Humoral and cellular immune responses in Aβ 42 -immunized mice

In 20-month-old transgenic mice that had received 13 immunizations (Fig. 1a), antibody levels reached 69.88 ± 11.08 μg of anti-Aβ 42 immunoglobulin G (IgG)/ml of plasma (63.77 ± 19.53 μg anti-Aβ 42 IgG/ml plasma in group 2) after DNA Aβ 42 immunization and 655.9 ± 9.58 μg/ml after Aβ 42 peptide immunization (763.4 ± 11.88 μg anti-Aβ 42 IgG/ml plasma in group 2). Similar antibody levels were found in parallel immunized 20-month-old wild-type control animals: 49.79 ± 6.35 μg/ml in DNA Aβ 42 -immunized mice and 659.7 ± 6.95 μg/ml in Aβ 42 peptide-immunized mice (Fig. 1b). DNA Aβ 42 trimer-immunized mice had high levels of IgG1 and IgG2b antibodies. The overall isotype composition was IgG1 = IgG2b > IgG2a/c (IgG1/IgG2a ratio of 1.61). Low levels of IgG2a/c antibodies were consistent with a noninflammatory Th2 immune response (Fig. 1c). All peptide-immunized mice had mixed isotype profiles with similar levels of IgG1, IgG2a/c, and IgG2b antibodies, indicative of a mixed Th1/Th2 immune response (Fig. 1d). This mixed profile was found at high plasma dilutions up to 1:20,000 (Fig. 1e).

ELISPOT assays from splenocyte cultures of 3xTg-AD mice and wild-type mice were performed to detect IFN-γ (Th1 cytokine), IL-17A (Th17 cytokine), and IL-4 (Th2 cytokine) upon Aβ 42 peptide restimulation in the immunized mice. Although we found high numbers of IFN-γ- and IL-17-secreting cells in peptide-immunized mice, low numbers of cells secreting these proinflammatory cytokines were found in DNA-immunized mice. In peptide immunized 3xTg-AD mice, IFN-γ-secreting cells were detected with 8 ± 11.27 spots in medium control wells and more than 1000 spots in the Aβ 42 peptide restimulated wells (p < 0.0001 by Mann-Whitney U test) (Fig. 1f). We counted 104.7 ± 47.9 spots in medium control wells of DNA Aβ 42 -immunized 3xTg-AD mice with no increase in peptide restimulated wells (92 ± 39.95 spots; p = 0.7428). A similar pattern was observed for IL-17-secreting cells with increased numbers in Aβ 42 peptide-immunized mice: 227 ± 15.52 spots after peptide restimulation compared with 5.3 ± 4.04 spots in medium control wells (p < 0.0001 by Mann Whitney U test) and no significant increase in IL-17-secreting cells after peptide restimulation in DNA Aβ 42 -immunized mice (87.33 ± 30.6 spots in Aβ 42 peptide-containing wells, 111.7 ± 24.58 spots in medium control wells; p = 0.3439) (Fig. 1g). For a peptide mix (Aβ 10–26 /Aβ 17–31 ) containing the T-cell epitope of several mouse major histocompatibility complex haplotypes (H2b, H2k, H2d, H2s), similar results were obtained in the ELISPOT assays (Fig. 1f).

Histology showing amyloid reduction from brain

In the initial studies, we used male and female mice and found large differences in the pathology between sexes. Figure 2a–d shows sections of 18- and 20-month-old mice for comparison of Aβ 42 pathology in females and males. In 20-month-old mice, large numbers of Aβ plaques were found in the subiculum of the hippocampus in female mice (Fig. 2a), whereas no plaques were found in the 20-month-old males (Fig. 2b). Also, for tau antibody staining (HT7, AT180) in parallel sections, much less pathology was found in male mice (data not shown), and therefore we continued immunotherapy in the following groups only in females. In 18-month-old mice, amyloid plaques were abundant in the female mice (Fig. 2c). In age-matched male mice, only a few neurons with intracellular Aβ 42 staining were found, but no plaques (Fig. 2d).

