InteractiveFly: GeneBrief

tau: Biological Overview | References


Gene name - tau

Synonyms -

Cytological map position - 98A14-98A13

Function - cytoskeleton

Keywords - microtubule-associated protein, regulation of delivery of synaptic proteins, vesicular axonal transport, a major target for PAR-1 in dendritic pruning, Dendrite severing, regulation of photoreceptor development and progressive neuronal degeneration

Symbol - tau

FlyBase ID: FBgn0266579

Genetic map position - chr3R:27,639,886-27,656,814

NCBI classification - Tubulin-binding: Tau and MAP protein, tubulin-binding repeat

Cellular location - cytoplasmic



NCBI links: EntrezGene, Nucleotide, Protein (one of multiple isoforms)
Recent literature
Kadas, D., Papanikolopoulou, K., Xirou, S., Consoulas, C. and Skoulakis, E. M. C. (2019). Human Tau isoform-specific presynaptic deficits in a Drosophila central nervous system circuit. Neurobiol Dis 124: 311-321. PubMed ID: 30529489
Summary:
Accumulation of normal or mutant human Tau isoforms in Central Nervous System (CNS) neurons of vertebrate and invertebrate models underlies pathologies ranging from behavioral deficits to neurodegeneration that broadly recapitulate human Tauopathies. Although some functional differences have begun to emerge, it is still largely unclear whether normal and mutant Tau isoforms induce differential effects on the synaptic physiology of CNS neurons. This study used the oligosynaptic Giant Fiber System in the adult Drosophila CNS to address this question and reveal that 3R and 4R isoforms affect distinct synaptic parameters. Whereas 0N3R increased failure rate upon high frequency stimulation, 0N4R compromised stimulus conduction and response speed at a specific cholinergic synapse in an age-dependent manner. In contrast, accumulation of the R406W mutant of 0N4R induced mild, age-dependent conduction velocity defects. Because 0N4R and its mutant isoform are expressed equivalently, this demonstrates that the defects are not merely consequent of exogenous human Tau accumulation and suggests distinct functional properties of 3R and 4R isoforms in cholinergic presynapses.
BIOLOGICAL OVERVIEW

The mechanisms regulating synapse numbers during development and aging are essential for normal brain function and closely linked to brain disorders including dementias. Using Drosophila, this study demonstrates roles of the microtubule-associated protein Tau in regulating synapse numbers, thus unravelling an important cellular requirement of normal Tau. In this context, it was found that Tau displays a strong functional overlap with microtubule-binding spectraplakins, establishing new links between two different neurodegenerative factors. Tau and the spectraplakin Short Stop act upstream of a three-step regulatory cascade ensuring adequate delivery of synaptic proteins. This cascade involves microtubule stability as the initial trigger, JNK signalling as the central mediator, and kinesin-3 (see Drosophila Unc-103) mediated axonal transport as the key effector. This cascade acts during development (synapse formation) and aging (synapse maintenance) alike. Therefore, these findings suggest novel explanations for intellectual disability in Tau deficient individuals, as well as early synapse loss in dementias including Alzheimer's disease (Voelzmann, 2016).

The correct formation and subsequent maintenance of synapses is a key prerequisite for brain development, function and longevity. Precocious loss of synapses is observed in late onset neurodegenerative diseases including Alzheimer's disease (AD) and Frontotemporal Dementia (FTD), likely contributing to the cognitive decline and neuronal decay observed in patients. Therefore, the characterisation of mechanisms maintaining synapses during ageing would have major implications for understanding of dementias (Voelzmann, 2016).

The development of synapses and their maintenance during ageing is dependent on sustained transport of synaptic proteins from the distant soma, driven by motor proteins which trail along the bundles of microtubules in axons and dendrites. Microtubules are regulated by microtubule binding proteins which are therefore in a key position to regulate synapse formation and maintenance (Voelzmann, 2016).

Tau is a microtubule associated protein (MAP) discovered in the mid-seventies. Reduction in Tau levels has been linked to intellectual disability (Sapir, 2012) and a class of brain disorders termed 'dementias which lack distinctive histopathology' (DLDH) (Zhukareva, 2001). Tau detachment from MTs is linked to prominent neurodegenerative diseases such as Alzheimer's disease, Frontotemporal Dementia and some forms of Parkinson's disease (Kovacs, 2015). In vitro, Tau has the ability to regulate microtubule properties including stability, cross-linkage and polymerisation (Morris, 2013). Through such functions, Tau would be expected to regulate multiple aspects of neuronal cell biology, but its physiological roles are still not understood and highly debated (Morris, 2013). This might partly be due to experimental challenges posed by functional redundancy, where other MAPs are proposed to mask physiological roles of Tau (Voelzmann, 2016).

A good model in which to deal with functional redundancy is the fruit fly Drosophila melanogaster. As is ideal for studies of Tau, Drosophila neurons provide access to powerful genetics, they are readily established for research on the neuronal cytoskeleton, on neuronal transport and on synapses. Importantly, concepts and mechanisms gained from work in flies are often well conserved in higher organisms (Voelzmann, 2016).

Work in Drosophila suggested that the spectraplakin Short Stop (Shot), a large actin-MT linker molecules and potent regulators of microtubules, could display potential functional overlap with Tau during microtubule stabilisation (Alves-Silva, 2012; Prokop, 2013). This hypothesis is attractive because the well-conserved mammalian spectraplakin Dystonin is already linked to a neurodegenerative disease (type VI hereditary sensory autonomic neuropathy; OMIM #614653), and its paralogue ACF7/MACF1 plays important roles during brain development). Since ACF7 continues to be expressed in the brain, it is tempting to speculate that it might be required for neuronal maintenance (Voelzmann, 2016).

