Developing a Drosophila–based model in which to study aspects of pathological Tau transfer and seeding

Abstract

Prion-like propagation through neuronal circuitry is believed to be the mechanism by which Tau pathology spreads throughout the brain in tauopathies such as Alzheimer's disease (AD). This is reflected in the neuropathological Braak-staging of disease and manifests in the progressive cognitive decline evident clinically. Though various synaptic proteins are implicated, the precise players and mechanism(s) mediating the trans-cellular spread of pathological Tau species remains unclear. Furthermore, although the trans-cellular spread of pathological Tau species has been demonstrated in many experimental models, the neurobiological consequences in recipient neurons are largely unknown. Moreover, in almost all such studies, the Tau species that propagates is invariably mutated or isolated from pathological fractions of brains of Tauopathy patients. However, in the majority of AD cases it is wild-type Tau that spontaneously becomes pathological and spreads in AD, with this process is accompanied by neurodegeneration. Therefore, there is a need to further understand the mechanisms of Tau spread and how wild-type Tau becomes pathological with time. Drosophila’s genetic tractability, combined with detailed mapping of connections and physiological readouts, offer an ideal organism in which to study aspects of Tau spread and seeding. Using the Gal4.UAS system a mCherry tagged Tau^0N4R construct was expressed in both large and small olfactory circuitry, creating two potential models of Tau spread. A third model type was also investigated, using the injection of an exogenously-characterised Tau species into a human Tau background to investigate the relationship between Tau seeding potential and spread. Immunohistochemistry was used on the Drosophila brains to amplify Tau signal and provide further information on conformation. Tau spread and misfolding in these brains was followed by confocal microscopy at select time points. In all potential models the spontaneous aggregation of Tau^0N4R was observed and evidence of spread beyond expressed or injected regions was seen. Expression in the small circuit lends itself to the study of Tau spread between interneurons due to the physiology of the olfactory bulb. The large circuit offers opportunities to study a system in which Tau spread has functional consequences for the organism, and spread itself can be modulated by neuronal activity mutants. Finally, injected Tau seeds were seen to rapidly spread beyond the recipient neuronal populations and showed the ability to convert naïve human Tau to a disease-relevant conformation in the fly brain. Further development of all three model types will allow for investigation into the key synaptic players and mechanism(s) underlying prion like spread. Deeper understanding of this will provide better diagnosis, treatments and prognosis to those affected by this disease.

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Identifiers

ORCID for Ben Batchelor: orcid.org/0009-0009-6722-9874

ORCID for Mariana Vargas-Caballero: orcid.org/0000-0003-2326-4001

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