Catherine M. Cowan

Soluble hyper-phosphorylated tau causes microtubule breakdown and functionally compromises normal tau in vivo

It has been hypothesised that tau protein, when hyper-phosphorylated as in Alzheimer’s disease (AD), does not bind effectively to microtubules and is no longer able to stabilise them; thus microtubules break down, and axonal transport can no longer proceed efficiently in affected brain regions in AD and related tauopathies (tau-microtubule hypothesis). We have used Drosophila models of tauopathy to test all components of this hypothesis in vivo. We have previously shown that upon expression of human 0N3R tau in Drosophila motor neurons it becomes highly phosphorylated, resulting in disruptions to both axonal transport and synaptic function which culminate in behavioural phenotypes. We now show that the mechanism by which the human tau mediates these effects is twofold: first, as predicted by the tau-microtubule hypothesis, the highly phosphorylated tau exhibits significantly reduced binding to microtubules; and second, it participates in a pathogenic interaction with the endogenous normal Drosophila tau and sequesters it away from microtubules. This causes disruption of the microtubular cytoskeleton as evidenced by a reduction in the numbers of intact correctly-aligned microtubules and the appearance of microtubules that are not correctly oriented within the axon. These deleterious effects of human tau are phosphorylation dependent because treatment with LiCl to suppress tau phosphorylation increases microtubule binding of both human and Drosophila tau and restores cytoskeletal integrity. Notably, all these phospho-tau-mediated phenotypes occur in the absence of tau filament/neurofibrillary tangle formation or neuronal death, and may thus constitute the mechanism by which hyper-phosphorylated tau disrupts neuronal function and contributes to cognitive impairment prior to neuronal death in the early stages of tauopathies.

Aβ exacerbates the neuronal dysfunction caused by human tau expression in a Drosophila model of Alzheimer's disease

Alzheimers disease (AD) is characterised by neurofibrillary tangles composed of hyper-phosphorylated tau, and neuritic plaques composed of misfolded amyloid peptide (Abeta(42)). It is generally believed that the hyper-phosphorylated tau and oligomeric Abeta(42) are responsible for the neuronal dysfunction and cognitive impairments that underlie the early stages of AD, but the mechanism by which they interact in the pathogenic process is not clear. Mounting evidence suggests that Abeta(42) pathology lies upstream of hyper-phosphorylated tau pathology. Similarly much is being learnt about how each protein affects neuronal function. However, the impact that either pathological protein has on neuronal dysfunction caused by the other is not extensively studied. We have investigated this in a Drosophila model of AD in which we express both phosphorylated human tau (tau(wt)) and oligomeric Abeta(42). We find that expression of tau(wt) causes neuronal dysfunction by disrupting axonal transport and synaptic structure, and that this leads to behavioural impairments and reduced lifespan. Co-expression of Abeta(42) with tau(wt) increases tau phosphorylation and exacerbates all these tau-mediated phenotypes. Treatment of tau(wt)/Abeta(42) and flies with LiCl ameliorates the exacerbating effect of Abeta(42), suggesting that GSK-3beta may be involved in the mechanism by which Abeta(42) and tau(wt) interact to cause neuronal dysfunction. Conversely to the effect of Abeta(42), mimicking the wingless signalling pathway by co-expression of dishevelled with tau(wt) reduces tau phosphorylation and suppresses the tau-mediated phenotypes. It is therefore possible to speculate that the mechanism by which Abeta(42) interacts with tau in the pathogenesis of AD is by down-regulating endogenous wnt signalling.

Are tau aggregates toxic or protective in tauopathies

Aggregation of highly phosphorylated tau into aggregated forms such as filaments and neurofibrillary tangles is one of the defining pathological hallmarks of Alzheimer’s disease and other tauopathies. Hence therapeutic strategies have focused on inhibition of tau phosphorylation or disruption of aggregation. However, animal models imply that tau-mediated dysfunction and toxicity do not require aggregation but instead are caused by soluble hyper-phosphorylated tau. Over the years, our findings from a Drosophila model of tauopathy have reinforced this. We have shown that highly phosphorylated wild-type human tau causes behavioral deficits resulting from synaptic dysfunction, axonal transport disruption, and cytoskeletal destabilization in vivo. These deficits are evident in the absence of neuronal death or filament/tangle formation. Unsurprisingly, both pharmacological and genetic inhibition of GSK-3β rescue these tau phenotypes. However, GSK-3β inhibition also unexpectedly increases tau protein levels, and produces insoluble granular tau oligomers. As well as underlining the growing consensus that tau toxicity is mediated by a highly phosphorylated soluble tau species, our findings further show that not all insoluble tau aggregates are toxic. Some tau aggregates, in particular tau oligomers, are non-toxic, and may even be protective against tau toxicity in vivo. This has serious implications for emerging therapeutic strategies to dissolve tau aggregates, which might be ineffective or even counter-productive. In light of this, it is imperative to identify the key toxic tau species and to understand how it mediates dysfunction and degeneration so that the effective disease-modifying therapies can be developed.