This study used Drosophila neurons, in culture and in vivo alike, to demonstrate novel roles of Tau in regulating the formation and maintenance of synapses during ageing, by coordinating the intracellular trafficking of synaptic proteins. Thus, this study shows that the role of Tau in synapse regulation occurs in functional overlap with Shot. The robust shot-tau double-mutant phenotypes enabled study of the mechanistic cascade composed of three steps: microtubule stability as the trigger, the JNK signalling pathway as the mediator and kinesin-3 mediated axonal transport of synaptic proteins as the key effector. It is propose that a new mechanism based on the loss of Tau function which could explain intellectual disability in MAPT (the human tau gene) mutant individuals and precocious synapse loss in tau-related neurodegeneration (Voelzmann, 2016).

The aim of these studies was to understand the role of endogenous Tau in neurons with particular attention to synapses. This effort was essentially aided by the finding that Tau and Shot are functionally redundant, and the subsequent incorporation of Shot into these studies. The robust phenotypes of shot-tau double-mutant neurons enabled this study to demonstrate roles of Shot-Tau during the formation and maintenance of pre-synaptic sites in axons, and unravel the underlying mechanistic cascade which involves three major steps. Firstly, the absence of Shot-Tau causes microtubule destabilisation. Secondly, this cytoskeletal stress causes aberrant JNK activity patterns with upregulation in somata and downregulation at axon tips. Thirdly, aberrant JNK activation leads to a somatic roadblock for kinesin-3 mediated transport, thus inhibiting the delivery of synaptic proteins and eventually causing synapse loss. Depending on whether the functions of Tau and/or Shot are removed during development or ageing, either the formation or the maintenance of synapses are affected, respectively (Voelzmann, 2016).

The model explaining the function of Tau and Shot in synapse establishment and maintenance by regulating intracellular transport, is supported by loss- and gain-of-function experiments, genetic interactions and cross-rescue experiments. The initial finding that shot-tau mutant neurons had reduced branch numbers, could have suggested that defects on synapse numbers is indirect. However, experiments with double knock-down in culture and in the adult brain clearly showed strong synapse reduction whilst maintaining normal branch patterns, and Unc-104 rescued synapse reduction in shot-tau mutant neurons without major increases of the branch pattern in these neurons. These results clearly demonstrate that changes in neuronal morphology are not the cause of changes in synapse number (Voelzmann, 2016).

Notably, the synaptic function of Tau described in this study for Drosophila might be conserved in higher animals or humans, since also aged Tau knock-out mice develop a reduction of synaptic proteins from the hippocampus (Ma, 2014; Voelzmann, 2016 and references therein).

These findings provide potential new mechanistic explanations for various tau related brain disorders. For example, microdeletions in the region of MAPT (the human tau gene) cause intellectual disability, and Tau's synapse-promoting roles may well contribute to this pathology. Furthermore, various tauopathies are characterised by precocious pathological loss of synapses. The currnet data suggest that loss of tau could lead to defective synapse maintenance and eventually synapse loss. For example, a prominent group of dementias which lacks distinctive histopathology (DLDH) are characterised by the loss of Tau. Further tauopathies including Alzheimer disease, typically involve hyper-phosphorylation and aggregate formation of Tau. In this scenario, there are two parallel, non-exclusive modalities through which Tau can cause pathology. Firstly, detached hyper-phosphorylated tau attains gain-of-function roles in the cytoplasm damaging neurons through a number of mechanisms. Secondly, hyper-phosphorylation of tau causes a loss-of-function condition by depleting Tau from microtubules. However, since Tau knock-out mouse models mostly failed to show significant phenotypes and the neuronal functions of endogenous tau remain little understood, the pathological importance of Tau loss from microtubules has been marginalised. The current results now re-emphasise the notion that loss of Tau from microtubules could contribute to neurodegenerative pathology and deliver mechanistic explanations (Voelzmann, 2016).

To unravel pathomechanisms caused by the loss of Tau, a combined depletion of Shot and Tau gave strong phenotypes, ideal for short-term experimental approaches. However, similar, yet milder phenotypes were found if only Tau was depleted, suggesting that the mechanisms described in this study could well contribute to slow disease progression in tauopathies. The discovery that spectraplakins are MAPs which functionally overlap with Tau, opens up new experimental avenues for Tau studies. So far, spectraplakins have been linked to the degeneration of sensory and autonomous neurons, and it remains to be elucidated whether they may have similar roles also in the brain. These results clearly hint at this possibility (Voelzmann, 2016).

The loss of Tau and/or Shot inhibits kinesin-3 mediated transport leading to accumulation of synaptic proteins in the soma of neurons. A road-block mechanism is proposed suppressing the initiation of axonal transport in somata of Shot-Tau depleted neurons, which is caused indirectly through microtubule stress and mediated by JNK (Voelzmann, 2016).

The involvement of microtubules in causing a transport block is supported by experiments using microtubule stabilising and de-stabilising drugs which rescued or mimicked the shot-tau mutant phenotypes, respectively. Similarly, axonal transport defects and cognitive deficits of PS19Tg mice (expressing the P301S mutant form of human tau) and various other mouse and fly tauopathy models were shown to be rescued by microtubule-stabilising drugs, suggesting that the mechanisms described may be conserved and relevant to disease (Voelzmann, 2016).

The somatic road-block is a novel mechanism through which the loss of Tau can interfere with the transport of synaptic proteins and provides potential explanations also for somatic accumulations of postsynaptic proteins such as PSD-95, AMPA and NMDA receptors observed in mouse tauopathy models. A likely mechanism causing a roadblock in intracellular transport could be the direct inactivation of Unc-104 or its associated adaptor proteins, for example through JNK or other kinases within its pathway. This mode of regulation has a clear precedent in kinesin-1 and its adaptor Jip which are directly phosphorylated by JNK leading to transport inhibition. Unfortunately, extensive attempts to co-immunoprecipitate JNK and Kinesin-3 were unsuccessful, leaving open for now the exact molecular mechanism (Voelzmann, 2016).

It is proposed that aberrant JNK activation downstream of microtubule destabilisation or stress is the ultimate cause for the defective delivery of synaptic proteins in Tau and/or Shot loss of function. Also in mouse, microtubule stress leads to somatic activation of the JNK pathway, suggesting this mechanism is likely to be conserved with vertebrates (Voelzmann, 2016).

