Allyson D. Roe

Neuronal uptake and propagation of a rare phosphorylated high-molecular-weight tau derived from Alzheimer’s disease brain

Tau pathology is known to spread in a hierarchical pattern in Alzheimers disease (AD) brain during disease progression, likely by trans-synaptic tau transfer between neurons. However, the tau species involved in inter-neuron propagation remains unclear. To identify tau species responsible for propagation, we examined uptake and propagation properties of different tau species derived from postmortem cortical extracts and brain interstitial fluid of tau-transgenic mice, as well as human AD cortices. Here we show that PBS-soluble phosphorylated high-molecular-weight (HMW) tau, though very low in abundance, is taken up, axonally transported, and passed on to synaptically connected neurons. Our findings suggest that a rare species of soluble phosphorylated HMW tau is the endogenous form of tau involved in propagation and could be a target for therapeutic intervention and biomarker development.

Removing endogenous tau does not prevent tau propagation yet reduces its neurotoxicity

In Alzheimers disease and tauopathies, tau protein aggregates into neurofibrillary tangles that progressively spread to synaptically connected brain regions. A prion‐like mechanism has been suggested: misfolded tau propagating through the brain seeds neurotoxic aggregation of soluble tau in recipient neurons. We use transgenic mice and viral tau expression to test the hypotheses that trans‐synaptic tau propagation, aggregation, and toxicity rely on the presence of endogenous soluble tau. Surprisingly, mice expressing human P301Ltau in the entorhinal cortex showed equivalent tau propagation and accumulation in recipient neurons even in the absence of endogenous tau. We then tested whether the lack of endogenous tau protects against misfolded tau aggregation and toxicity, a second prion model paradigm for tau, using P301Ltau‐overexpressing mice with severe tangle pathology and neurodegeneration. Crossed onto tau‐null background, these mice had similar tangle numbers but were protected against neurotoxicity. Therefore, misfolded tau can propagate across neural systems without requisite templated misfolding, but the absence of endogenous tau markedly blunts toxicity. These results show that tau does not strictly classify as a prion protein.

Brain interstitial oligomeric amyloid β increases with age and is resistant to clearance from brain in a mouse model of Alzheimer's disease

There is a growing body of evidence that soluble oligomeric forms of amyloid β (Aβ) play a critical role in Alzheimers disease (AD). Despite the importance of soluble Aβ oligomers as a therapeutic target for AD, the dynamic metabolism of these Aβ species in vivo has not been elucidated because of the difficulty in monitoring brain Aβ oligomers in living animals. Here, using a unique large pore‐sized membrane microdialysis, we characterized soluble Aβ oligomers in brain interstitial fluid (ISF) of awake, freely moving APP/PS1 transgenic and control WT mice. We could detect high‐molecular‐weight (HMW) and low‐molecular‐weight (LMW) Aβ oligomers in the brain ISF of living animals, which increased dramatically in an age‐dependent manner (5‐ to 8‐fold increase, 4 vs. 17‐18 mo). Notably, HMW Aβ decreased more slowly than other forms of Aβ after acute γ‐secretase inhibition [% decrease from the baseline (HMW vs. LMW) was 36.9 vs. 74.1% (Aβ40, P<0.05) and 25.4 vs. 88.0% (Aβ42, P<0.01)], suggesting that HMW Aβ oligomers clear more slowly than other forms from the brain. These data reveal the dynamic metabolism of neurotoxic Aβ oligomers in AD brain and could provide new insights into Aβ‐targeted therapies for AD.—Takeda, S., Hashimoto, T., Roe, A. D., Hori, Y., Spires‐Jones, T. L.and Hyman, B. T., Brain interstitial oligomeric amyloid β increases with age and is resistant to clearance from brain in a mouse model of Alzheimers disease. FASEB J. 27, 3239–3248 (2013). www.fasebj.org