NAP (davunetide) rescues neuronal dysfunction in a Drosophila model of tauopathy

Alzheimer’s disease (AD) is a devastating neurodegenerative disease causing irreversible cognitive decline in the elderly. There is no disease-modifying therapy for this condition and the mechanisms underpinning neuronal dysfunction and neurodegeneration are unclear. Compromised cytoskeletal integrity within neurons is reported in AD. This is believed to result from loss-of-function of the microtubule-associated protein tau, which becomes hyper-phosphorylated and deposits into neurofibrillary tangles in AD. We have developed a Drosophila model of tauopathy in which abnormal human tau mediates neuronal dysfunction characterised by microtubule destabilisation, axonal transport disruption, synaptic defects and behavioural impairments. Here we show that a microtubule-stabilising drug, NAPVSIPQ (NAP), prevents as well as reverses these phenotypes even after they have become established. Moreover, it does not alter abnormal tau levels indicating that it by-passes toxic tau altogether. Thus, microtubule stabilisation is a disease-modifying therapeutic strategy protecting against tau-mediated neuronal dysfunction, which holds great promise for tauopathies like AD.

What is the pathological significance of tau oligomers

Insoluble aggregates of the microtubule-associated protein tau characterize a number of neurodegenerative diseases collectively termed tauopathies. These aggregates comprise abnormally hyperphosphorylated and misfolded tau proteins. Research in this field has traditionally focused on understanding how hyperphosphorylated and aggregated tau mediates dysfunction and toxicity in tauopathies. Recent findings from both Drosophila and rodent models of tauopathy suggest that large insoluble aggregates such as tau filaments and tangles may not be the key toxic species in these diseases. Thus some investigators have shifted their focus to study pre-filament tau species such as tau oligomers and hyperphosphorylated tau monomers. Interestingly, tau oligomers can exist in a variety of states including hyperphosphorylated and unphosphorylated forms, which can be both soluble and insoluble. It remains to be determined which of these oligomeric states of tau are causally involved in neurodegeneration and which signal the beginning of the formation of inert/protective filaments. It will be important to better understand this so that tau-based therapeutic interventions can target the most toxic tau species.

Disruption of neuronal function by soluble hyperphosphorylated tau in a Drosophila model of tauopathy

Axonal microtubules are essential for transport of materials to the synapse. Compromised microtubules and synaptic loss have been demonstrated in AD (Alzheimers disease), which is believed to contribute to cognitive dysfunction before neuronal death in the early stages of the disease. The mechanism by which hyperphosphorylated tau, the building block of neurofibrillary tangles, one of the pathological hallmarks of AD, disrupts neuronal and synaptic function is unclear. There is a theory that hyperphosphorylated tau does not bind effectively to microtubules and is no longer able to function in stabilizing them, thus axonal transport can no longer proceed efficiently. This leads to synaptic dysfunction. We have tested this theory in a Drosophila model of tauopathies in which we expressed human tau (h-tau). Using this model, we have tested all aspects of this hypothesis and have demonstrated that axonal transport does become compromised in the presence of hyperphosphorylated h-tau and this leads to synaptic and behavioural defects. We are currently investigating the mechanism by which hyperphosphorylated h-tau mediates this effect and are preliminary data indicate that this entails phospho-tau-mediated effects that are predicted by the tau-microtubule hypothesis, as well as novel effects. These deleterious effects of h-tau occur in the absence of tau filaments and before neuronal death. This sequence of pathogenic events may constitute the mechanism by which abnormal tau disrupts neuronal and synaptic function and contributes to cognitive impairment before neuronal death in the early stages of tauopathies such as AD.

Insights from Drosophila models of Alzheimer's disease

AD (Alzheimers disease) is a neurodegenerative disorder characterized by the abnormal hyperphosphorylation and aggregation of the microtubule-associated protein tau and the misfolding and deposition of Abeta peptide. The mechanisms by which tau and Abeta become abnormal is not clearly understood, neither is it known what role either protein plays in the neurodegenerative process underlying AD. We have modelled aspects of AD in Drosophila melanogaster to shed light on these processes and to further our understanding of the relationship between tau and amyloid in this disease.

Rescue from tau-induced neuronal dysfunction produces insoluble tau oligomers

Aggregation of highly phosphorylated tau is a hallmark of Alzheimer’s disease and other tauopathies. Nevertheless, animal models demonstrate that tau-mediated dysfunction/toxicity may not require large tau aggregates but instead may be caused by soluble hyper-phosphorylated tau or by small tau oligomers. Challenging this widely held view, we use multiple techniques to show that insoluble tau oligomers form in conditions where tau-mediated dysfunction is rescued in vivo. This shows that tau oligomers are not necessarily always toxic. Furthermore, their formation correlates with increased tau levels, caused intriguingly, by either pharmacological or genetic inhibition of tau kinase glycogen-synthase-kinase-3beta (GSK-3β). Moreover, contrary to common belief, these tau oligomers were neither highly phosphorylated, and nor did they contain beta-pleated sheet structure. This may explain their lack of toxicity. Our study makes the novel observation that tau also forms non-toxic insoluble oligomers in vivo in addition to toxic oligomers, which have been reported by others. Whether these are inert or actively protective remains to be established. Nevertheless, this has wide implications for emerging therapeutic strategies such as those that target dissolution of tau oligomers as they may be ineffective or even counterproductive unless they act on the relevant toxic oligomeric tau species.

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.

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.