The JNK pathway is emerging as a central player in neurodegenerative diseases. Its activation is prompted by various neurodegeneration risk factors including oxidative stress, inflammation, and ageing. Furthermore, JNK is activated in AD patients and in several AD models where it triggers progression of the pathology. The new link between Tau/spectraplakins, JNK and synapses proposed in this study, is therefore likely to provide mechanistic explanations for synaptic pathology observed in AD and other tauopathies (Voelzmann, 2016).

This study has delivered an important conceptual advance by revealing a new mechanistic cascade which can explain synaptic decay as the consequence of Tau loss from microtubules. Furthermore, a previously unknown functional redundancy with spectraplakins was identified as a promising new avenue for research on Tau. These findings emphasize that Tau detachment from microtubules can be an important aspect contributing to the pathology of tauopathies in parallel to roles of hyper-phosphorylated Tau in the cytoplasm. Synaptic decay, axonal transport and alterations in the JNK pathway are emerging as central players in a wider range of adult-onset neurodegenerative diseases, and here this study has aligned these factors into a concrete mechanistic cascade (Voelzmann, 2016).

PAR-1 promotes microtubule breakdown during dendrite pruning in Drosophila

Pruning of unspecific neurites is an important mechanism during neuronal morphogenesis. Drosophila sensory neurons prune their dendrites during metamorphosis. Pruning dendrites are severed in their proximal regions. Prior to severing, dendritic microtubules undergo local disassembly, and dendrites thin extensively through local endocytosis. Microtubule disassembly requires a katanin homologue, but the signals initiating microtubule breakdown are not known. This study shows that the kinase PAR-1 is required for pruning and dendritic microtubule breakdown. The data show that neurons lacking PAR-1 fail to break down dendritic microtubules, and PAR-1 is required for an increase in neuronal microtubule dynamics at the onset of metamorphosis. Mammalian PAR-1 is a known Tau kinase, and genetic interactions suggest that PAR-1 promotes microtubule breakdown largely via inhibition of Tau also in Drosophila. Finally, PAR-1 is also required for dendritic thinning, suggesting that microtubule breakdown might precede ensuing plasma membrane alterations. These results shed light on the signaling cascades and epistatic relationships involved in neurite destabilization during dendrite pruning (Herzmann, 2017).

The physiological degeneration of synapses, axons, or dendrites without loss of the parent neuron is known as pruning. Pruning is an important developmental mechanism that is used to ensure specificity of neuronal connections, and to remove developmental intermediates. While the mechanisms of neurite outgrowth and synapse formation have been studied in some detail, comparably little is known about the mechanisms underlying pruning (Herzmann, 2017).

In holometabolous insects, the nervous system is remodeled at a large scale during metamorphosis. In the peripheral nervous system (PNS) of Drosophila, several types of sensory neurons undergo either apoptosis or prune their larval processes in an ecdysone-dependent manner. The sensory class IV dendritic arborization (c4da) neurons completely and specifically prune their long and branched larval dendrites at the onset of the pupal phase, while their axons stay intact. Pruning proceeds in a stereotypical fashion: Dendrites are first severed at proximal sites close to the cell body between 5 and 10 h after puparium formation (h APF). Severed dendrites are then fragmented and phagocytosed by the epidermal cells surrounding them. First signs of dendrite pruning are visible at 2-3 h APF when dendrites start to display swellings and thinned regions in their proximal parts where they are subsequently severed. Proximal dendrites are destabilized by local disassembly of the cytoskeleton through the microtubule-severing enzyme Katanin p60-like 1 (Kat-60L1) (Lee, 2009) and possibly the actin-severing enzyme Mical. Furthermore, the plasma membrane of proximal dendrites is thinned through increased local endocytosis (Herzmann, 2017).

How proximal dendrite destabilization is orchestrated is one of the most intriguing questions in the field. Local microtubule breakdown is one of the first apparent signs of pruning before plasma membrane severing in dendrites. However, not much is known about the signals leading to microtubule breakdown. For example, Kat-60L1 is already expressed at the larval stage in c4da neurons (Stewart, 2012), opening up the question as to how it is activated temporally for dendrite pruning (Herzmann, 2017).

This study shows that the kinase PAR-1 is required for dendrite pruning and dendritic microtubule breakdown. PAR-1 is known to phosphorylate microtubule-associated proteins (MAPs) including Tau, thus leading to microtubule destabilization (Drewes, 1997). This study found that PAR-1 is required for an increase in c4da neuron microtubule dynamics during the early pupal phase. Furthermore, PAR-1 was found to interact genetically with Drosophila Tau in a manner consistent with Tau being a PAR-1 target during dendrite pruning. Tau is also known to inhibit katanin (Qiang, 2006), and this study found that PAR-1 interacts genetically with Kat-60L1. Finally, local microtubule breakdown is linked to loss of membrane stabilizing factors and dendritic membrane collapse. Thus, the results suggest a mechanism for local microtubule disassembly and the relationship between early local events during pruning (Herzmann, 2017).

The kinase PAR-1 is part of a pathway for microtubule disassembly during dendrite pruning. The data show that PAR-1 acts to enhance microtubule dynamics specifically during the early pupal phase. In the absence of PAR-1, c4da neurons accumulate stable microtubules at a time when control neurons have already degraded most of their dendritic microtubules. The genetic data suggest that Tau is a major target for PAR-1 in this process and that PAR-1 is required during pruning to remove, or inactivate Tau. It is known that Tau itself stabilizes microtubules; therefore, Tau inhibition likely serves to destabilize microtubules. Interestingly, Tau removal might also serve to activate the katanin homologue Kat-60L1 during dendrite pruning. This is an attractive possibility because Tau, but not the Futsch homolog MAP1B, has been shown to be a potent katanin inhibitor in mammalian cells (Qiang, 2006), exactly matching the observed genetic interactions with PAR-1 during dendrite pruning. Tau also becomes depleted from mammalian sensory neuron axons after trophic support withdrawal in an in vitro pruning model system (Maor-Nof, 2013). Tau depletion was not sufficient to induce pruning in mammalian sensory neurons (Maor-Nof, 2013), matching the observations in c4da neurons. However, while not sufficient, it is interesting to speculate that Tau inactivation might also be required for pruning in mammalian neurons (Herzmann, 2017).