Fig. 2 Amyloid-β (Aβ) immunization results in removal of amyloid plaques in brains of triple-transgenic Alzheimer’s disease (3xTg-AD) mice. Brain sections of mice aged 18 months (c and d) and 20 months (a, b, e–h) were stained with a NeuN antibody (red) to detect neurons and an anti-Aβ antibody (McSA1, brown) to detect numerous plaques in the subiculum of the hippocampus in 3xTg-AD mice. a The hippocampus of a 20-month-old female control 3xTg-AD mouse with numerous amyloid plaques is shown (5× magnification). b Hippocampus of a 20-month-old male control 3xTg-AD mouse showing no plaque pathology. c The subiculum of an 18-month-old female control 3xTg-AD mouse is shown at higher magnification (20×). d Aβ staining in the subiculum of an 18-month-old male mouse. Only intraneuronal Aβ can be detected (indicated with arrow and shown at higher magnification in inset). e Numerous plaques in the hippocampus of an untreated 20-month-old 3xTg-AD female mouse. f No plaques were found in 20-month-old wild-type mice. Both immunization regimens, Aβ 42 peptide (g) and DNA Aβ 42 (h), led to a reduction of plaques in 20-month-old 3xTg-AD mice compared with the control mouse (e). i Images were counted for plaques ≥ 10 μm in a 1-mm2 area of the subiculum/CA1 of the hippocampus by two blinded experimenters. Blue bars show plaque count in DNA Aβ 42 trimer-immunized mice (n = 7), and yellow bars show plaque count found in brains of Aβ 42 peptide-immunized mice (n = 8). Black bars show the numbers found in age- and gender-matched 3xTg-AD control mice (n = 15). * indicates p value of ≤ 0.05 (unpaired Student's t test) Full size image

Aβ 42 immunotherapy led to a reduction of the number of amyloid plaques in the hippocampus of treated mice. In Fig. 2e–h, staining for NeuN, which stains neurons (red color), and an Aβ antibody (McSA1), which stains amyloid plaques (brown color) are shown for the hippocampal area for representative examples of the different mouse groups in one experimental cohort. The mAb McSA1 recognizes the N-terminal region of the human Aβ peptide (Aβ 1–12 ). This epitope is present in β-C-terminal fragment and amyloid precursor protein (APP) as well, but McSA1 has been reported as highly specific for Aβ as opposed to APP or soluble APP following competition studies with these antigens [33, 34]. Figure 2e shows staining of the hippocampus subiculum of a 20-month-old 3xTg-AD control female mouse. Figure 2f shows this area stained for neurons and amyloid in a 20-month-old wild-type control mouse. A reduction of amyloid plaques was seen in all mice that had received Aβ immunotherapy. Representative sections are shown for one Aβ 42 peptide-immunized mouse (Fig. 2g) and one DNA Aβ 42 -immunized 3xTg-AD mouse (Fig. 2h).

Immunohistological staining of plaques in the brains of these mice was subjected to the counting of plaques > 10 μm in corresponding 1-mm2 areas (subiculum/CA1) of 15 control mice (7 DNA Aβ 42 -immunized mice and 8 Aβ 42 peptide-immunized mice) by two blinded experimenters. These analyses showed significantly reduced plaque numbers in the DNA Aβ 42 -immunized mice (p = 0.0238 by Student’s t test compared with control mice) and a nonsignificant reduction in the Aβ 42 peptide-immunized mice (p = 0.6809). Also, the difference in plaque numbers between the DNA Aβ 42 - and peptide-immunized mice was significant (p = 0.0487) (Fig. 2i).

