Manuela Polydoro

Propagation of Tau Pathology in a Model of Early Alzheimer's Disease

Neurofibrillary tangles advance from layer II of the entorhinal cortex (EC-II) toward limbic and association cortices as Alzheimers disease evolves. However, the mechanism involved in this hierarchical pattern of disease progression is unknown. We describe a transgenic mouse model in which overexpression of human tau P301L is restricted to EC-II. Tau pathology progresses from EC transgene-expressing neurons to neurons without detectable transgene expression, first to EC neighboring cells, followed by propagation to neurons downstream in the synaptic circuit such as the dentate gyrus, CA fields of the hippocampus, and cingulate cortex. Human tau protein spreads to these regions and coaggregates with endogenous mouse tau. With age, synaptic degeneration occurs in the entorhinal target zone and EC neurons are lost. These data suggest that a sequence of progressive misfolding of tau proteins, circuit-based transfer to new cell populations, and deafferentation induced degeneration are part of a process of tau-induced neurodegeneration.

The intersection of amyloid beta and tau in glutamatergic synaptic dysfunction and collapse in Alzheimer's disease

The synaptic connections that form between neurons during development remain plastic and able to adapt throughout the lifespan, enabling learning and memory. However, during aging and in particular in neurodegenerative diseases, synapses become dysfunctional and degenerate, contributing to dementia. In the case of Alzheimers disease (AD), synapse loss is the strongest pathological correlate of cognitive decline, indicating that synaptic degeneration plays a central role in dementia. Over the past decade, strong evidence has emerged that oligomeric forms of amyloid beta, the protein that accumulates in senile plaques in the AD brain, contribute to degeneration of synaptic structure and function. More recent data indicate that pathological forms of tau protein, which accumulate in neurofibrillary tangles in the AD brain, also cause synaptic dysfunction and loss. In this review, we will present the case that soluble forms of both amyloid beta and tau protein act at the synapse to cause neural network dysfunction, and further that these two pathological proteins may act in concert to cause synaptic pathology. These data may have wide-ranging implications for the targeting of soluble pathological proteins in neurodegenerative diseases to prevent or reverse cognitive decline.

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.

Synaptic alterations in the rTg4510 mouse model of tauopathy

Synapse loss, rather than the hallmark amyloid‐β (Aβ) plaques or tau‐filled neurofibrillary tangles (NFT), is considered the most predictive pathological feature associated with cognitive status in the Alzheimers disease (AD) brain. The role of Aβ in synapse loss is well established, but despite data linking tau to synaptic function, the role of tau in synapse loss remains largely undetermined. Here we test the hypothesis that human mutant P301L tau overexpression in a mouse model (rTg4510) will lead to age‐dependent synaptic loss and dysfunction. Using array tomography and two methods of quantification (automated, threshold‐based counting and a manual stereology‐based technique) we demonstrate that overall synapse density is maintained in the neuropil, implicating synapse loss commensurate with the cortical atrophy known to occur in this model. Multiphoton in vivo imaging reveals close to 30% loss of apical dendritic spines of individual pyramidal neurons, suggesting these cells may be particularly vulnerable to tau‐induced degeneration. Postmortem, we confirm the presence of tau in dendritic spines of rTg4510‐YFP mouse brain by array tomography. These data implicate tau‐induced loss of a subset of synapses that may be accompanied by compensatory increases in other synaptic subtypes, thereby preserving overall synapse density. Biochemical fractionation of synaptosomes from rTg4510 brain demonstrates a significant decrease in expression of several synaptic proteins, suggesting a functional deficit of remaining synapses in the rTg4510 brain. Together, these data show morphological and biochemical synaptic consequences in response to tau overexpression in the rTg4510 mouse model. J. Comp. Neurol., 521:1334–1353, 2013.

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.