The data suggest that PAR-1 acts specifically during the pupal phase, but PAR-1 protein levels do not seem to increase at this stage. PAR-1 can be activated through phosphorylation by upstream kinases such as LKB1. lkb1 mutants showed only mild pruning defects that likely cannot fully explain the stronger defects caused by PAR-1 downregulation. Interestingly, this study found that PAR-1 interacts genetically with ik2, another kinase required for dendrite pruning. Thus, PAR-1 activation during dendrite pruning might depend on the interplay of several kinases. Given the temporal specificity of the PAR-1 effect , it is interesting to speculate that PAR-1 might be directly activated by a ecdysone-responsive factor (Herzmann, 2017).

This study also found that loss of PAR-1 prevents several processes at the dendritic plasma membrane during the pruning process: It prevented the local loss of membrane-associated Ank2XL from proximal dendrites, abrogated Ca2+ transients, and displayed strong enhancing genetic interactions with the thinning factor Shibire. As the genetic data indicate that Tau is the primary PAR-1 target during dendrite pruning, this suggests that microtubule breakdown is required for these plasma membrane alterations. In this scenario, the data actually suggest that microtubule disruption is closely linked to plasma membrane alterations, such that it is interesting to speculate that microtubule loss might trigger local endocytosis and thinning formation during dendrite pruning. Thus, a model is proposed where PAR-1, via Tau and possibly Kat-60L1, promotes microtubule disruption. In this model, these processes are placed epistatically over plasma membrane alterations during dendrite pruning (Herzmann, 2017).

Spatial regulation of microtubule disruption during dendrite pruning in Drosophila

Large scale neurite pruning is an important specificity mechanism during neuronal morphogenesis. Drosophila sensory neurons prune their larval dendrites during metamorphosis. Pruning dendrites are severed in their proximal regions, but how this spatial information is encoded is not clear. Dendrite severing is preceded by local breakdown of dendritic microtubules through PAR-1-mediated inhibition of Tau. This study investigated spatial aspects of microtubule breakdown during dendrite pruning. Live imaging of fluorescently tagged tubulin shows that microtubule breakdown first occurs at proximal dendritic branchpoints, followed by breakdown at more distal branchpoints, suggesting that the process is triggered by a signal emanating from the soma. In fly dendrites, microtubules are arranged in uniformly oriented arrays where all plus ends face towards the soma. Mutants in kinesin-1 and -2, which are required for uniform microtubule orientation, cause defects in microtubule breakdown and dendrite pruning. These data suggest that the local microtubule organization at branch points determines where microtubule breakdown occurs. Local microtubule organization may therefore contribute spatial information for severing sites during dendrite pruning (Herzmann, 2018).

The physiological degeneration of synapses, axons or dendrites without loss of the parent neuron is known as pruning. Pruning is an important developmental mechanism that is used to ensure specificity of neuronal connections and to remove developmental intermediates. Whereas the mechanisms of neurite outgrowth and synapse formation have been studied in some detail, comparably little is known about the mechanisms underlying pruning (Herzmann, 2018).

In holometabolous insects, the nervous system is remodeled on a large scale during metamorphosis. In the peripheral nervous system (PNS) of Drosophila, several types of sensory neurons undergo either apoptosis or prune their larval processes in an ecdysone-dependent manner. The sensory class IV dendritic arborization (c4da) neurons completely and specifically prune their long and branched larval dendrites at the onset of the pupal phase by a degenerative mechanism, while their axons remain intact. Dendrite pruning is induced in a cell-autonomous fashion by the steroid hormone ecdysone and proceeds in a stereotypical fashion. Dendrites are first severed at proximal sites close to the cell body between 5 and 10 h after puparium formation (APF). Severed dendrites are then fragmented and phagocytosed by the epidermal cells surrounding them. First signs of dendrite pruning are visible at 2-3 h APF, when proximal dendrites take on an irregular appearance with beadings and thinnings. In these regions, microtubules are disassembled locally. Plasma membrane retrieval through increased local endocytosis also contributes to thinning of proximal dendrites. Live imaging and genetic data suggest that local microtubule breakdown precedes membrane thinning. Microtubule disassembly is therefore the earliest known local sign of dendrite pruning. Microtubule disassembly requires the kinase PAR-1, which mediates inhibition of the microtubule-associated protein Tau. Furthermore, dendrite pruning also requires the microtubule-severing enzyme Katanin p60-like 1 (Kat-60L1), possibly also downstream of PAR-1 and Tau (Herzmann, 2018).

Microtubules are polar rods with so-called 'plus' and 'minus' ends. This nomenclature reflects the fact that the rates of both growth and shrinkage are greater at the plus ends. Microtubules in larval c4da neuron dendrites are oriented uniformly with their plus ends facing the soma. This uniform orientation is particularly prevalent in the primary and secondary dendrites, while higher order dendrites can also have microtubules with mixed orientation. Uniform dendritic plus-end-in microtubule orientation is known to depend on plus-end-directed motors of the kinesin family. In c4da neurons, uniform plus-end-in orientation depends on kinesin-2, which can bind to microtubule plus ends via EB1 and thereby promotes their orientation. In C. elegans, kinesin-1 was shown to be required for plus-end-in dendritic microtubule orientation. Kinesin-1 cannot bind to plus ends, and it was suggested that it instead moved wrongly oriented microtubules out of dendrites via microtubule sliding, or that it could be anchored to the dendritic cell cortex and could thus move microtubules out of the dendrite via its motor domain (Herzmann, 2018).