Histology showing reduction in levels of phospho-tau

The use of the 3xTg-AD mouse model allowed us to analyze a second pathology of human AD, which is the hyperphosphorylation of tau and development of neurofibrillary tangles. IHC of 3xTg-AD brain sections with different antibodies specific for tau molecules phosphorylated at specific residues (AT180, AT8, AT270, pT404, pS212, Tyr18) showed that Aβ 42 immunotherapy also led to a significant reduction in the levels of tau phosphorylation. In Fig. 3a, the age progression for tau phosphorylation in the 3xTg-AD mouse model is shown. Brains from 2-, 4-, 7-, 9-, 12-, and 18-month-old mice (n = 4/group) were harvested, and PFA-fixed, paraffin-embedded sections were analyzed with the mAb AT180, which detects tau phosphorylated at residue T231. In the comparison of the staining pattern with brains from 18-month-old 3xTg-AD mice, which had received DNA Aβ 42 immunizations, we observed that the AT180 staining intensity of the immunized 18-month-old mice appeared more like the staining intensity in brains from 7- or 9-month-old mice (Fig. 3b). Sections from four 18-month-old Luc immunized control mice, five 18-month-old DNA Aβ 42 -immunized mice, and six 18-month-old Aβ 42 peptide-immunized mice were semiquantitatively analyzed with the area measure tool in ImageJ software. The results showed an about 40% reduction after DNA Aβ 42 immunization and an approximately 20% reduction after Aβ 42 peptide immunization (Fig. 3c). However, owing to high SDs and the small number of control animals, the results were not statistically significant.

Fig. 3 IHC staining of T231p (AT180) and T202p/S205p (AT8) in triple-transgenic Alzheimer’s disease (3xTg-AD) mouse brains and Western blots for total tau. a The age progression of T231p (AT180) in 3xTg-AD mouse brains is shown by IHC and staining in 2-, 4-, 7-, 9-, 12-, and 18-month-old mice. b T231p staining in the hippocampus of three brains from 18-month-old 3xTg-AD mice that had received DNA amyloid-β 1–42 peptide (Aβ 42 ) trimer immunizations is shown for comparison. c Semiquantitative analyses for pT231 staining in the hippocampus of four 18-month-old control mice, five 18-month-old DNA Aβ 42 immunized mice, six 18-month-old Aβ 42 peptide-immunized mice, and four wild-type mice using ImageJ software (National Institutes of Health, Bethesda, MD, USA). Blue bars show positive areas found in DNA Aβ 42 trimer-immunized mice, and yellow bars show areas found in brains of Aβ 42 peptide-immunized mice. Black bars show the values of age- and gender-matched 3xTg-AD control mice. d–f DNA Aβ 42 trimer immunization decreased AT8 staining in hippocampal sections from 20-month-old 3xTg-AD mice. d Representative sections from two control mice. e Sections from Aβ 42 peptide-immunized mice. f Staining of AT8+ tangles in the hippocampus of two DNA Aβ 42 -immunized mice. All pictures are in 10× magnification (hippocampus); insets are in 40× magnification (subiculum). g A representative Western blot from detergent-soluble brain lysates of 20-month-old 3xTg-AD control mice (labeled C1–C4), DNA Aβ 42 -immunized mice (labeled D1–D4), Aβ 42 peptide-immunized mice (labeled P1, P2), and a wild-type control (wt) mouse is shown. h Gray value intensities of human tau bands (indicated with an arrowhead, missing in the wt control, at 50 kD) were semiquantitatively analyzed using ImageJ software. Black bars show the values in 3xTg-AD control mice; yellow bars represent the peptide-immunized mice; and blue bars show values found in DNA Aβ 42 -immunized mice Full size image

Staining with the AT8 antibody specific for pS201/pT205, which is a late tau phosphorylation site [35], was less prominent in 18-month-old mice, but good staining was observed in 20-month-old mice, which showed reduction of AT8 staining in DNA Aβ 42 trimer-immunized mice. Figure 3d shows that AT8-positive neurons were detected in the hippocampus of 20-month-old 3xTg-AD control mice (sections from two mice). Two representative sections from the Aβ 42 peptide-immunized 20-month-old 3xTg-AD mice are shown in Fig. 3e. The brains showed fewer AT8-positive neurons than in the control animals. Much less staining was found in DNA Aβ 42 trimer-immunized mice. Figure 3 shows the respective brain sections of the hippocampus from two mice (insets show higher magnification of subiculum in Fig. 3f).

