Synergistic Interactions between Abeta, tau, and alpha-synuclein: acceleration of neuropathology and cognitive decline

Abstract

Alzheimer's disease (AD), the most prevalent age-related neurodegenerative disorder, is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and tau-laden neurofibrillary tangles. Interestingly, up to 50% of AD cases exhibit a third prevalent neuropathology: the aggregation of alpha-synuclein into Lewy bodies. Importantly, the presence of Lewy body pathology in AD is associated with a more aggressive disease course and accelerated cognitive dysfunction. Thus, Abeta, tau, and alpha-synuclein may interact synergistically to promote the accumulation of each other. In this study, we used a genetic approach to generate a model that exhibits the combined pathologies of AD and dementia with Lewy bodies (DLB). To achieve this goal, we introduced a mutant human alpha-synuclein transgene into 3xTg-AD mice. As occurs in human disease, transgenic mice that develop both DLB and AD pathologies (DLB-AD mice) exhibit accelerated cognitive decline associated with a dramatic enhancement of Abeta, tau, and alpha-synuclein pathologies. Our findings also provide additional evidence that the accumulation of alpha-synuclein alone can significantly disrupt cognition. Together, our data support the notion that Abeta, tau, and alpha-synuclein interact in vivo to promote the aggregation and accumulation of each other and accelerate cognitive dysfunction.

Figure 1.

Coexpression of α-syn, Aβ, and tau accelerates cognitive decline in DLB-AD mice. A, BCM acquisition. DLB-AD mice exhibit accelerated cognitive decline during BCM acquisition. Two-month-old 3xTg-AD and DLB-AD mice have significantly longer escape latencies than non-Tg and M83-h mice on days 2 and 3 of acquisition (*p < 0.05). Four-, 6-, 9-, and 12-month-old 3xTg-AD and DLB-AD are significantly worse than non-Tg and M83-h mice on all 4 d of acquisition (*p < 0.05). An interesting difference emerges between the 3xTg-AD and DLB-AD mice at 6 months of age whereby the DLB-AD mice have significantly longer group escape latency on day 4 of acquisition (^#p < 0.05). This difference is further exacerbated at 9 months of age when the DLB-AD mice have significantly longer escape latencies than age-matched 3xTg-AD mice on days 3 and 4 of acquisition (^#p < 0.05), but this difference is no longer evident at 12 months of age (p > 0.05). Interestingly, 12-month-old M83-h mice are significantly impaired compared with age-matched non-Tg mice on all 4 d of acquisition (^†p < 0.05), supporting the notion that mutant α-syn alone can disrupt cognition. B, No genotype differences were apparent on the cued BCM. Both young (2–6 months) and older (7–12 months) perform equivalently on the cued version of the BCM (p > 0.05). C, Retention trials. 3xTg-AD and DLB-AD mice have significantly longer escape latencies on both the 24 h and 7 d retention trials compared with age-matched non-Tg and M83-h mice at 4, 6, 9, and 12 months of age (*p < 0.05). 3xTg-AD and DLB-AD mice are significantly impaired compared with non-Tg and M83-h at the 24 h retention at 2 months of age but are unimpaired on the 7 d retention trial. Six-month-old DLB-AD mice are significantly impaired compared with age-matched 3xTg-AD mice at the 24 h retention trial (^#p < 0.05), and 9-month-old DLB-AD have significantly worse short- and long-term retention compared with age-matched 3xTg-AD mice (^#p < 0.05). D, Errors made during BCM acquisition. There are no significant genotype differences at 2 months of age (p > 0.05). Four-month-old 3xTg-AD and DLB-AD mice visit significantly more incorrect holes compared with age-matched non-Tg and M83-h mice (*p < 0.05). Six-month-old DLB-AD make significantly more errors compared with age-matched 3xTg-AD mice on day 4 (^#p < 0.05) and age-matched non-Tg and M83-h mice on day 4 (^†p < 0.05). Nine-month-old DLB-AD mice make significantly more errors than non-Tg and M83-h mice on days 3 and 4 (^†p < 0.05). Twelve-month-old 3xTg-AD and DLB-AD mice visit significantly more incorrect holes than age-matched non-Tg and M83-h mice (*p < 0.05). Error bars indicate SEM.

Figure 2.

DLB-AD exhibit deficits in an inhibitory avoidance task at earlier ages than 3xTg-AD, M83-h, and control mice. All genotypes are unimpaired at 2 and 4 months of age. Six-month-old DLB-AD mice cross over to the dark (shock) compartment significantly faster compared with age-matched 3xTg-AD mice (^#p < 0.05) and non-Tg/M83-h mice (^†p < 0.05) at the 1.5 h probe trial. Nine-month-old DLB-AD mice cross over to the dark (shock) compartment significantly faster compared with age-matched 3xTg-AD mice (^#p < 0.05) and non-Tg/M83-h mice (^†p < 0.05) at both the 1.5 and 24 h probe trial. Twelve-month-old 3xTg-AD and DLB-AD mice have significantly impaired short- and long-term retention compared with age-matched non-Tg and M83-h mice (*p < 0.05). Error bars indicate SEM.