Given that microtubule breakdown occurs very early during the pruning process in a spatially confined manner, it is likely that this process involves spatial cues for pruning, such as the restriction of the destructive process to proximal over distal dendrites, or the sparing of the axon. This study addressed the apparent spatial regulation of microtubule disassembly during dendrite pruning by live imaging of fluorescently marked tubulin markers as well as in genetic studies. Gaps in dendritic microtubules were found to occur first at proximal dendrite branchpoints and later at more distal ones. Kinesin-2 is required for efficient c4da neuron dendrite pruning and microtubule disassembly, and EB1 manipulation was found to modulate the phenotypes associated with other microtubule disassembly factors, indicating that the uniform plus-end-in orientation of dendritic microtubules is also required for efficient microtubule breakdown and dendrite pruning. This is further supported by the observation that mutation of kinesin-1 also affects dendritic microtubule orientation, disassembly, and dendrite pruning in c4da neurons. It is proposed that microtubule disassembly depends on a signal emanating from the soma, and that the local microtubule organization at dendrite branchpoints, including the uniform plus-end-in orientation, favors microtubule disassembly there. The data indicate that dendritic microtubule organization may represent an important spatial cue for severing site selection during dendrite pruning (Herzmann, 2018).

Microtubule disassembly is the earliest known local destabilizing process during c4da neuron dendrite pruning and therefore a likely candidate to determine proximal severing. This study has begun to address the spatial regulation of microtubule disassembly during c4da neuron dendrite pruning. Using time-lapse live imaging of fluorescently tagged tubulin, it was found that gaps in the fluorescent microtubule signal develop preferentially at branchpoints of primary dendrites, and usually extend into the smaller side branches, rather than in the primary branch. Moreover, the temporal occurrence of these gaps correlates with proximity to the soma, i.e. microtubule gaps initially develop at the first branchpoint and later at the more distal branchpoints. These observations are consistent with a microtubule-destabilizing signal emanating from the soma. Previous work has shown that microtubule disassembly is induced by PAR-1-mediated Tau inhibition (Herzmann, 2017). PAR-1 activity is often induced by phosphorylation. Previous data suggested that PAR-1 is specifically activated for microtubule disassembly at the onset of the pupal phase, possibly by ecdysone (Herzmann, 2017). It is therefore plausible that the signal emanating from the soma is activated PAR-1. In such a model, activation of PAR-1 in the soma would contribute to the preference for microtubule disruption (and later dendrite severing) in proximal dendrites (Herzmann, 2018).

This study also found that the uniform plus-end-in orientation of dendritic microtubules is required for efficient microtubule disassembly and dendrite pruning. kinesin-2 and kinesin-1, both affecting dendritic microtubule orientation, are required for dendrite pruning. Manipulation of EB1, which caused less severe dendrite pruning defects by itself, strongly modified the defects seen upon manipulation of the microtubule dynamics regulator PAR-1. Interestingly, microtubule orientation (and hence the localization of the outermost microtubule plus ends) correlates well with the sites of degeneration and the degeneration mode in several models of large-scale pruning. For example, degeneration often starts from the distal end in axons, where microtubules have uniform plus-end-out orientation. As in c4da neurons, microtubule disassembly is an early event during large-scale axon pruning and is therefore likely to carry intrinsic spatial information crucial for pruning as well. This was shown to be the case in pruning axons of Drosophila mushroom body γ neurons and for pruning motoneuron axons at the mammalian neuromuscular junction. These axons retract, i.e. they shrink from their distal ends. Intriguingly, and in an exactly opposite manner to c4da neuron dendrites, these axons degenerate from the distal tips, and degeneration stops at the next branchpoint, again implying input from neurite branchpoints in the spatial regulation of large-scale pruning (Herzmann, 2018).

What could be special about microtubules at branchpoints? Although branchpoints could be localization sites for spatial pruning regulators, it is interesting to speculate that the local microtubule organization at branchpoints might suffice. For dendrites, uniformly oriented microtubules within the microtubule array would presumably enable the formation of larger gaps if the plus ends of the array microtubules have similar positions. Alternatively, microtubules at branchpoints could be more likely to be bent, and might therefore be more susceptible to shrinkage or catastrophe. Also, the overlap between the microtubule arrays of the primary and side branches may be smaller than that within an array in a straight branch, enabling gaps to occur at these sites even with relatively little microtubule shrinkage. Of note, previous data suggest that PAR-1 acts through inhibition of Tau to promote microtubule breakdown (Herzmann, 2017), and Tau was recently shown to promote microtubule bundling. Thus, PAR-1 activation might lead to 'unbundling' of microtubules at branchpoints. Furthermore, it could also be speculated that local concentrations of activated PAR-1 might increase as the diameter of the dendrite side branch decreases, which would also favor microtubule disassembly at proximal branchpoints. In order to distinguish between these models, it will be important to better characterize the local microtubule organization at dendritic branchpoints (Herzmann, 2018).

Previously it has been proposed that microtubule disassembly acts upstream of membrane thinning. Dendrite thinning was observed to be ~55% during the time of microtubule breakdown. Importantly, dendrites can often thin out to ~20% of their original diameter before rupturing, indicating that microtubule loss correlates with the onset of dendrite thinning (Herzmann, 2018).

Taken together, these data reveal that the spatial regulation of neurite pruning depends on the local microtubule organization. The data are consistent with a model in which microtubule disassembly carries the spatial information for pruning and therefore contributes to the selection of both degeneration mode and severing sites (Herzmann, 2018).

Vesicular axonal transport is modified in vivo by Tau deletion or overexpression in Drosophila

Structural microtubule associated protein Tau is found in high amount in axons and is involved in several neurodegenerative diseases. Although many studies have highlighted the toxicity of an excess of Tau in neurons, the in vivo understanding of the endogenous role of Tau in axon morphology and physiology is poor. Indeed, knock-out mice display no strong cytoskeleton or axonal transport phenotype, probably because of some important functional redundancy with other microtubule-associated proteins (MAPs). Here, advantage was taken of the model organism Drosophila, which has only one homologue of the Tau/MAP2/MAP4 family, to decipher (endogenous) Tau functions. Tau depletion was found to lead to a decrease in microtubule number and microtubule density within axons, while Tau excess leads to the opposite phenotypes. Analysis of vesicular transport in tau mutants showed altered mobility of vesicles, but no change in the total amount of putatively mobile vesicles, whereas both aspects were affected when Tau was overexpressed. In conclusion, this study shows that loss of Tau in tau mutants not only leads to a decrease in axonal microtubule density, but also impairs axonal vesicular transport, albeit to a lesser extent compared to the effects of an excess of Tau (Talmat-Amar, 2018).