The histological data indicating a possible reduction of tau in the Aβ 42 -immunized mice led to further substantiation of this finding by Western blot analysis and a panel of commercially available tau ELISAs that allowed testing for statistical significance of reduction of different tau phosphorylation patterns.

Western blot analysis of total tau

The reduction of tau in mice that had received Aβ 42 immunotherapy was further analyzed using Western blotting of the brain lysates. In the comparison of total tau detected with the mAb Tau12, it was found that both immunotherapies led to a reduction in tau. The reduction was not significant in Aβ 42 peptide-immunized mice and was higher and significant in DNA Aβ 42 -immunized mice (p values of 0.0302, 0.0142, and 0.0023 from three independently performed Western blot analyses with detergent-soluble brain lysates). Figure 3g and h shows the results of one of these experiments (Western blot and ImageJ analysis of gray-level intensities of the bands, respectively). Total tau and phosphorylated tau were further analyzed by Western blotting, and the results are shown in Fig. 4. All band intensities were normalized to band intensities found in the reprobing of the Western blots with antibodies to housekeeping proteins. In the detergent-soluble fractions, tau detected with the mAb Tau12 was significantly reduced in brain lysates from DNA Aβ 42 -immunized mice (p = 0.0059 by Student’s unpaired t test) (Fig. 4a). The intensity of the Western blot band reactive to the mAb AT8 was only slightly reduced in DNA Aβ 42 -immunized mice (p = 0.3224, nonsignificant). The AT8-reactive protein band was found at higher molecular weight (about 65 kDa), which might correspond to the 64 kDa tau, a Tris-buffered saline-extractable hyperphosphorylated tau species described in the rTg4510 mouse brain (Fig. 4a, middle panel) [36]. In Fig. 4b, two different total human tau antibodies, 43D and HT7, were directly compared in parallel-run SDS-PAGE. Significant reductions of tau were found in the brain samples from DNA Aβ 42 -immunized mice (HT7 antibody, p = 0.0152; 43D antibody, p = 0.0138).

Fig. 4 Western blot analyses for total and phosphorylated tau. Equal amounts of proteins from detergent-soluble brain lysates of 20-month-old triple-transgenic Alzheimer’s disease (3xTg-AD) mice (D1–D5 = DNA Aβ 42 -immunized mice, P1–P4 = amyloid-β 1–42 [Aβ 42 ] peptide-immunized mice, C1–C5 = 3xTg-AD control mice, wt = wild-type controls) were separated by SDS-PAGE, blotted onto nitrocellulose filters, and probed using antibodies specific for total human tau (a, upper panel), and phosphorylated tau AT8 (a, middle panel), and β-tubulin as a loading control (a, bottom panel). The graph on the right-hand side of the SDS-PAGE pictures shows analyses of the band intensities performed with ImageJ software. All gray-level intensities of tau protein bands were normalized to the gray-level intensities of protein bands of the housekeeping proteins β-tubulin or β-actin, respectively. The reduction of total tau in the DNA-immunized mice compared with the 3xTg-AD control animals was highly significant (p = 0.0059). Of note, gray-level intensities for sample D2 were not included in thess calculations, because the loading control for this sample indicated a much lower protein content (a, bottom panel). b A comparison of total tau levels in DNA-immunized mice, 3xTg-AD control mice, and wt control mice in Western blots is shown using two different antibodies. In the upper panel, 43D (Tau1–100) was used for detection; in the middle panel, antibody HT7 was used; and in the lower panel, the same membrane was probed with a β-actin antibody as a protein loading control. The graph on the right-hand side of the panels shows the analyses of gray-level intensities for the protein bands with ImageJ software normalized to gray-level intensities of the housekeeping protein β-actin. Differences were statistically significant with p values of 0.0152 (HT7) and 0.0138 (43D). * and ** indicate p values of ≤ 0.05 and ≤ 0.01, respectively (Mann-Whitney U test) Full size image