Figure 3.

Aβ solubility and plaque load is increased in the DLB-AD mice. A, Detergent-soluble whole-brain Aβ₄₂ in 6-month-old DLB-AD compared with age-matched 3xTg-AD mice (*p < 0.05); no change in soluble Aβ₄₀. B, No change in detergent-insoluble Aβ₄₀ and Aβ₄₂ (p > 0.05). C, Detergent-soluble Aβ₄₀ is increased in 12-month-old DLB-AD mice compared with age-matched 3xTg-AD mice (*p < 0.05); no change in Aβ₄₂. D, Detergent-insoluble whole-brain Aβ₄₀ and Aβ₄₂ levels are significantly increased in DLB-AD mice compared with age-matched 3xTg-AD mice (*p < 0.05). E, Twelve-month-old DLB-AD mice have more plaques based on 6E10 immunoreactivity and many of these plaques are thioflavin S positive. F, Steady-state levels of APP, C99, and C83 are comparable between 12-month-old 3xTg-AD and DLB-AD mice, and quantification (G) reveals no significant differences (p > 0.05). Error bars indicate SEM.

Figure 4.

Tau solubility and phosphorylation is exacerbated in the DLB-AD mice. A, There are no differences in soluble steady-state tau levels in 6-month-old 3xTg-AD and DLB-AD. There is also no insoluble tau at this age. B, Quantification of soluble tau levels indicates there are no differences in steady-state levels (p > 0.05). C, Soluble steady-state tau levels appear to decrease in 12-month-old DLB-AD mice with a concomitant increase in detergent-insoluble tau. D, Quantification of steady-state soluble and insoluble tau indicates there is a significant decrease and increase, respectively, in 12-month-old DLB-AD mice compared with age-matched 3xTg-AD mice (*p < 0.05). E, 3xTg-AD and DLB-AD mice exhibit somatodendritic tau accumulation in the septal CA1 (I and III, respectively), and the morphology of the tau accumulation appears to be indistinguishable as shown at higher magnification (II vs IV). F, There are no AT8-postive cells in the septal CA1 of the hippocampus in the 3xTg-AD mice (shown in I), whereas there are AT8-positive tangles in the same region of the DLB-AD mice (shown in III). There is a similar disparity in the neocortex where AT8 cells are not apparent in the 3xTg-AD mice, but AT8-positive tangles are present in the DLB-AD mice (II and IV, respectively). Error bars indicate SEM.

Figure 5.

α-syn accumulation, solubility, and phosphorylation is altered in the DLB-AD mice. A, Comparison of soluble and insoluble α-syn and pS129 α-syn in 6-month-old M83-h, 3xTg-AD, and DLB-AD mice. B, Significantly more detergent-soluble α-syn in both the M83-h and DLB-AD mice compared with age-matched 3xTg-AD mice (*p < 0.05). Significantly, detergent-insoluble α-syn is present in DLB-AD mice but absent in both M83-h and 3xTg-AD mice. C, Soluble and insoluble α-syn and pS129 α-syn in 12-month-old M83-h, 3xTg-AD, and DLB-AD mice. D, Significantly more detergent-soluble α-syn in both the M83-h and DLB-AD mice compared with age-matched 3xTg-AD mice (*p < 0.05). There is also the appearance of high-molecular-weight α-syn that is only apparent in the DLB-AD mice. Insoluble α-syn is still only apparent in the DLB-AD mice (*p < 0.05). Soluble pS129 levels are significantly increased in the DLB-AD mice compared with age-matched M83-h and 3xTg-AD mice (*p < 0.05). Error bars indicate SEM.

Figure 6.

LB-like pathology is increased by coexpression of APP and tau. Normal neuropil staining of α-syn in 24-month-old M83-h brain (representative photomicrograph from the cortex) is shown in A. Twelve-, 18-, and 24-month-old DLB-AD mice develop LB-like inclusions in the cortex as depicted in B–D, respectively. Representative human LB is shown in E. Eighteen-month-old 3xTg-AD mice maintain normal neuropil staining of α-syn despite the development of plaques and AT8-positive cells (F–J). Eighteen-month-old DLB-AD mice develop LB-like deposits, plaques, and AT8-positive cells in the subiculum (K–T). Scale bar: F–I, K–N, 12 μm; A–E, J, O, 5 μm; P–S, 2.5 μm; T, 0.6 μm.