Deletion of endogenous Tau proteins is not detrimental in Drosophila
Human Tau (hTau) is a highly soluble and natively unfolded protein that binds to microtubules within neurons. Its dysfunction and aggregation into insoluble paired helical filaments is involved in the pathogenesis of Alzheimer's disease (AD), constituting, together with accumulated β-amyloid (Aβ) peptides, a hallmark of the disease. Deciphering both the loss-of-function and toxic gain-of-function of hTau proteins is crucial to further understand the mechanisms leading to neurodegeneration in AD. As the fruit fly Drosophila melanogaster expresses Tau proteins (dTau) that are homologous to hTau, this study aimed to better comprehend dTau functions by generating a specific tau knock-out (KO) fly line using homologous recombination. It was observed that the specific removal of endogenous dTau proteins does not lead to overt, macroscopic phenotypes in flies. Indeed, survival, climbing ability and neuronal function are unchanged in tau KO flies. In addition, any overt positive or negative effect of dTau removal on human Aβ-induced toxicity were not found. Altogether, these results indicate that the absence of dTau proteins has no major functional impact on flies, and suggest that the tau KO strain is a relevant model to further investigate the role of dTau proteins in vivo, thereby giving additional insights into hTau functions (Burnouf, 2016).

Loss of Tau results in defects in photoreceptor development and progressive neuronal degeneration in Drosophila.

Accumulations of Tau, a microtubule-associated protein (MAP), into neurofibrillary tangles is a hallmark of Alzheimer's disease and other tauopathies. However, the mechanisms leading to this pathology are still unclear: the aggregates themselves could be toxic or the sequestration of Tau into tangles might prevent Tau from fulfilling its normal functions, thereby inducing a loss of function defect. Surprisingly, the consequences of losing normal Tau expression in vivo are still not well understood, in part due to the fact that Tau knockout mice show only subtle phenotypes, presumably due to the fact that mammals express several MAPs with partially overlapping functions. In contrast, flies express fewer MAP, with Tau being the only member of the Tau/MAP2/MAP4 family. Therefore, this study used Drosophila to address the physiological consequences caused by the loss of Tau. Reducing the levels of fly Tau (dTau) ubiquitously resulted in developmental lethality, whereas deleting Tau specifically in neurons or the eye caused progressive neurodegeneration. Similarly, chromosomal mutations affecting dTau also caused progressive degeneration in both the eye and brain. Although photoreceptor cells initially developed normally in dTau knockdown animals, they subsequently degenerated during late pupal stages whereas weaker dTau alleles caused an age-dependent defect in rhabdomere structure. Expression of wild type human Tau partially rescued the neurodegenerative phenotype caused by the loss of endogenous dTau, suggesting that the functions of Tau proteins are functionally conserved from flies to humans (Bolkan, 2014).

Identification and characterization of the Drosophila tau homolog

A pathological hallmark of neurodegenerative tauopathies, including Alzheimer's disease and a group of clinically heterogeneous frontotemporal dementias, is the presence of intracellular neurofibrillary protein lesions. The principal component of these structures is the microtubule-associated protein tau. Although tau is normally a highly soluble protein enriched in axons, in these deposits, it is abnormally hyperphosphorylated, insoluble, and redistributed to the somatodendritic compartments of neurons. Through ultrastructual analyses, it has been determined that the tau protein in these lesions is filamentous and organized into paired-helical filaments, straight filaments, or ribbon-like filaments. By the dynamic binding of microtubules, tau is thought to promote the structural stability of axons, but whether tau aggregates contribute to neurodegeneration through a direct toxicity on normal cellular functions such as organelle transport or an indirect effect on microtubule stability, is currently unknown. The identification of mutations in the tau locus in patients with familial frontotemporal dementia and Parkinsonism linked to chromosome 17 has demonstrated that mutations in tau are sufficient to cause neurodegenerative disease). To elucidate the mechanisms by which tau dysfunction contributes to neuronal loss, this study sought to model human tauopathies in a genetically tractable organism. This study describes the isolation of a Drosophila tau cDNA, the production of antibodies that recognize the encoded protein, and their use in determining the expression and subcellular localization of the fly tau protein (Heidary, 2001).

Drosophila tauopathy models

Differential effects of 14-3-3 dimers on Tau phosphorylation, stability and toxicity in vivo

Tauopathies involve aberrant phosphorylation and aggregation of the neuronal protein Tau. The largely neuronal 14-3-3 proteins are also elevated in the Central Nervous System (CNS) and Cerebrospinal Fluid of Tauopathy patients, suggesting functional linkage. This study used the Drosophila system to investigate in vivo whether 14-3-3s are causal or synergistic with Tau accumulation in precipitating pathogenesis. Both Drosophila 14-3-3 proteins interact with human wild type and mutant Tau on multiple sites irrespective of their phosphorylation state. 14-3-3 dimers regulate steady state phosphorylation of both Wild Type and the R406W mutant Tau, but they are not essential for toxicity of either variant. Moreover, 14-3-3 elevation itself is not pathogenic, but recruitment of dimers on accumulating Wild Type Tau increases its steady state levels ostensibly by occluding access to proteases in a phosphorylation-dependent manner. In contrast, the R406W mutant, which lacks a putative 14-3-3 binding site, responds differentially to elevation of each 14-3-3 isoform. Although excess 14-3-3zeta stabilizes the mutant protein, elevated D14-3-3epsilon has a destabilizing effect probably because of altered 14-3-3 dimer composition. These collective data demonstrate the complexity of 14-3-3/Tau interactions in vivo and suggest that 14-3-3 attenuation is not appropriate ameliorative treatment of Tauopathies. Finally, it is suggested that 'bystander' 14-3-3s are recruited by accumulating Tau with the consequences depending on the composition of available dimers within particular neurons and the Tau variant (Papanikolopoulou, 2018).