These results are consistent with the ELISA results described in the “Quantification of tau in ELISAs” section below. Although the reductions in the detergent-soluble brain lysate fractions were obvious but statistically not significant, reductions in the nonsoluble brain lysates were highly significant. However, the nonsoluble fractions could not be tested, owing to the extraction method used with 5 M guanidine for the solubilization of the pellet. These samples are not compatible with SDS-PAGE. In future mouse cohorts, we will use a different extraction protocol allowing the nonsoluble brain lysate fractions to be analyzed in Western blots (SDS-PAGE).

Quantification of Aβ x-42 and Aβ x-40 in ELISAs

After analysis of brain histology as shown in Fig. 2, ELISAs were used for semiquantitative analyses of reduction of Aβ x-40 and Aβ x-42 peptides in DNA Aβ 42 trimer- and Aβ 42 peptide-immunized female 3xTg-AD mice. An increase of Aβ 42 and Aβ 40 peptides in brains from 3xTg-AD mice with age is shown in Fig. 5a. ELISAs were also used to quantify the reduction of Aβ x-40 and Aβ x-42 peptides due to DNA Aβ 42 trimer and Aβ 42 peptide immunization (Fig. 5b and c). Statistical significance for reduction of Aβ 42 and Aβ 40 was reached in the comparison of DNA Aβ 42 trimer-immunized mice (n = 7, blue bars) compared with control animals (n = 14, black bars) in the nonsoluble fractions (p = 0.0461, Mann-Whitney U test, for Aβ x-42 ; p = 0.0125 for Aβ x-40 ). These reductions were nonsignificant in the one-way ANOVA (Fig. 5b). In the soluble brain lysate fractions, a reduction of both Aβ peptides was highly significant (p < 0.0008, Mann-Whitney U test; p = 0.0123, one-way-ANOVA, for Aβ x-42 ; p = 0.0017, Mann-Whitney U test; p = 0.0028, one-way ANOVA for Aβ x-40 ) (Fig. 5c) in DNA Aβ 42 -immunized mice. Aβ x-42 peptides were also reduced in brains from Aβ 42 peptide-immunized 3xTg-AD mice in the nonsoluble lysate and detergent-soluble lysates, but levels did not reach statistical significance (p = 0.2766 for nonsoluble Aβ x-42 , p = 0.0815 for soluble Aβ x-42 ). Much less removal was found for Aβ x-40 peptides in brains from Aβ 42 peptide-immunized mice (Fig. 5b and c, yellow bars, right-hand graphs).

Fig. 5 Quantitative enzyme-linked immunosorbent assay (ELISA) analyses for amyloid-β 1–42 peptide (Aβ 42 ) and Aβ 40 in brain lysates from triple-transgenic Alzheimer’s disease (3xTg-AD) mice. a Analyses of an increase of Aβ 42 and Aβ 40 peptides in brains from 3xTg-AD mice with age (12-month-, 18-month-, and 20-month-old female control mice). b Reduction of Aβ 42 and Aβ 40 peptide concentrations in the nonsoluble fractions of the brain lysates owing to Aβ 42 immunotherapy. Blue bars show Aβ 42 peptide concentrations found in brains from DNA Aβ 42 trimer-immunized mice; yellow bars show the concentrations found in brains from Aβ 42 peptide-immunized mice. The black bars show Aβ 42 peptide concentrations in age- and gender-matched 3xTg-AD control mice. The left-hand graph displays data for Aβ 42 peptides, and the right-hand graph shows data for Aβ 40 peptides. c Reduction of Aβ 42 and Aβ 40 peptide concentrations in the soluble fractions of the brain lysates owing to Aβ 42 immunotherapy. The left-hand graph shows data for Aβ 42 peptides, and the right-hand graph displays data for Aβ 40 peptides. ELISAs for the nonsoluble brain lysates were performed three times (dilution 1:10,000), and ELISAs for the detergent-soluble brain lysates were performed twice (dilution 1:2) for this particular group of mice and confirmed the data shown. * p ≤0.05, ** p ≤ 0.01, *** p ≤ 0.005, and **** p ≤ 0.001 (Mann-Whitney U test) Full size image

