Genetic suppression of transgenic APP rescues Hypersynchronous network activity in a mouse model of Alzeimer's disease - PubMed (original) (raw)
. 2014 Mar 12;34(11):3826-40.
doi: 10.1523/JNEUROSCI.5171-13.2014.
Ji-Yoen Kim, Ricky R Savjani, Pritam Das, Yuri A Dabaghian, Qinxi Guo, Jong W Yoo, Dorothy R Schuler, John R Cirrito, Hui Zheng, Todd E Golde, Jeffrey L Noebels, Joanna L Jankowsky
Affiliations
- PMID: 24623762
- PMCID: PMC3951689
- DOI: 10.1523/JNEUROSCI.5171-13.2014
Genetic suppression of transgenic APP rescues Hypersynchronous network activity in a mouse model of Alzeimer's disease
Heather A Born et al. J Neurosci. 2014.
Abstract
Alzheimer's disease (AD) is associated with an elevated risk for seizures that may be fundamentally connected to cognitive dysfunction. Supporting this link, many mouse models for AD exhibit abnormal electroencephalogram (EEG) activity in addition to the expected neuropathology and cognitive deficits. Here, we used a controllable transgenic system to investigate how network changes develop and are maintained in a model characterized by amyloid β (Aβ) overproduction and progressive amyloid pathology. EEG recordings in tet-off mice overexpressing amyloid precursor protein (APP) from birth display frequent sharp wave discharges (SWDs). Unexpectedly, we found that withholding APP overexpression until adulthood substantially delayed the appearance of epileptiform activity. Together, these findings suggest that juvenile APP overexpression altered cortical development to favor synchronized firing. Regardless of the age at which EEG abnormalities appeared, the phenotype was dependent on continued APP overexpression and abated over several weeks once transgene expression was suppressed. Abnormal EEG discharges were independent of plaque load and could be extinguished without altering deposited amyloid. Selective reduction of Aβ with a γ-secretase inhibitor has no effect on the frequency of SWDs, indicating that another APP fragment or the full-length protein was likely responsible for maintaining EEG abnormalities. Moreover, transgene suppression normalized the ratio of excitatory to inhibitory innervation in the cortex, whereas secretase inhibition did not. Our results suggest that APP overexpression, and not Aβ overproduction, is responsible for EEG abnormalities in our transgenic mice and can be rescued independently of pathology.
Keywords: EEG; amyloid precursor protein; epilepsy; seizure; sharp wave discharge; transgene suppression.
Figures
Figure 1.
EEG recording reveals abnormal SWDs in tet-off APP-transgenic mice. A, B, EEG recordings made via wired dual-lead electrodes (A) or by wireless radiofrequency transmitters (B) uncovered frequent SWDs and irregular seizure episodes in APP/TTA mice. Scale bars: A, 0.5 mV and 2 s; B, 0.4 mV and 2 s. C, Recordings made with dual-lead electrodes were used to assess the percentage of spikes that would be counted in error by moving to a single-lead setup. The graph illustrates the percentage of SWDs recorded bilaterally as a function of SWDs recorded unilaterally. As the number of spikes increased, so also did the percentage of spikes recorded bilaterally. One data point (circled) was identified as an outlier and excluded from analysis. D, APP/TTA mice display a significantly higher number of SWDs/h than age-matched NTG and TTA controls. ***p < 0.001. E, SWD frequency was inversely related to locomotor activity in APP/TTA mice. Black circles, SWDs; open circles, locomotor activity.
Figure 2.
Transgene suppression reduces the frequency of SWDs in APP/TTA mice. A, Mice were implanted with radiofrequency transmitters at 7–8 months of age. After baseline recordings were completed, all mice were treated with DOX while continuing weekly EEG recordings. B, C, Western blotting for transgenic APP (6E10) confirmed that untreated APP/TTA mice expressed high levels of transgenic protein. Expression was substantially decreased within 2 d of DOX treatment and remained low at the end of the experiment 4–5 weeks later. D, Quantification of the Western blots shown in B and C. Transgenic APP has been normalized to GAPDH and all values are expressed relative to baseline expression in untreated bigenic animals. ***p < 0.001. E, In vivo microdialysis showed that ISF Aβ reached new steady-state levels within days after transgene suppression. Black circles, Aβ40; gray circles, Aβ42. F, Tissue sections immunostained for Aβ confirmed that plaque loads were similar in untreated and DOX-treated APP/TTA mice used for EEG analysis. G, EEG abnormalities in APP/TTA mice were substantially reduced after 4 weeks of transgene suppression. Scale bar, 0.4 mV, 5 s. H, Comparison of SWD frequency over time in APP/TTA mice before and after 4 weeks of transgene suppression revealed that the decrease in SWD frequency was most pronounced during daylight hours. Filled circles, baseline; open circles, DOX 4 weeks. I, Rate of SWDs in APP/TTA mice declined steadily from the start of treatment and became statistically indistinguishable from controls after 4 weeks of transgene suppression. Black circles, APP/TTA; gray circles, Control. **p < 0.01, ***p < 0.001 vs Control; †p < 0.05, ††p < 0.01, †††p < 0.001 vs Baseline. J, Locomotor activity measured during EEG recordings suggests that nighttime movement is slightly diminished in both genotypes by DOX treatment. Circles, APP/TTA; squares, Control; filled symbols, Baseline; open symbols, DOX 4 weeks.