Tau-induced nuclear envelope invagination causes a toxic accumulation of mRNA in Drosophila

The nucleus is a spherical dual-membrane bound organelle that encapsulates genomic DNA. In eukaryotes, messenger RNAs (mRNA) are transcribed in the nucleus and transported through nuclear pores into the cytoplasm for translation into protein. In certain cell types and pathological conditions, nuclei harbor tubular invaginations of the nuclear envelope known as the "nucleoplasmic reticulum." Nucleoplasmic reticulum expansion has recently been established as a mediator of neurodegeneration in tauopathies, including Alzheimer's disease. While the presence of pore-lined, cytoplasm-filled, nuclear envelope invaginations has been proposed to facilitate the rapid export of RNAs from the nucleus to the cytoplasm, the functional significance of nuclear envelope invaginations in regard to RNA export in any disorder is currently unknown. This study reports that polyadenylated RNAs accumulate within and adjacent to tau-induced nuclear envelope invaginations in a Drosophila model of tauopathy. Genetic or pharmacologic inhibition of RNA export machinery reduces accumulation of polyadenylated RNA within and adjacent to nuclear envelope invaginations and reduces tau-induced neuronal death. These data are the first to point toward a possible role for RNA export through nuclear envelope invaginations in the pathogenesis of a neurodegenerative disorder and suggest that nucleocytoplasmic transport machinery may serve as a possible novel class of therapeutic targets for the treatment of tauopathies (Cornelison, 2018).

Beta-sheet assembly of Tau and neurodegeneration in Drosophila melanogaster

The assembly of Tau into abundant beta-sheet-rich filaments characterizes human tauopathies. A pathological pathway leading from monomeric to filamentous Tau is believed to be at the heart of these diseases. However, in Drosophila models of Tauopathy, neurodegeneration has been observed in the absence of abundant Tau filaments. This study investigated the role of Tau assembly into beta-sheets by expressing wild-type and Delta306-311 human Tau-383 in the retina and brain of Drosophila. Both lines were examined for eye abnormalities, brain vacuolization, Tau phosphorylation and assembly, as well as climbing activity and survival. Flies expressing wild-type Tau-383 showed MC-1 staining, Tau hyperphosphorylation, and neurodegeneration. By contrast, flies expressing Delta306-311 Tau-383 had less MC-1 staining, reduced Tau hyperphosphorylation, and no detectable neurodegeneration. Their climbing ability and lifespan were similar to those of nontransgenic flies. Fluorescence spectroscopy after addition of Thioflavin T, a dye that interacts with beta-sheets, showed no signal when Delta306-311 Tau-383 was incubated with heparin. These findings demonstrate that the assembly of Tau into beta-sheets is necessary for neurodegeneration (Passarella, 2018).

Developmental Expression of 4-Repeat-Tau Induces Neuronal Aneuploidy in Drosophila Tauopathy Models

Tau-mediated neurodegeneration in Alzheimer's disease and tauopathies is generally assumed to start in a normally developed brain. However, several lines of evidence suggest that impaired Tau isoform expression during development could affect mitosis and ploidy in post-mitotic differentiated tissue. Interestingly, the relative expression levels of Tau isoforms containing either 3 (3R-Tau) or 4 repeats (4R-Tau) play an important role both during brain development and neurodegeneration. This study used genetic and cellular tools to study the link between 3R and 4R-Tau isoform expression, mitotic progression in neuronal progenitors and post-mitotic neuronal survival. The results illustrated that the severity of Tau-induced adult phenotypes depends on 4R-Tau isoform expression during development. As recently described, a mitotic delay was observed in 4R-Tau expressing cells of larval eye discs and brains. Live imaging revealed that the spindle undergoes a cycle of collapse and recovery before proceeding to anaphase. Furthermore, a high level of aneuploidy was found in post-mitotic differentiated tissue. Finally, it was shown that overexpression of wild type and mutant 4R-Tau isoform in neuroblastoma SH-SY5Y cell lines is sufficient to induce monopolar spindles. Taken together, these results suggested that neurodegeneration could be in part linked to neuronal aneuploidy caused by 4R-Tau expression during brain development (Malmanche, 2017).

A conserved cytoskeletal signaling cascade mediates neurotoxicity of FTDP-17 tau mutations in vivo

The microtubule binding protein tau is strongly implicated in multiple neurodegenerative disorders, including frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), which is caused by mutations in tau. In vitro, FTDP-17 mutant versions of tau can reduce microtubule binding and increase aggregation of tau, but the mechanism by which these mutations promote disease in vivo is not clear. This study took a combined biochemical and in vivo modeling approach to define functional properties of tau driving neurotoxicity in vivo. Wild type human tau and five FTDP-17 mutant forms of tau were expressed in Drosophila using a site-directed insertion strategy to ensure equivalent levels of expression. Multiple markers of neurodegeneration and neurotoxicity were analyzed in transgenic animals, including analysis of both males and females. FTDP-17 mutations act to enhance phosphorylation of tau and thus promote neurotoxicity in an in vivo setting. Further, it was demonstrated that phosphorylation-dependent excess stabilization of the actin cytoskeleton is a key phosphorylation-dependent mediator of the toxicity of wild type tau, and of all the FTDP-17 mutants tested. Finally, it was shown that important downstream pathways, including autophagy and the unfolded protein response, are co-regulated with neurotoxicity and actin cytoskeletal stabilization in brains of flies expressing wild type human and various FTDP-17 tau mutants, supporting a conserved mechanism of neurotoxicity of wild type tau and FTDP-17 mutant tau in disease pathogenesis (Bardai, 2017).