Quantification of tau in ELISAs

Histological analyses of the mouse brains with tau antibodies AT180 and AT8 (early and late tau phosphorylation) showed reduced staining in the immunized mice (Fig. 3). ELISAs were used for detection of total tau, pT231 tau, p396 tau, pT181 tau, and pS199 tau in the semiquantitative analyses of tau reduction in DNA Aβ 42 trimer- and Aβ 42 peptide-immunized 3xTg-AD mice. Tau was reduced in both mouse groups, which had received Aβ 42 immunotherapy or DNA or peptide vaccine (Fig. 6a–e, Table 1). However, statistical significance was reached only in the DNA Aβ 42 trimer-immunized mice (Table 1).

Fig. 6 DNA amyloid-β 1–42 (Aβ 42 ) immunization reduces total and phosphorylated tau in brains of triple-transgenic Alzheimer’s disease (3xTg-AD) mice. Quantitative enzyme-linked immunosorbent assay analyses for tau in detergent-soluble and nonsoluble fractions of brain lysates from 20-month-old 3xTg-AD mice. a Analysis of total concentrations of human tau. Blue bars show concentrations found in DNA Aβ 42 trimer-immunized mice, and yellow bars show concentrations found in brains of Aβ 42 peptide-immunized mice. Black bars show the values of age- and gender-matched 3xTg-AD control mice. The left-hand graph shows the analyses in detergent-soluble fractions from hemibrain lysates; the right-hand graph represents the analyses from nonsoluble fractions. b Analysis of tau phosphorylated at residue T231 (pT231), c Analysis of tau phosphorylated at residue S396 (pS396). d Analysis of tau phosphorylated at residue T181 (pT181). e Analysis of tau phosphorylated at residue S199 (pS199). All data are based on the analyses and comparison of 7 DNA Aβ 42 trimer-immunized mice, 9 Aβ 42 peptide-immunized mice, and 14 age- and gender-matched 3xTg-AD control mice. All samples were run in duplicates, and the assay was repeated twice. * p ≤ 0.05, ** p ≤ 0.01, and *** p ≤ 0.005 (Mann-Whitney U test) Full size image

Table 1 Tau reductions in 20-month-old female 3xTg-AD mice following Aβ42 immunotherapy Full size table

High levels of tau protein were found in the detergent soluble fractions: 1.235 × 105 ± 0.556 × 105 pg/ml brain lysate in the 20-month-old female 3xTg-AD control mice (n = 14), with small reductions in the mouse groups that had received Aβ 42 immunotherapy (1.201 × 105 ± 0.44 × 105 pg/mg in the peptide-immunized mice [p = 0.9754, n = 9], and 0.928 × 105 ± 0.324 × 105 pg/mg [p = 0.1285] in DNA-immunized mice [n = 7]). Higher reductions of total tau were found in the nonsoluble brain lysate fractions: control mice had mean values of 7.322 × 105 ± 3.301 × 105 pg/mg; Aβ 42 peptide-immunized mice had levels of 6.879 × 105 ± 3.153 × 105 pg/mg brain weight (p = 0.8446); and DNA Aβ 42 trimer-immunized mice had significantly reduced levels of 3.793 × 105 ± 1.096 × 106 pg/mg brain weight of total human tau (p = 0.0411, one-way ANOVA) (Fig. 6a).