Identification of Small Molecule Inhibitors of Neurite Loss Induced by Aβ peptide using High Content Screening

Background: Amyloid-β-induced degeneration of neurites is a key event in Alzheimer disease. Results: We describe NeuriteIQ high content screening platform for analysis of neurite degeneration. Conclusion: We identified multiple cyclooxygenase inhibitors and agonists of PPARγ as suppressors of Aβ-induced neurite loss. Significance: Our study demonstrates the feasibility of using NeuriteQ to discover inhibitors of neurite loss and provide a new insight into neurite degeneration. Multiple lines of evidence indicate a strong relationship between Αβ peptide-induced neurite degeneration and the progressive loss of cognitive functions in Alzheimer disease (AD) patients and in AD animal models. This prompted us to develop a high content screening assay (HCS) and Neurite Image Quantitator (NeuriteIQ) software to quantify the loss of neuronal projections induced by Aβ peptide neurons and enable us to identify new classes of neurite-protective small molecules, which may represent new leads for AD drug discovery. We identified thirty-six inhibitors of Aβ-induced neurite loss in the 1,040-compound National Institute of Neurological Disorders and Stroke (NINDS) custom collection of known bioactives and FDA approved drugs. Activity clustering showed that non-steroidal anti-inflammatory drugs (NSAIDs) were significantly enriched among the hits. Notably, NSAIDs have previously attracted significant attention as potential drugs for AD; however their mechanism of action remains controversial. Our data revealed that cyclooxygenase-2 (COX-2) expression was increased following Aβ treatment. Furthermore, multiple distinct classes of COX inhibitors efficiently blocked neurite loss in primary neurons, suggesting that increased COX activity contributes to Aβ peptide-induced neurite loss. Finally, we discovered that the detrimental effect of COX activity on neurite integrity may be mediated through the inhibition of peroxisome proliferator-activated receptor γ (PPARγ) activity. Overall, our work establishes the feasibility of identifying small molecule inhibitors of Aβ-induced neurite loss using the NeuriteIQ pipeline and provides novel insights into the mechanisms of neuroprotection by NSAIDs.

Tau-amyloid interactions in the rTgTauEC model of early Alzheimer's disease suggest amyloid-induced disruption of axonal projections and exacerbated axonal pathology.

Early observations of the patterns of neurofibrillary tangles and amyloid plaques in Alzheimers disease suggested a hierarchical vulnerability of neurons for tangles, and a widespread nonspecific pattern of plaques that nonetheless seemed to correlate with the terminal zone of tangle‐bearing neurons in some instances. The first neurofibrillary cortical lesions in Alzheimers disease occur in the entorhinal cortex, thereby disrupting the origin of the perforant pathway projection to the hippocampus, and amyloid deposits are often found in the molecular layer of the dentate gyrus, which is the terminal zone of the entorhinal cortex. We modeled these anatomical changes in a transgenic mouse model that overexpresses both P301L tau (uniquely in the medial entorhinal cortex) and mutant APP/PS1 (in a widespread distribution) to examine the anatomical consequences of early tangles, plaques, or the combination. We find that tau uniformly occupies the terminal zone of the perforant pathway in tau‐expressing mice. By contrast, the addition of amyloid deposits in this area leads to disruption of the perforant pathway terminal zone and apparent aberrant distribution of tau‐containing axons. Moreover, human P301L tau‐containing axons appear to increase the extent of dystrophic axons around plaques. Thus, the presence of amyloid deposits in the axonal terminal zone of pathological tau‐containing neurons profoundly impacts their normal connectivity. J. Comp. Neurol. 521:4236–4248, 2013.

Endogenous tau aggregates in oligodendrocytes of rTg4510 mice induced by human P301L tau.

Tau belongs to the microtubule-associated family of proteins that maintain cytoskeletal structure by regulating microtubule dynamics. In certain neurodegenerative diseases termed tauopathies, tau is abnormally phosphorylated and accumulates as filamentous inclusions. Transgenic mouse models that overexpress human tau have been widely used to investigate tau pathogenesis. Although many studies have attempted to elucidate the pathological function of transgenic human tau, it remains unknown whether endogenous mouse tau is involved in disease progression. Here we generated an mTau antibody that selectively recognizes mouse and rat tau, but not human tau. In rTg4510 tau transgenic mice, we identified a higher molecular weight mouse tau (~60-kDa) in sarkosyl-insoluble fractions. mTau antibody started to recognize intracellular aggregates and thread-like structures in 4- to 6-month-old rTg4510 mice. Tau inclusions appeared earlier, being detected in 2.5-month-old rTg4510 mice with MC1 antibody. Immunoelectron microscopy confirmed the presence of filamentous aggregates of mouse tau, which were abundant in oligodendrocytes but rare in neurons. Mouse tau inclusions in oligodendrocytes were confirmed by double-labeling with an oligodendrocyte marker. Our data indicate that mouse tau has potential aggregation properties in neurons and non-neurons. The mTau antibody will be useful for investigating the role of mouse tau in mouse models of tauopathy.