TDP-43 pathology is associated with increased tau burdens and seeding.

BACKGROUND: Most Alzheimer's Disease (AD) cases also exhibit limbic predominant age-related TDP-43 encephalopathy neuropathological changes (LATE-NC), besides amyloid-ß plaques and neurofibrillary tangles (NFTs) containing hyperphosphorylated tau (p-tau). LATE-NC is characterized by cytoplasmic aggregates positive for pathological TDP-43 and is associated with more severe clinical outcomes in AD, compared to AD cases lacking TDP-43 pathology TDP-43: AD(LATE-NC-). Accumulating evidence suggests that TDP-43 and p-tau interact and exhibit pathological synergy during AD pathogenesis. However, it is not yet fully understood how the presence of TDP-43 affects p-tau aggregation in symptomatic AD. METHODS: In this study, we investigated the impact of TDP-43 proteinopathy on p-tau pathology with different approaches: histologically, in a human post-mortem cohort (n = 98), as well as functionally using a tau biosensor cell line and TDP-43A315T transgenic mice. RESULTS: We found that AD cases with comorbid LATE-NC, AD(LATE-NC+), have increased burdens of pretangles and/or NFTs as well as increased brain levels of p-tau199, compared to AD(LATE-NC-) cases and controls. The burden of TDP-43 pathology was also correlated with the Braak NFT stages. A tau biosensor cell line treated with sarkosyl-insoluble, brain-derived homogenates from AD(LATE-NC+) cases displayed exacerbated p-tau seeding, compared to control and AD(LATE-NC-)-treated cells. Consistently, TDP-43A315T mice injected with AD(LATE-NC+)-derived extracts also exhibited a more severe hippocampal seeding, compared to the remaining experimental groups, albeit no TDP-43 aggregation was observed. CONCLUSIONS: Our findings extend the current knowledge by supporting a functional synergy between TDP-43 and p-tau. We further demonstrate that TDP-43 pathology worsens p-tau aggregation in an indirect manner and increases its seeding potential, probably by increasing p-tau levels. This may ultimately contribute to tau-driven neurotoxicity and cell death. Because most AD cases present with comorbid LATE-NC, this study has an impact on the understanding of TDP-43 and tau pathogenesis in AD and LATE, which account for the majority of dementia cases worldwide. Moreover, it highlights the need for the development of a biomarker that detects TDP-43 during life, in order to properly stratify AD and LATE patients.

Beta-synuclein in cerebrospinal fluid as an early diagnostic marker of Alzheimer's disease.

OBJECTIVE: Synaptic loss plays a major role in Alzheimer's disease (AD). However so far no neurochemical marker for synaptic loss has been introduced into clinical routine. By mass spectrometry beta-synuclein was established as a candidate marker. We now aimed to set up a novel ELISA for beta-synuclein for evaluation of its potential as a diagnostic and predictive marker for AD. METHODS: We analysed in total 393 patients from four specialised centres. The diagnostic groups comprised: AD (n=151), behavioural variant frontotemporal dementia (bvFTD, n=18), Parkinson syndrome (n=46), Creutzfeldt-Jakob disease (CJD, n=23), amyotrophic lateral sclerosis (ALS, n=29), disease control (n=66) and 60 non-neurodegenerative control patients. Results were compared with core AD biomarkers (total tau, phospho-tau and amyloid-ß peptide 1-42). Additionally, coexistence of beta-synuclein with vesicular glutamate transporter 1 (VGLUT1) was determined and beta-synuclein levels were quantified in brain homogenates. RESULTS: Beta-synuclein levels quantified with the newly established ELISA correlated strongly with antibody-free quantitative mass spectrometry data (r=0.92 (95% CI: 0.89 to 0.94), p<0.0001). Cerebrospinal fluid (CSF) beta-synuclein levels were increased in AD-mild cognitive impairment (p<0.0001), AD dementia (p<0.0001) and CJD (p<0.0001), but not in bvFTD, Parkinson syndrome or ALS. Furthermore, beta-synuclein was localised in VGLUT1-positive glutamatergic synapses, and its expression was significantly reduced in brain tissue from patients with AD (p<0.01). CONCLUSION: We successfully established a sensitive and robust ELISA for the measurement of brain-enriched beta-synuclein, which we could show is localised in glutamatergic synapses. We confirmed previous, mass spectrometry-based observations of increased beta-synuclein levels in CSF of patients with AD and CJD supporting its potential use as a marker of synaptic degeneration.

Aß-induced acceleration of Alzheimer-related τ-pathology spreading and its association with prion protein.

Extracellular deposition of amyloid ß-protein (Aß) in amyloid plaques and intracellular accumulation of abnormally phosphorylated τ-protein (p-τ) in neurofibrillary tangles (NFTs) represent pathological hallmark lesions of Alzheimer's disease (AD). Both lesions develop in parallel in the human brain throughout the preclinical and clinical course of AD. Nevertheless, it is not yet clear whether there is a direct link between Aß and τ pathology or whether other proteins are involved in this process. To address this question, we crossed amyloid precursor protein (APP) transgenic mice overexpressing human APP with the Swedish mutation (670/671 KM → NL) (APP23), human wild-type APP (APP51/16), or a proenkephalin signal peptide linked to human Aß42 (APP48) with τ-transgenic mice overexpressing human mutant 4-repeat τ-protein with the P301S mutation (TAU58). In 6-month-old APP23xTAU58 and APP51/16xTAU58 mice, soluble Aß was associated with the aggravation of p-τ pathology propagation into the CA1/subiculum region, whereas 6-month-old TAU58 and APP48xTAU58 mice neither exhibited significant amounts of p-τ pathology in the CA1/subiculum region nor displayed significant levels of soluble Aß in the forebrain. In APP23xTAU58 and APP51/16xTAU58 mice showing an acceleration of p-τ propagation, Aß and p-τ were co-immunoprecipitated with cellular prion protein (PrPC). A similar interaction between PrPC, p-τ and Aß was observed in human AD brains. This association was particularly noticed in 60% of the symptomatic AD cases in our sample, suggesting that PrPC may play a role in the progression of AD pathology. An in vitro pull-down assay confirmed that PrPC is capable of interacting with Aß and p-τ. Using a proximity ligation assay, we could demonstrate proximity (less than ~ 30-40 nm distance) between PrPC and Aß and between PrPC and p-τ in APP23xTAU58 mouse brain as well as in human AD brain. Proximity between PrPC and p-τ was also seen in APP51/16xTAU58, APP48xTAU58, and TAU58 mice. Based on these findings, it is tempting to speculate that PrPC is a critical player in the interplay between Aß and p-τ propagation at least in a large group of AD cases. Preexisting p-τ pathology interacting with PrPC, thereby, appears to be a prerequisite for Aß to function as a p-τ pathology accelerator via PrPC.