Prion protein and Abeta-related synaptic toxicity impairment

Abstract

Alzheimer's disease (AD), the most common neurodegenerative disorder, goes along with extracellular amyloid-beta (Abeta) deposits. The cognitive decline observed during AD progression correlates with damaged spines, dendrites and synapses in hippocampus and cortex. Numerous studies have shown that Abeta oligomers, both synthetic and derived from cultures and AD brains, potently impair synaptic structure and functions. The cellular prion protein (PrP(C)) was proposed to mediate this effect. We report that ablation or overexpression of PrP(C) had no effect on the impairment of hippocampal synaptic plasticity in a transgenic model of AD. These findings challenge the role of PrP(C) as a mediator of Abeta toxicity.

Figure 1. CA1 hippocampal LTP impairment in APPPS1⁺ mice occurs at 4 months of age and is not regulated by PrP^C expression

1. CA1 hippocampal LTP was induced in acute slices from 4-month-old Prnp^+/+ mice (black, n = 7), but was abolished in slices from age-matched APPPS1⁺Prnp^+/+ (dark blue, n = 6), APPPS1⁺Prnp^+/o (blue, n = 5) and APPPS1⁺Prnp^o/o mice (light blue n = 5).

2. fEPSP traces before (red) and after (black) LTP induction. Calibration: 1 mV; 10 ms.

3. Input–output curves (stimulus intensity vs. fEPSP slope) indicative of normal basal synaptic transmission.

4. Unaffected LTP in slices derived from 2-month-old APPPS1⁺Prnp^+/+ (n = 5), APPPS1⁺Prnp^+/o (n = 5), APPPS1⁺Prnp^o/o (n = 4) and Prnp^+/o mice (n = 5). These results indicate that LTP impairment in APPPS1⁺ mice was not a developmental defect, and occurred only after 2 months of age independently of Prnp gene dosage.

Figure 2. LTP in 4-month-old APPPS1⁺ mice expressing a PrP^C transgene

1. At 4 months of age, LTP was impaired in slices from both APPPS1⁺tga20^tg/−Prnp^o/o (n = 4) and APPPS1⁺tga20^−/−Prnp^o/o (n = 5) but not in Prnp^+/+ slices (n = 7; LTP mean ± SEM from Fig 1A represented as grey ribbon). Basal synaptic transmission was normal as indicated by normal input–output curve (stimulus intensity vs. fEPSP slope).

2. Average fEPSP slopes (percentage of baseline) at 10–25 min post-LTP plotted against the average number of 129/Sv specific markers for mice depicted in panel A and Fig 1A. In all investigated paradigms, LTP suppression by the APPPS1 transgene was independent of the genetic background.

Figure 3. Analysis of 4-month-old APPPS1⁺ mice with supraphysiological levels of PrP^C

1. Percentage of strain-specific microsatellites in APPPS1⁺tga20^tg/−Prnp^+/o (n = 6) and APPPS1⁺tga20^−/−Prnp^+/o (n = 4) mice is displayed by box plot. No significant difference in the genetic background of the two mouse strains was detected (Mann–Whitney U-test, two-tailed, p > 0.05).

2. At 4 months of age, slices of both APPPS1⁺tga20^tg/−Prnp^+/o (n = 6) and APPPS1⁺tga20^−/−Prnp^+/o mice (n = 4) displayed reduced LTP when compared to Prnp^+/+ mice (n = 7); LTP mean ± SEMfrom Fig 1A represented as grey ribbon. Basal synaptic transmission was normal as indicated by normal input–output curve (stimulus intensity vs. fEPSP slope). All error bars: standard errors of the mean.

3. APP expression and processing by secretases were similar in 4-month-old APPPS1⁺ tga20^tg/−Prnp^+/o and APPPS1⁺tga20^−/−Prnp^+/o mice. Left panel: representative SDS–PAGE followed by immunoblotting using an APP C-terminal antibody detecting full-length APP and αβ-CTF; actin was used as loading control. Right panel: quantitation of chemiluminescence for APP, α-CTF and β-CTF.

4. TRIS-soluble (left panel), detergent-soluble (middle panel) and insoluble (right panel) human Aβ₄₂ levels as assessed by ELISA. Each symbol denotes one individual mouse.

Figure 4. Anchorless soluble PrP^C reduces hippocampal LTP impairment in APPPS1⁺ mice

1. Percentage of strain-specific microsatellites in APPPS1⁺tg44^tg/−Prnp^−/o (n = 5) and APPPS1⁺tg44^−/−Prnp^−/o (n = 5) mice is displayed by box plot. No significant difference in the genetic background was detected (Mann–Whitney U-test, two-tailed, p > 0.05).

2. LTP was induced in slices prepared from 4-month-old tg44^tg/−Prnp^−/o (n = 5) and tg44^−/−Prnp^−/o (n = 7) mice, but was impaired in slices from APPPS1⁺tg44^−/−Prnp^−/o mice (n = 6) and partially rescued in APPPS1⁺tg44^tg/−Prnp^−/o (n = 7) mice. Basal synaptic transmission was normal as indicated by normal input–output curve (stimulus intensity vs. fEPSP slope). All mice were compound heterozygotes for the ‘Zurich-I’ (Prnp^o) and the ‘Edbg’ (Prnp⁻) knockout alleles of Prnp.

3. APP expression and processing by secretases were similar in APPPS1⁺tg44^tg/−Prnp^−/o and APPPS1⁺tg44^−/−Prnp^−/o mice at 4 months of age. Left panel: representative SDS–PAGE followed by immunoblotting using an APP C-terminal antibody detecting full-length APP and C-terminal fragments (αβ-CTF); actin was used as loading control. Right panel: quantitation of chemiluminescence revealed no difference in APP, α-CTF and β-CTF between the two groups.

4. TRIS-soluble (left panel), detergent-soluble (middle panel) and insoluble (right panel) human Aβ₄₂ levels as assessed by ELISA. Each symbol denotes one individual mouse.