pT231 tau reached mean values of 1793 ± 490.3 U/mg brain weight in the detergent-soluble brain lysate fractions of 3xTg-AD control mice, 1454 ± 390.6 U/mg in Aβ 42 peptide-immunized mice, and 1199 ± 221.5 U/mg in DNA Aβ 42 trimer-immunized mice. Although the reduction in the peptide-immunized mice was not significant (p = 0.4767), the reduction in DNA-immunized mice was highly significant (p = 0.0091). In the nonsoluble brain fractions, a mean value of 1.296 × 105 ± 0.282 × 105 U/mg pT231 tau was found for control mice, 1.313 ± 0.338 × 105 U/mg was found in peptide-immunized mice, and 0.809 × 105 ± 0.192 × 105 U/mg was found in DNA-immunized mice (Fig. 6b). Thus, Aβ immunotherapy reduced the nonsoluble pT231 only in DNA Aβ 42 trimer-immunized mice (p = 0.0017).

pS396 tau was slightly reduced in the detergent-soluble fraction for DNA Aβ 42 trimer- and Aβ 42 peptide-immunized female mice compared with 3xTg-AD female control mice (889.2 ± 273.2 pg/mg, 1264 ± 389.1 pg/mg, and 1441 ± 566 pg/mg, respectively; p = ns by one-way ANOVA). pS396 was significantly reduced in DNA Aβ 42 trimer-immunized mice in the nonsoluble fractions with mean levels of 0.631 × 105 ± 0.121 × 105 pg/mg (p = 0.0007) compared with 1.136 × 105 ± 0.272 × 105 pg/mg in the 3xTg-AD control mice (Fig. 6c).

For pT181 tau, mean levels of 3.869 × 104± 1.774 × 104 pg/mg in the detergent-soluble brain lysates of 3xTg-AD control mice were reduced to 3.098 × 104 ± 0.99 × 104 pg/mg in Aβ 42 peptide-immunized mice (p = 0.3686) and to 1.969 × 104 ± 0.507 × 104 pg/mg in DNA Aβ 42 trimer-immunized mice (p = 0.0198). In the nonsoluble brain fractions, a level of 1.876 × 105 ± 0.591 × 105 pg/mg was measured for female 3xTg-AD control mice, which was reduced to 1.672 × 105 ± 0.661 × 105 pg/mg (p = 0.5123) in Aβ 42 peptide-immunized mice and to 0.911 × 105 ± 0.248 × 105 pg/mg (p = 0.002, one-way ANOVA) in DNA Aβ 42 trimer-immunized 3xTg-AD mice (Fig. 6d).

pS199 tau was also reduced after DNA Aβ 42 immunotherapy: 20-month-old 3xTg-AD control mice had a mean 5341 ± 1208 pg/mg wet brain weight in the detergent-soluble fractions, and DNA Aβ 42 trimer-immunized mice had a reduced level of 3227 ± 730.5 pg/mg wet brain weight (p = 0.0012). Aβ 42 peptide-immunized mice showed no reduction (5094 ± 1246 pg/mg, p = 0.5995). Significant differences were present in the nonsoluble brain lysate fractions between female control and DNA Aβ 42 trimer-immunized mice with mean levels of 2.69 × 105 ± 5.46 × 104 pg/mg in control mice and 1.58 × 105 ± 2.32 × 104 pg/mg in DNA-immunized mice (p = 0.0007) (Fig. 6e, Table 1). In peptide-immunized mice the reduction was not significant, with 2.34 × 105 ± 6.86 × 104 pg pS199/mg (p = 0.2496).

In comparison of the two Aβ immunotherapies, a better reduction with high significance for phosphorylated tau molecules was found in DNA Aβ 42 trimer-immunized mice. Percentages of reduction were calculated for the groups, and the results are shown in Table 1. A greater than 20% higher reduction was found in the detergent-soluble brain fractions of DNA-immunized mice for pT181 and pS396. This was statistically significant in the comparison of DNA- and peptide-immunized mice for pS396 (p < 0.0311) (Table 1). For the nonsoluble brain fractions, 12–25% higher reductions were found in lysates from DNA Aβ 42 trimer-immunized mice for total tau, pT231, pT181, and pS396. These values were statistically significant for the comparisons with the age- and gender-matched control mice (Fig. 6) and also in the comparison between the differently immunized groups of mice (Table 1).

