Samantha B. Nicholls

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.

Amyloid accelerates tau propagation and toxicity in a model of early Alzheimer's disease

IntroductionIn early stages of Alzheimer’s disease (AD), neurofibrillary tangles (NFT) are largely restricted to the entorhinal cortex and medial temporal lobe. At later stages, when clinical symptoms generally occur, NFT involve widespread limbic and association cortices. At this point in the disease, amyloid plaques are also abundantly distributed in the cortex. This observation from human neuropathological studies led us to pose two alternative hypotheses: that amyloid in the cortex is permissive for the spread of tangles from the medial temporal lobe, or that these are co-occurring but not causally related events simply reflecting progression of AD pathology.ResultsWe now directly test the hypothesis that cortical amyloid acts as an accelerant for spreading of tangles beyond the medial temporal lobe. We crossed rTgTauEC transgenic mice that demonstrate spread of tau from entorhinal cortex to other brain structures at advanced age with APP/PS1 mice, and examined mice with either NFTs, amyloid pathology, or both. We show that concurrent amyloid deposition in the cortex 1) leads to a dramatic increase in the speed of tau propagation and an extraordinary increase in the spread of tau to distal brain regions, and 2) significantly increases tau-induced neuronal loss.ConclusionsThese data strongly support the hypothesis that cortical amyloid accelerates the spread of tangles throughout the cortex and amplifies tangle-associated neural system failure in AD.

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.

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.

Propagation of tau pathology in Alzheimer's disease: identification of novel therapeutic targets

Accumulation and aggregation of the microtubule-associated protein tau are a pathological hallmark of neurodegenerative disorders such as Alzheimer’s disease (AD). In AD, tau becomes abnormally phosphorylated and forms inclusions throughout the brain, starting in the entorhinal cortex and progressively affecting additional brain regions as the disease progresses. Formation of these inclusions is thought to lead to synapse loss and cell death. Tau is also found in the cerebrospinal fluid (CSF), and elevated levels are a biomarker for AD. Until recently, it was thought that the presence of tau in the CSF was due to the passive release of aggregated tau from dead or dying tangle-bearing neurons. However, accumulating evidence from different AD model systems suggests that tau is actively secreted and transferred between synaptically connected neurons. Transgenic mouse lines with localized expression of aggregating human tau in the entorhinal cortex have demonstrated that, as these animals age, tau becomes mislocalized from axons to cell bodies and dendrites and that human tau-positive aggregates form first in the entorhinal cortex and later in downstream projection targets. Numerous in vitro and in vivo studies have provided insight into the mechanisms by which tau may be released and internalized by neurons and have started to provide insight into how tau pathology may spread in AD. In this review, we discuss the evidence for regulated tau release and its specific uptake by neurons. Furthermore, we identify possible therapeutic targets for preventing the propagation of tau pathology, as inhibition of tau transfer may restrict development of tau tangles in a small subset of neurons affected in early stages of AD and therefore prevent widespread neuron loss and cognitive dysfunction associated with later stages of the 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.

Hypothermia mediates age-dependent increase of tau phosphorylation in db/db mice.

Accumulating evidence from epidemiological studies suggest that type 2 diabetes is linked to an increased risk of Alzheimers disease (AD). However, the consequences of type 2 diabetes on AD pathologies, such as tau hyperphosphorylation, are not well understood. Here, we evaluated the impact of type 2 diabetes on tau phosphorylation in db/db diabetic mice aged 4 and 26weeks. We found increased tau phosphorylation at the CP13 epitope correlating with a deregulation of c-Jun. N-terminal kinase (JNK) and Protein Phosphatase 2A (PP2A) in 4-week-old db/db mice. 26-week-old db/db mice displayed tau hyperphosphorylation at multiple epitopes (CP13, AT8, PHF-1), but no obvious change in kinases or phosphatases, no cleavage of tau, and no deregulation of central insulin signaling pathways. In contrast to younger animals, 26-week-old db/db mice were hypothermic and restoration of normothermia rescued phosphorylation at most epitopes. Our results suggest that, at early stages of type 2 diabetes, changes in tau phosphorylation may be due to deregulation of JNK and PP2A, while at later stages hyperphosphorylation is mostly a consequence of hypothermia. These results provide a novel link between diabetes and tau pathology, and underlie the importance of recording body temperature to better understand the relationship between diabetes and AD.

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.

Measuring caspase activity in vivo.

Caspases are a family of integral proteases playing a role in apoptosis. The importance of apoptosis in disease has made these proteases not only an attractive drug target but also a focal point for measuring apoptosis in vivo. The critical role caspases play in determining cell death has led to the development of a wide array of technologies to measure caspase activity in vivo, ranging from small molecule PET imaging reagents to fluorescent and luminescent protein-based reporters used in whole animal and cell-based applications. This chapter reviews this wide range of technologies available as well as the most appropriate applications for each reagent and the mechanism of how it measures caspase activity in vivo.