An evaluation of Drosophila as a model system for studying tauopathies such as Alzheimer's disease

Work spanning almost two decades using the fruit fly, Drosophila melanogaster, to study tau-mediated neurodegeneration has provided valuable and novel insights into the causes and mechanisms of tau-mediated toxicity and dysfunction in tauopathies such as Alzheimer's disease (AD). The fly has proven to be an excellent model for human diseases because of its cost efficiency, and the availability of powerful genetic tools for use in a comparatively less-complicated, but evolutionarily conserved, in vivo system. In this review, we provide a critical evaluation of the insights provided by fly models, highlighting both the advantages and limitations of the system. The fly has contributed to a greater understanding of the causes of tau abnormalities, the role of these abnormalities in mediating toxicity and/or dysfunction, and the nature of causative species mediating tau-toxicity. However, it is not possible to perfectly model all aspects of human degenerative diseases. What sets the fly apart from other animal models is its genetic tractability, which makes it highly amenable to overcoming experimental limitations. The explosion of genetic technology since the first fly disease models were established has translated into fly lines that allow for greater temporal control in restricting tau expression to single neuron types, and lines that can label and monitor the function of subcellular structures and components; thus, fly models offer an unprecedented view of the neurodegenerative process. Emerging genetic technology means that the fly provides an ever-evolving experimental platform for studying disease.</p

Microtubule stabilising peptides rescue tau phenotypes in-vivo

The microtubule cytoskeleton is a highly dynamic, filamentous network underpinning cellular structure and function. In Alzheimer’s disease, the microtubule cytoskeleton is compromised, leading to neuronal dysfunction and eventually cell death. There are currently no disease-modifying therapies to slow down or halt disease progression. However, microtubule stabilisation is a promising therapeutic strategy that is being explored. We previously investigated the disease-modifying potential of a microtubule-stabilising peptide NAP (NAPVSIPQ) in a well-established Drosophila model of tauopathy characterised by microtubule breakdown and axonal transport deficits. NAP prevented as well as reversed these phenotypes even after they had become established. In this study, we investigate the neuroprotective capabilities of an analogous peptide SAL (SALLRSIPA). We found that SAL mimicked NAP’s protective effects, by preventing axonal transport disruption and improving behavioural deficits, suggesting both NAP and SAL may act via a common mechanism. Both peptides contain a putative ‘SIP’ (Ser-Ile-Pro) domain that is important for interactions with microtubule end-binding proteins. Our data suggests this domain may be central to the microtubule stabilising function of both peptides and the mechanism by which they rescue phenotypes in this model of tauopathy. Our observations support microtubule stabilisation as a promising disease-modifying therapeutic strategy for tauopathies like Alzheimer’s disease

Modelling Tauopathies in Drosophila: Insights from the Fruit Fly

Drosophila melanogaster is an experimentally tractable model organism that has been used successfully to model aspects of many human neurodegenerative diseases. Drosophila models of tauopathy have provided valuable insights into tau-mediated mechanisms of neuronal dysfunction and death. Here we review the findings from Drosophila models of tauopathy reported over the past ten years and discuss how they have furthered our understanding of the pathogenesis of tauopathies. We also discuss the multitude of technical advantages that Drosophila offers, which make it highly attractive as a model for such studies

Tau-mediated axonal degeneration is prevented by activation of the WldS pathway

Tauopathy is characterized by neuronal dysfunction and degeneration occurring as a result of changes to the microtubule-associated protein tau. The neuronal changes evident in tauopathy bear striking morphological resemblance to those reported in models of Wallerian degeneration. The mechanisms underpinning Wallerian degeneration are not fully understood although it can be delayed by the expression of the slow Wallerian degeneration (WldS) protein, which has also been demonstrated to delay axonal degeneration in some models of neurodegenerative disease. Given the morphological similarities between tauopathy and Wallerian degeneration, this study investigated whether tau-mediated phenotypes can be modulated by co-expression of WldS. In a Drosophila model of tauopathy in which expression of human 0N3R tau protein leads to progressive age-dependent phenotypes, WldS was expressed with and without activation of the downstream pathway. The olfactory receptor neuron circuit OR47b was used for these studies in adults, and the larval motor neuron system was employed in larvae. Tau phenotypes studied included neurodegeneration, axonal transport, synaptic deficits and locomotor behaviour. Impact on total tau was ascertained by assessing total, phosphorylated and misfolded tau levels by immunohistochemistry. Activation of the pathway downstream of WldS completely suppressed tau-mediated degeneration. This protective effect was evident even if the pathway downstream of WldS was activated several weeks after tau-mediated degeneration had become established. Though total tau levels were not altered, the protected neurons displayed significantly reduced MC1 immunoreactivity suggestive of clearance of misfolded tau, as well as a trend for a decline in tau species phosphorylated at the AT8 and PHF1 epitopes. In contrast, WldS expression without activation of the downstream protective pathway did not rescue tau-mediated degeneration in adults or improve tau-mediated neuronal dysfunction including deficits in axonal transport, synaptic alterations and locomotor behaviour in tau-expressing larvae. This collectively implies that the pathway mediating the protective effect of WldS intersects with the mechanism(s) of degeneration initiated by tau and can effectively halt tau-mediated degeneration at both early and late stages. Understanding the mechanisms underpinning this protection could identify much-needed disease-modifying targets for tauopathies.</p

Distinct phenotypes of three-repeat and four-repeat human tau in a transgenic model of tauopathy.