Figure 3.
Transgene suppression normalizes the power spectrum, entropy, and autocorrelation of EEG signal in APP/TTA mice. A, Spectral analysis of the EEG signal revealed a shift toward higher power in the theta range and diminished power in the delta range of untreated APP/TTA mice compared with controls. After 4 weeks of transgene suppression, the power distribution was identical between genotypes. B, Interpeak timing in untreated APP/TTA mice has a lower entropy distribution compared with control mice, suggesting that their electrical activity is less stochastic than normal. The entropy distribution returns to control levels after 4 weeks of transgene suppression. C, Autocorrelation within the EEG signal is lower in untreated APP/TTA mice than controls, indicating that the signal is intrinsically more structured. This, too, returns to control levels after DOX treatment. Red, APP/TTA; blue, Control.
Figure 4.
Selective Aβ reduction does not diminish SWD frequency in APP/TTA mice. A, Mice were implanted at 8–9 months for baseline EEG recordings before GSI treatment with continued weekly recordings. B, EEG recordings made before and during GSI treatment show that SWDs in APP/TTA mice were unchanged after 4 weeks of selective Aβ reduction. Scale bar, 0.4 mV, 5 s. C, EEG analysis confirmed that the rate of SWDs in APP/TTA mice did not change during treatment and remained statistically different from controls after 6 weeks of GSI treatment. Black, APP/TTA; gray, Controls. *p < 0.05, **p < 0.01, ***p < 0.001. D, Dose–response analysis for LY411575 in young, predeposit APP/TTA mice revealed that both acute and chronic treatment reduced Aβ levels in the brain. **p < 0.01, ***p < 0.001. E, Western blotting with 6E10 shows that expression of full-length transgenic APP is maintained during GSI treatment, however, the levels of C-terminal fragment (CTF) increase as expected after γ-secretase inhibition. F, Analysis of SWD frequency over time reveals that the circadian pattern of EEG activity in APP/TTA mice was unchanged by GSI treatment. Filled circles, Baseline; open circles, GSI 4 weeks. G, Locomotor hyperactivity was also unchanged by GSI treatment. Circles, APP/TTA; squares, Control; filled symbols, Baseline; open symbols, GSI 4 weeks.
Figure 5.
APP/PS1 DKI mice show normal EEG activity despite elevated Aβ production. A, APPSwe/Lon/PS1M146V mice and age-matched WT animals were implanted with transmitters for EEG recording at 23–24 months of age. B, SWDs are not evident in aged DKI animals. Scale bar, 0.4 mV, 5 s. C, Silver histology indicates that early amyloid pathology was present by the time of EEG recording.
Figure 6.
APP/TTA and control mice were challenged with kainic acid (A, C, D, E) or picrotoxin (B, F, G, H) and scored for seizure induction by EEG and behavioral observation. A separate cohort was treated with DOX for 5 weeks before picrotoxin challenge (I–K). A, B, Example EEG traces from APP/TTA mice challenged with kainic acid or picrotoxin. Scale bars: A, 0.4 mV, 1 s; B, 0.4 mV, 5 s. C, F, I, Seizure severity was scored using a modified Racine scale. D, G, J, Latency to first seizure. E, H, K, Total number of seizures observed in the first hour after drug challenge. *p < 0.05, **p < 0.01, ***p < 0.001.
Figure 7.
Transgene suppression restores cortical markers of glutamatergic and GABAergic innervation to control levels. A, Tissue from TTA and APP/TTA mice used for EEG recording was coimmunostained for vGLUT1 and vGAT. Images show cortex layer 5 from untreated TTA (control) mice along with untreated, DOX-treated, and GSI-treated APP/TTA mice. Scale bar, 50 μm. B, Percent area occupied by vGLUT1 and vGAT staining within layers 2/3 and 5 was measured using a custom MATLAB script. Untreated APP/TTA mice had a lower density of vGLUT1 and higher density of vGAT in layer 5 than control animals, with a similar trend in layer 2/3. Transgene suppression restored these excitatory/inhibitory markers to control levels, whereas GSI had no effect. *p < 0.05, **p < 0.01.
Figure 8.
Delaying the onset of APP overexpression protects against early EEG abnormalities. A, EEG activity was compared between juvenile-onset mice that express transgenic APP from birth and adult-onset mice in which APP overexpression was delayed with DOX until the mice reached 6 weeks of age. B, SWDs were common in juvenile-onset APP/TTA mice at both 6 and 9 months of age, but were found in adult-onset mice only after 9 months of APP overexpression. Scale bar, 0.4 mV, 5 s. C, Transgenic APP expression from birth resulted in elevated SWD frequency by the earliest ages we could examine (3 months), whereas mice with delayed transgene expression develop SWDs considerably later (9 months). Filled circles, juvenile onset; open circles, adult onset. D, After 9 months of APP overexpression, adult onset mice were treated either with DOX or with GSI for 5 weeks. The frequency of SWDs decreased to control levels within 2 weeks of transgene suppression, but did not change significantly with secretase inhibition. Filled circles, DOX; open circles, GSI. *p < 0.05, **p < 0.01.
References
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