A, HFS-induced LTP (dark gray triangles, n = 8) was blocked by Aβ1-42 (500 nm) in wild-type (WT) hippocampal slices (light gray triangles, n = 7). In slices from SOD-2 transgenic mice, HFS was able to induce LTP in the presence of Aβ1-42 (500 nm; gray circles, n = 5). B, Cumulative data showing mean fEPSP slopes 80 min post-HFS.

Figure 6.

Aβ-induced increases in mitochondrial superoxide were prevented in SOD-2 transgenic mice. Aβ-induced increases in the MitoSOX fluorescent (red) signal were absent in slices from SOD-2 transgenic mice. MitoSOX fluorescent intensity was slightly elevated in SOD-2 transgenic mice compared with wild-type (WT) littermates. Results are representative of three independent experiments. Scale bar, 50 μm.