Abstract Body

Abstract Body

Objectives: Independent lines of investigation have implicated tau and microglial dysfunction in the pathogenesis of Alzheimer’s disease (AD) and other disabling disorders. Whether and how these pathogenic factors interact remains to be determined. We investigated whether neural network dysfunction links tau to the innate immune system in AD-relevant contexts.

Methods: Genetically modified mouse models were analyzed by behavioral testing, electrophysiology, histopathology, biochemical assays, and transcriptomics.

Results: Like many patients with AD, human amyloid precursor protein (hAPP) transgenic mice and App knock-in mice have nonconvulsive epileptiform activity. In such mouse models, this activity correlated with peripheral cytokine levels and with changes in the hippocampal and cortical expression of inflammation-related genes, including many genes that are prominently expressed in microglia. Tau reduction, which prevents epileptiform activity in experimental models of diverse conditions, effectively prevented many of these gene expression changes in hAPP mice. Suppressing epileptiform activity with the antiepileptic drug levetiracetam reversed the aberrant expression of microglial gene products. Single-cell transcriptomics revealed both neuronal and microglial gene expression changes that predicted learning and memory deficits in these models. Ongoing studies focus on identifying the mechanisms by which tau enables neural network dysfunction and on differentiating between adaptive versus maladaptive microglial responses to aberrant neuronal activities.

Conclusions: Tau contributes to the abnormal expression of inflammation-related genes in AD-relevant contexts, at least in part, by enabling neural network dysfunction. Microglia respond to this dysfunction in ways that are either beneficial, curtailing aberrant neuronal activities, or detrimental, promoting neurodegeneration and cognitive decline.

Support: The NIH

Abstract Body

Tauopathies comprise a growing number of neurodegenerative diseases characterized by the presence of tau neuropathology. Caspase-2 (Casp2) is a cysteine aspartic protease which segregates in evolutionary trees with caspases that mediate inflammation and cell differentiation. Its only known physiological function in the adult brain is to facilitate the internalization of AMPA receptors in dendritic spines in response to low-frequency stimulation, resulting in long-term depression (LTD). In the tauopathies, Huntington’s disease, Alzheimer’s disease, and Lewy body disease, levels of Casp2 and the N-terminal tau fragment generated by Casp2 cleavage at aspartate 314 (Δtau314) are elevated. In cell and mouse models of frontotemporal dementia and Alzheimer’s disease, the cleavage of tau by Casp2, combined with its phosphorylation in the C-terminal tail and proline-rich domain, lead to the accumulation of tau in dendritic spines and the internalization of AMPA receptors, producing an excessive LTD-like synaptic state. This cleavage event may also contribute to neuron loss, as data in transgenic mice overexpressing tau variants suggest that rendering tau resistant to Casp2 cleavage confers protection from brain atrophy until 15 months of age. Taken altogether, these results suggest that abnormalities in postsynaptic excitatory transmission in tauopathies may ensue when pathological processes co-opt and overstimulate a physiological Casp2-LTD signaling pathway, and support the development of Casp2 inhibitors to regulate and repair synaptic transmission in tauopathies.

Aims

In few tauopathies such as Alzheimer’s disease (AD), tau pathology first affects a specific region before spreading to other cells following specific neural pathways. It has recently been considered that this staging could be linked to a prion-like propagation¹. According to this hypothesis, seed-competent tau species might be transferred to healthy cells and recruit endogenous tau protein leading to the formation of aggregates. Several mechanisms mediating their transfer have been highlighted such as extracellular vesicles (EVs)². In this context, we wondered if EV’s, isolated from human derived interstitial fluids (ISF), might contain tau seeds that should induce tau aggregation in recipient cells before analyzing their seeding capability in vivo.

Methods

ISF are purified from humans brains (AD, Pick’s disease (PiD), progressive supranuclear palsy (PSP) patients and controls) or mice brains (Tg30tau and WT mice). EV’s are then isolated from ISF and characterized using size-exclusion columns before transferred by lipofection to CFP-/YFP-Tau-RD(P301S) HEK293 cells³. FRET signal is measured by flow cytometry. EV’s with FRET-positive signal are injected into the hippocampus of young Tg30tau and WT mice.

Results

FRET assay shows significant FRET-positive cells treated with EV’s from Tg30tau ISF compared to cells treated with WT EV’s. Moreover, EV’s from AD ISF induce significant FRET-positive signal compared to PSP, PiD and controls. Once injected into the brain of Tau30 mice, EV’s with FRET-positive signal mediate the seeding of endogenous human tau.

Conclusions

These results support that EV’s transport seed-competent tau species that participate to the spreading of tau pathology with different process among tauopathies.