Analyses of kinase variations

Western blot analyses were performed to detect whether different enzymatic kinase patterns could be found in brains from immunized mice. Significantly reduced levels of phosphorylated MEK (MAP2K), and phosphorylated ERK1/2 (p44/p42 mitogen-activated protein kinase [MAPK]), as well as reduced levels for the activated form of GSK3β (Y216), were found in brains from DNA-immunized mice. Figure 7 shows the detection MEK1/2 and phospho-MEK1/2 (Fig. 7a), as well as ERK1/2 and phosphorylated ERK1/2 (Fig. 7b), in brain lysates from seven DNA Aβ 42 -immunized mice compared with seven age- and gender-matched 3xTg-AD control mice and two 20-month-old wild-type mice.

Fig. 7 Significant changes in enzymes of the Ras/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) signaling pathway and glycogen synthase kinase 3β (GSK3β) following DNA amyloid-β 1–42 peptide (Aβ 42 ) immunization. Equal amounts of proteins from soluble brain lysates of 20-month-old triple-transgenic Alzheimer’s disease (3xTg-AD) mice (D1–D7 = DNA Aβ 42 -immunized mice, C1–C7 = 3xTg-AD control mice, wt = wild-type control mice) were separated by SDS-PAGE, blotted onto nitrocellulose filters, and probed using antibodies specific for MEK (a, upper panel) and its active form phosphorylated MEK (a lower panel), total ERK1/2 (b, upper panel) and the phosphorylated forms of ERK1/2 (b lower panel), and GSK3α/β (c, upper panel) and activated GSK3β (c, lower panel). Of note, a blot with GSK3α/β is shown in the comparison for activated GSK3β because it appears that there was weak cross-reactivity of this specific antibody with both GSK3 bands (c, lower panel), but differences were seen only for the strong reactivity with GSK3β phosphorylated at residue Y216 (46 kD band). As a loading control, the blots were reprobed with the housekeeping protein β-tubulin (d). All assays were performed three times in independent experiments. Shown are representative results from one of these assays Full size image

Results from the semiquantitative analysis of gray-level intensities (ImageJ software) are depicted in Fig. 8. Reductions in protein levels of phospho-MEK1/2 (Fig. 8a), total ERK1/2, and phospho-ERK1/2 (Fig. 8b) in DNA Aβ 42 -immunized mice were significant (p values of 0.0379, 0.0006, and 0.0087, respectively, by Mann-Whitney U test). Significant reductions were also found for protein levels of activated GSK3β (p = 0.0006) (Fig. 8c). These data were further normalized against the protein levels of total MEK1/2, total ERK1/2, and total GSKα/β for each of the bands individually to compensate for the possibility of different overall protein levels for the tested enzymes in the brain lysates and shown as a percentage of protein (percentage of phosphorylated MEK, ERK, and GSKα/β). The percentage difference for phospho-MEK1/2 was highly significant between control and DNA Aβ 42 -immunized mice (p = 0.0031). In the comparison of phosho-ERK1/2 with total ERK1/2 in the DNA Aβ 42 -immunized mice, the reduction was not significant, because these mice already had less total ERK1/2. The percentage reduction of phospho-GSK3β (Y216) in the DNA Aβ 42 -immunized mice was highly significant (p = 0.006). No differences in protein levels between the mouse groups were observed for the proteins MEK1/2 (Fig. 7a), GSK3α/β (Fig. 7c), and the housekeeping protein β-tubulin (Fig. 7d). Of note, a blot with GSK3α/β is shown in the comparison for activated GSK3β because it appears that there is weak cross-reactivity of this specific antibody with both GSK3 bands (Fig. 7c, lower panel), but differences were seen only for the strong reactivity with GSK3β phosphorylated at residue Y216 (46 kD band). Only this activated form of GSK3β is described and discussed. No significant differences were found for total GSK3β protein levels in brain lysates from control and immunized mice (data not shown).