Tau exists as six closely related protein isoforms in the adult human brain. These are generated from alternative splicing of a single mRNA transcript and they differ in the absence or presence of two N-terminal and three or four microtubule binding domains. Typically all six isoforms have been considered functionally similar. However, their differential involvement in particular tauopathies raises the possibility that there may be isoform-specific differences in physiological function and pathological role. To explore this, we have compared the phenotypes induced by the 0N3R and 0N4R isoforms in Drosophila. Expression of the 3R isoform causes more profound axonal transport defects and locomotor impairments, culminating in a shorter lifespan than the 4R isoform. In contrast, the 4R isoform leads to greater neurodegeneration and impairments in learning and memory. Furthermore, the phosphorylation patterns of the two isoforms are distinct, as is their ability to induce oxidative stress. These differences are not consequent to different expression levels and are suggestive of bona fide physiological differences in isoform biology and pathological potential. They may therefore explain isoform-specific mechanisms of tau-toxicity and the differential susceptibility of brain regions to different tauopathies

Age-related changes in Tau and Autophagy in human brain in the absence of neurodegeneration

Tau becomes abnormally hyper-phosphorylated and aggregated in tauopathies like Alzheimers disease (AD). As age is the greatest risk factor for developing AD, it is important to understand how tau protein itself, and the pathways implicated in its turnover, change during aging. We investigated age-related changes in total and phosphorylated tau in brain samples from two cohorts of cognitively normal individuals spanning 19-74 years, without overt neurodegeneration. One cohort utilised resected tissue and the other used post-mortem tissue. Total soluble tau levels declined with age in both cohorts. Phosphorylated tau was undetectable in the post-mortem tissue but was clearly evident in the resected tissue and did not undergo significant age-related change. To ascertain if the decline in soluble tau was correlated with age-related changes in autophagy, three markers of autophagy were tested but only two appeared to increase with age and the third was unchanged. This implies that in individuals who do not develop neurodegeneration, there is an age-related reduction in soluble tau which could potentially be due to age-related changes in autophagy. Thus, to explore how an age-related increase in autophagy might influence tau-mediated dysfunctions in vivo, autophagy was enhanced in a Drosophila model and all age-related tau phenotypes were significantly ameliorated. These data shed light on age-related physiological changes in proteins implicated in AD and highlights the need to study pathways that may be responsible for these changes. It also demonstrates the therapeutic potential of interventions that upregulate turnover of aggregate-prone proteins during aging

Peptide-based inhibitors of Tau aggregation as a potential therapeutic for Alzheimer’s disease and other Tauopathies

There are currently no disease altering drugs available for Tauopathies such as Alzheimer’s disease, which alone is predicted to affect ~88 million people worldwide by 2050. As Tau aggregation underpins its toxicity, aggregation inhibitors are likely to have disease-modifying potential. Guided by in-silico mutagenesis studies, we developed a potent retro-inverso peptide inhibitor of Tau aggregation, RI-AG03 [Ac-rrrrrrrrGpkyk(ac)iqvGr-NH2], based on the 306VQIVYK311 hotspot. Aggregation of recombinant Tau was reduced by &gt;90% with equimolar RI-AG03 and no fibrils were observed by EM. When added during the growth phase, RI-AG03 blocked seeded aggregation. Fluorescein-tagged RI-AG03 efficiently penetrated HEK-293 cells over 24 hours and was non-toxic at doses up to 30 μM. In transgenic Drosophila, RI-AG03 significantly improves neurodegenerative and behavioural phenotypes caused by expression of human Tau. Collectively this shows that RI-AG03 can effectively reduce Tau aggregation in vitro and block aggregation-dependent phenotypes in vivo, raising possibilities for exploring its translational potential

Curcumin as a Holistic Treatment for Tau Pathology

Global forecasts for prevalence of Alzheimer’s Disease (AD) estimate that 152.8 million people will have dementia in 2050, a sharp rise from 57.4 million in 2019 (GBD 2019). This rise can be attributable to increases in population growth and aging, but in the absence of disease-modifying therapies it poses a huge societal challenge that must be addressed urgently. One way to combat this challenge is to explore the utility of holistic treatments that may protect against AD, including traditional herbs, spices and other nutraceuticals that are pharmacologically safe, inexpensive and readily available. In this light, the spice turmeric, and its active ingredient curcumin, has been investigated as a potential holistic treatment for AD over the past 2 decades; however, promising results with animal studies have not translated to success in clinical trials. One issue is that most animal models examining the effects of curcumin and curcumin derivatives in AD have been done with a focus at ameliorating amyloid pathology. Due to the limited success of Amyloid-β-based drugs in recent clinical trials, tau-focused therapeutics provide a promising alternative. In this article, we aim to provide a clearer picture of what is currently known about the effectiveness of curcumin and curcumin derivatives to ameliorate tau pathology. Tau focused studies may help inform more successful clinical studies by placing greater emphasis on the development and optimised delivery of curcumin derivatives that more effectively target tau pathology