Aims

We hypothesize that tau-containing exosomes derived from Alzheimer’s affected human brains can serve as a seed for the spread of tauopathy in recipient animal brains.

Methods

Exosome-enriched fractions were isolated from unfixed frozen human brain samples from Alzheimer’s disease (AD) and control (CTRL) cases, as well as from tau knockout (TKO) mouse brains. Tau oligomer epitopes were determined by dot blot using multiple oligomeric antibodies. EVs were further examinedthe atomic force microscopy (AFM), and the efficiency for the neuronal uptake in vitro. Moreover, aged C57BL/6 mice were inoculated with human brain exosomes containing tau, comparing with the fibril or oligomeric tau, into the right dorsal hippocampus. After injection, the brains were incubated for 18 weeks. The brains were then subjected to immunohistochemistry for phosphorylated-tau (p-tau) epitopes.

Results

The inoculation of AD or prodromal AD extracellular vesicles (EVs) containing only 300 pg of tau into the OML of the DG resulted in the accumulation of abnormally phosphorylated tau by 4.5 months, whereas inoculation of an equal amount of tau from control EVs, isolated tau oligomers, or fibrils showed little tau pathology. Unexpectedly, phosphorylated tau was primarily accumulated in GABAergic interneurons and, to a lesser extent, GluR 2/3-positive excitatory mossy cells, showing preferential EV-mediated GABAergic interneuronal tau propagation. Whole-cell patch clamp recordings of CA1 pyramidal cells showed significant reduction in the amplitude of spontaneous inhibitory post-synaptic currents.

Conclusions

This is the first time comprehensively characterized the physicochemical structure and pathogenic function of human brain-derived EVs isolated from AD, prodromal AD, and non-demented control cases.

Aims

Neuropathologists defined neurodegenerative specific pathways in tauopathies suggesting the existence of vulnerable neurons. The spreading of extracellular tau species play a major role in this progression and the main remaining questions are now around the molecular species responsible for spreading. Although tau can be secreted through unconventional secretion, we focused on extracellular vesicles (EVs) found in human fluids and compare their role in the transport of seed-competent species among tauopathies.

Methods

Material: Viral vectors, cells, murine models, Human-derived materials (CSF, plasma, brain samples, ISF) from AD, PSP, PiD and controls. Methods: IHC, ELISA, EM, NTA, WB, FRET, intracranial injections.

Results

We demonstrated that 1) EVs-containing tau are secreted in many cell and animal models but also in human fluids (ISF, CSF, plasma) and 2) These secreted EVs are capable to transfer tau in neurons. The seed competency of these EVs was then investigated and, among human fluids, EVs derived from ISF are the most toxic one with a progressive loss of seeding effect from the central to the peripheral fluids. Additionally, whereas tau is detected in EVs-ISF from AD, PSP, PiD and controls, only EVs derived from AD’s ISF are highly converters.

Conclusions

To conclude, seeds from central human fluids are secreted in EVs and transmitted to receiving cells to induce tau lesions, especially in AD. Our data also suggested that secreted species or spreading mechanisms are probably different among tauopathies and identify EVs as good extracellular targets for seeds depletion in AD.

Aims

Brains of AD patients are characterized by the presence of amyloid pathology, Tau-pathology and neurodegeneration, referred to as A-, T- and N-pathology respectively. These stages develop in a characteristic spatio-temporal pattern in AD patients. In this work, we aimed to analyze the effect of amyloid pathology on Tau-seeded pathology, its propagation and subsequent neurodegeneration in crosses of TauP301S and 5xFAD mice. We thereby aimed to recapitulate aspects of the A/T/N pathology.

Methods

Using stereotaxic injections, Tau-seeding was performed in TauP301S mice in the presence or absence of amyloid pathology. The extent of Tau-pathology, propagation of Tau-pathology, and subsequent cerebral atrophy and microgliosis were measured using standard analyses.

Results

Tau-seeded Tau-pathology was significantly increased in the presence of amyloid pathology compared to absence. Tau-pathology propagated more efficiently to functionally connected brain areas remote from the injection site in the presence of amyloid pathology. Most strikingly, in the presence of amyloid pathology, Tau-seeding induced significant hippocampal and cortical atrophy, correlating with the level of Tau-pathology. Finally, microgliosis significantly increased at the different stages of the ATN axis in this model.

Conclusions

Our in vivo model displays amyloid facilitated propagation of Tau-seeded pathology and Tau-induced neurodegeneration. We here present a model robustly recapitulating the ATN pathologies, providing a tool to gain mechanistic insights in progression along the ATN axis.