Temporal T807 binding correlates with CSF tau and phospho-tau in normal elderly

Objective: To better understand cross-sectional relationships between CSF and PET measures of tau pathology, we compared regional and global measures of 18F-T807 (AV-1451) PET to CSF protein levels of total tau (t-tau), phosphorylated tau (p-tau), and β-amyloid 1–42 (Aβ42). Methods: T-tau, p-tau, and Aβ42 levels were assessed using INNOTEST xMAP immunoassays. Linear regression was used to compare regional and global measures of 18F-T807 standardized uptake value ratios (SUVR) to CSF protein levels using data from 31 cognitively unimpaired elderly participants in the Harvard Aging Brain study. Results: After controlling for sex and age, total cortical 18F-T807 binding was significantly correlated with p-tau (partial r = 0.48; p < 0.01) and at trend level with t-tau (partial r = 0.30; p = 0.12). Regional 18F-T807 measures were more strongly correlated with CSF protein levels than the global measure, with both t-tau and p-tau significantly correlated with 18F-T807 SUVR in entorhinal, parahippocampal, and inferior temporal cortical regions (partial r = 0.53–0.73). Peak correlations between CSF and PET measures of tau were similar to those between CSF and PET measures of amyloid burden. Finally, we observed significantly higher temporal T807 SUVR in individuals with high amyloid burden. Conclusions: These data support the link between 18F-T807 PET and CSF measures of tau pathology. In these cognitively normal individuals with 18F-T807 binding largely restricted to the temporal lobe, 18F-T807 SUVR in temporal regions appeared more reflective of CSF t-tau and p-tau than a total cortical measure.

Reversal of Neurofibrillary Tangles and Tau-Associated Phenotype in the rTgTauEC Model of Early Alzheimer's Disease

Neurofibrillary tangles (NFTs), a marker of neuronal alterations in Alzheimers disease (AD) and other tauopathies, are comprised of aggregates of hyperphosphorylated tau protein. We recently studied the formation of NFTs in the entorhinal cortex (EC) and their subsequent propagation through neural circuits in the rTgTauEC mouse model (de Calignon et al., 2012). We now examine the consequences of suppressing transgene expression with doxycycline on the NFT-associated pathological features of neuronal system deafferentation, NFT progression and propagation, and neuronal loss. At 21 months of age we observe that EC axonal lesions are associated with an abnormal sprouting response of acetylcholinesterase (AChE)-positive fibers, a phenotype reminiscent of human AD. At 24 months, NFTs progress, tau inclusions propagate to the dentate gyrus, and neuronal loss is evident. Suppression of the transgene expression from 18 to 24 months led to reversal of AChE sprouting, resolution of Gallyas-positive and Alz50-positive NFTs, and abrogation of progressive neuronal loss. These data suggest that propagation of NFTs, as well as some of the neural system consequences of NFTs, can be reversed in an animal model of NFT-associated toxicity, providing proof in principle that these lesions can be halted, even in established disease.

Seed-competent high-molecular-weight tau species accumulates in the cerebrospinal fluid of Alzheimer's disease mouse model and human patients

Cerebrospinal fluid (CSF) tau is an excellent surrogate marker for assessing neuropathological changes that occur in Alzheimers disease (AD) patients. However, whether the elevated tau in AD CSF is just a marker of neurodegeneration or, in fact, a part of the disease process is uncertain. Moreover, it is unknown how CSF tau relates to the recently described soluble high‐molecular‐weight (HMW) species that is found in the postmortem AD brain and can be taken up by neurons and seed aggregates.

Seed‐competent HMW tau species accumulates in the cerebrospinal fluid of Alzheimer's disease mouse model and human patients

Cerebrospinal fluid (CSF) tau is an excellent surrogate marker for assessing neuropathological changes that occur in Alzheimers disease (AD) patients. However, whether the elevated tau in AD CSF is just a marker of neurodegeneration or, in fact, a part of the disease process is uncertain. Moreover, it is unknown how CSF tau relates to the recently described soluble high‐molecular‐weight (HMW) species that is found in the postmortem AD brain and can be taken up by neurons and seed aggregates.

