Decreased brain-derived neurotrophic factor depends on amyloid aggregation state in transgenic mouse models of Alzheimer's disease

Abstract

Downregulation of brain-derived neurotrophic factor (BDNF) in the cortex occurs early in the progression of Alzheimer's disease (AD). Since BDNF plays a critical role in neuronal survival, synaptic plasticity, and memory, BDNF reduction may contribute to synaptic and cellular loss and memory deficits characteristic of AD. In vitro evidence suggests that amyloid-beta (A beta) contributes to BDNF downregulation in AD, but the specific A beta aggregation state responsible for this downregulation in vivo is unknown. In the present study, we examined cortical levels of BDNF mRNA in three different transgenic AD mouse models harboring mutations in APP resulting in A beta overproduction, and in a genetic mouse model of Down syndrome. Two of the three A beta transgenic strains (APP(NLh) and TgCRND8) exhibited significantly decreased cortical BDNF mRNA levels compared with wild-type mice, whereas neither the other strain (APP(swe)/PS-1) nor the Down syndrome mouse model (Ts65Dn) was affected. Only APP(NLh) and TgCRND8 mice expressed high A beta(42)/A beta(40) ratios and larger SDS-stable A beta oligomers (approximately 115 kDa). TgCRND8 mice exhibited downregulation of BDNF transcripts III and IV; transcript IV is also downregulated in AD. Furthermore, in all transgenic mouse strains, there was a correlation between levels of large oligomers, A beta(42)/A beta(40), and severity of BDNF decrease. These data show that the amount and species of A beta vary among transgenic mouse models of AD and are negatively correlated with BDNF levels. These findings also suggest that the effect of A beta on decreased BDNF expression is specific to the aggregation state of A beta and is dependent on large oligomers.

Figure 1.

BDNF mRNA is downregulated in APP^NLh/PS-1^P264L and TgCRND8 but not in APPswe/PS-1^M146V or Ts65Dn mice compared with controls. Total BDNF mRNA and β-actin were measured by absolute quantitative real-time PCR. Means of triplicates were evaluated by one-way ANOVA and post hoc Tukey test for APPNLh/PS-1P^264L and APPswe/PS-1^M146V mice or unpaired Student's t test for TgCRND8 and Ts65Dn mice. Data are expressed as a ratio of copies BDNF/copies β-actin. *p < 0.05 and **p < 0.01 compared with wild type. Group sizes are shown in Table 2. Error bars represent SEM. wt, Wild-type littermates of each transgenic mouse group; tg, transgenic mice of each group; APP, transgenic mice with only a mutant APP gene; DBL, transgenic mice with both mutant APP and PS-1 genes; PS1, transgenic mice with only a mutant PS-1 gene; 2N, normal disomic controls; 3N, trisomic Ts65Dn mice.

Figure 2.

A, Ca²⁺ treatment of transgenic mouse cortical homogenates. Western blot of APP^NLh/PS-1^P264L homogenates treated with calcium to precipitate large oligomers/protofibrils as described in Materials and Methods and detected with antibody 6E10. Oligomers of ∼115 kDa (“No Ca²⁺”) disappeared from the soluble fraction after Ca²⁺ treatment (“10 mm Ca²⁺”) and appeared as insoluble species (“Pellet”). B, Comparison of Aβ species in four strains of mice with three different monoclonal antibodies, an ADDL-specific antibody (NU-2), Aβ-cross-reactive antibody 6E10, and the APP-specific antibody 22C11. Lane 1, APP^NLh/PS-1^P264L mice; lane 2, TgCRND8 mice; lane 3, APP^swe/PS-1^M146V mice; lane 4, Ts65Dn mice. Note: each lane contained 30 μg of total protein.

Figure 3.

2Aβ complement of different transgenic mouse strains by Western blotting. A shows a representative Western blot of ∼115 kDa oligomers in APP^NLh and TgCRND8 mice but not in APP^swe/PS-1^M146V mice. B shows the corresponding β-actin blot. C shows no ∼115 kDa oligomer signals in wild-type mice from each group, but there are weak signals of Aβ monomers (data not shown) and other conformations. D shows no ∼115 kDa signals in Ts65Dn mice compared with other AD transgenic mice.

Figure 4.

Correlation between Aβ₄₂/Aβ₄₀ ratio and BDNF mRNA levels in transgenic mice. BDNF and β-actin mRNA was measured by real-time RT-PCR, and Aβ₄₀ and Aβ₄₂ in cortex of transgenic mice were measured by sandwich ELISA. The ratio of Aβ₄₂/Aβ₄₀ was negatively correlated with BDNF mRNA levels in these transgenic mice. r = −0.54, p = 0.045, Spearman rank correlation. R Sq, R².

Figure 5.

Expression of BDNF transcripts containing exons I, III, IV, and VI in TgCRND8 (tg) compared with wild-type (wt) littermates. Means of triplicates were evaluated by unpaired Student's t test. n = 8 for wild type and n = 6 for TgCRND8. Error bars represent SEM.