Tau protein liquid–liquid phase separation can initiate tau aggregation

The transition between soluble intrinsically disordered tau protein and aggregated tau in neurofibrillary tangles in Alzheimers disease is unknown. Here, we propose that soluble tau species can undergo liquid–liquid phase separation (LLPS) under cellular conditions and that phase‐separated tau droplets can serve as an intermediate toward tau aggregate formation. We demonstrate that phosphorylated or mutant aggregation prone recombinant tau undergoes LLPS, as does high molecular weight soluble phospho‐tau isolated from human Alzheimer brain. Droplet‐like tau can also be observed in neurons and other cells. We found that tau droplets become gel‐like in minutes, and over days start to spontaneously form thioflavin‐S‐positive tau aggregates that are competent of seeding cellular tau aggregation. Since analogous LLPS observations have been made for FUS, hnRNPA1, and TDP43, which aggregate in the context of amyotrophic lateral sclerosis, we suggest that LLPS represents a biophysical process with a role in multiple different neurodegenerative diseases.

Synaptic phosphorylated α-synuclein in dementia with Lewy bodies

Dementia with Lewy bodies is characterized by the accumulation of Lewy bodies and Lewy neurites in the CNS, both of which are composed mainly of aggregated α-synuclein phosphorylated at Ser129. Although phosphorylated α-synuclein is believed to exert toxic effects at the synapse in dementia with Lewy bodies and other α-synucleinopathies, direct evidence for the precise synaptic localization has been difficult to achieve due to the lack of adequate optical microscopic resolution to study human synapses. In the present study we applied array tomography, a microscopy technique that combines ultrathin sectioning of tissue with immunofluorescence allowing precise identification of small structures, to quantitatively investigate the synaptic phosphorylated α-synuclein pathology in dementia with Lewy bodies. We performed array tomography on human brain samples from five patients with dementia with Lewy bodies, five patients with Alzheimers disease and five healthy control subjects to analyse the presence of phosphorylated α-synuclein immunoreactivity at the synapse and their relationship with synapse size. Main analyses were performed in blocks from cingulate cortex and confirmed in blocks from the striatum of cases with dementia with Lewy bodies. A total of 1 318 700 single pre- or postsynaptic terminals were analysed. We found that phosphorylated α-synuclein is present exclusively in dementia with Lewy bodies cases, where it can be identified in the form of Lewy bodies, Lewy neurites and small aggregates (<0.16 µm3). Between 19% and 25% of phosphorylated α-synuclein deposits were found in presynaptic terminals mainly in the form of small aggregates. Synaptic terminals that co-localized with small aggregates of phosphorylated α-synuclein were significantly larger than those that did not. Finally, a gradient of phosphorylated α-synuclein aggregation in synapses (pre > pre + post > postsynaptic) was observed. These results indicate that phosphorylated α-synuclein is found at the presynaptic terminals of dementia with Lewy bodies cases mainly in the form of small phosphorylated α-synuclein aggregates that are associated with changes in synaptic morphology. Overall, our data support the notion that pathological phosphorylated α-synuclein may disrupt the structure and function of the synapse in dementia with Lewy bodies.

Characterization of TauC3 antibody and demonstration of its potential to block tau propagation

The spread of neurofibrillary tangle (NFT) pathology through the human brain is a hallmark of Alzheimer’s disease (AD), which is thought to be caused by the propagation of “seeding” competent soluble misfolded tau. “TauC3”, a C-terminally truncated form of tau that is generated by caspase-3 cleavage at D421, has previously been observed in NFTs and has been implicated in tau toxicity. Here we show that TauC3 is found in the seeding competent high molecular weight (HMW) protein fraction of human AD brain. Using a specific TauC3 antibody, we were able to substantially block the HMW tau seeding activity of human AD brain extracts in an in vitro tau seeding FRET assay. We propose that TauC3 could contribute to the templated tau misfolding that leads to NFT spread in AD brains.