An AMPK-dependent regulatory pathway in tau-mediated toxicity

Neurodegenerative tauopathies are characterized by accumulation of hyperphosphorylated tau aggregates primarily degraded by autophagy. The 5'AMP-activated protein kinase (AMPK) is expressed in most cells, including neurons. Alongside its metabolic functions, it is also known to be activated in Alzheimer's brains, phosphorylate tau, and be a critical autophagy activator. While stress conditions can result in AMPK activation enhancing tau-mediated toxicity, AMPK activation is not always concomitant with autophagic induction. Using a Drosophila in vivo quantitative approach, this study has analysed the impact of AMPK and autophagy on tau-mediated toxicity, recapitulating the AMPK-mediated tauopathy condition: increased tau phosphorylation, without corresponding autophagy activation. It was demonstrated that AMPK, binding to and phosphorylating tau at Ser-262, a site reported to facilitate soluble tau accumulation, affects its degradation. This phosphorylation results in exacerbation of tau toxicity and is ameliorated via rapamycin-induced autophagy stimulation. These findings support the development of combinatorial therapies effective at reducing tau toxicity targeting tau phosphorylation and AMPK-independent autophagic induction. The proposed in vivo tool represents an ideal readout to perform preliminary screening for drugs promoting this process (Galasso, 2017).

Altered protein glycosylation predicts Alzheimer's disease and modulates its pathology in disease model Drosophila

The pathological hallmarks of Alzheimer's disease (AD) are pathogenic oligomers and fibrils of misfolded amyloidogenic proteins (e.g., beta-amyloid and hyper-phosphorylated tau in AD), which cause progressive loss of neurons in the brain and nervous system. In an analysis of available expression data sets this study indicates that many glycosylation-related genes are differentially expressed in brains of AD patients compared with healthy controls. The robust differences found enabled prediction of the occurrence of AD with remarkable accuracy in a test cohort and identification of a set of key genes whose expression determines this classification. Then the effect of reducing expression of homologs of 6 of these genes in was studied in transgenic Drosophila overexpressing human tau, a well-established invertebrate AD model. These experiments have led to the identification of glycosylation genes that may augment or ameliorate tauopathy phenotypes. These results indicate that OstDelta, l(2)not and beta4GalT7 are tauopathy suppressors, whereas pgnat5 and CG33303 are enhancers, of tauopathy. These results suggest that specific alterations in protein glycosylation may play a causal role in AD etiology (Frenkel-Pinter, 2017).

Astrocyte transport of glutamate and neuronal activity reciprocally modulate tau pathology in Drosophila

Abnormal buildup of the microtubule associated protein tau is a major pathological hallmark of Alzheimer's disease (AD) and various tauopathies. The mechanisms by which pathological tau accumulates and spreads throughout the brain remain largely unknown. It is known that a restoration of the major astrocytic glutamate transporter, GLT1, ameliorates a buildup of tau pathology and rescues cognition in a mouse model of AD. In this study, it was hypothesized that aberrant extracellular glutamate and abnormal neuronal excitatory activities promote tau pathology. Consequently, the genetic interactions between tau and the GLT1 homolog dEaat1 were investigated in Drosophila melanogaster. Neuronal-specific overexpression of human wildtype tau markedly shortens lifespan and impairs motor behavior. RNAi depletion of dEaat1 in astrocytes worsens these phenotypes, whereas overexpression of dEaat1 improves them. However, the synaptic neuropil appears unaffected, and there is no major neuronal loss with tau overexpression in combination with dEaat1 depletion. To mimic glutamate-induced aberrant excitatory input in neurons, repeated depolarization of neurons via transgenic TrpA1 was applied to the adult Drosophila optic nerves, and the change of tau deposits was examined. Repeated depolarization significantly increases the accumulation of tau in these neurons. The study propose that increased neuronal excitatory activity exacerbates tau-mediated neuronal toxicity and behavioral deficits (Kilian, 2017).

Salidroside reduces tau hyperphosphorylation via up-regulating GSK-3β phosphorylation in a tau transgenic Drosophila model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related and progressive neurodegenerative disease that causes substantial public health care burdens. Intensive efforts have been made to find effective and safe treatment against AD. The plant product Salidroside (Sal) is the main effective component of Rhodiola rosea L., which has several pharmacological activities. The objective of this study was to investigate the efficacy of Sal in the treatment of AD transgenic Drosophila and the associated mechanisms. Microtubule associated protein tau transgenic Drosophila line (TAU) was used in which tau protein is expressed in the central nervous system and eyes by the Gal4/UAS system. After feeding flies with Sal, the lifespan and locomotor activity were recorded. The appearance of vacuoles in the mushroom body was examined using immunohistochemistry, and the levels of total glycogen synthase kinase 3β (t-GSK-3β), phosphorylated GSK-3β (p-GSK-3β), t-tau and p-tau was detected in the brain by western blot analysis. The results showed that the longevity was improved in salidroside-fed Drosophila groups as well as the locomotor activity. Less vacuoles in the mushroom body, upregulated level of p-GSK-3β and downregulated p-tau were detected following Sal treatment. These data presented the evidence that Sal was capable of reducing the neurodegeneration in tau transgenic Drosophila and inhibiting neuronal loss. The neuroprotective effects of Sal were associated with its up-regulation of the p-GSK-3β and down-regulation of the p-tau (Zhang, 2016).

Stabilization of microtubule-unbound Tau via Tau phosphorylation at Ser262/356 by Par-1/MARK contributes to augmentation of AD-related phosphorylation and Aβ42-induced Tau toxicity

To prevent the cascade of events leading to neurodegeneration in Alzheimer's disease (AD), it is essential to elucidate the mechanisms underlying the initial events of tau mismetabolism. In this study, using transgenic Drosophila co-expressing human tau and Aβ, tau phosphorylation at AD-related Ser262/356 stabilized microtubule-unbound tau was found in the early phase of tau mismetabolism, leading to neurodegeneration. Aβ increased the level of tau detached from microtubules, independent of the phosphorylation status at GSK3-targeted SP/TP sites. Such mislocalized tau proteins, especially the less phosphorylated species, were stabilized by phosphorylation at Ser262/356 via PAR-1/MARK. Levels of Ser262 phosphorylation were increased by Aβ42, and blocking this stabilization of tau suppressed Aβ42-mediated augmentation of tau toxicity and an increase in the levels of tau phosphorylation at the SP/TP site Thr231, suggesting that this process may be involved in AD pathogenesis. In contrast to PAR-1/MARK, blocking tau phosphorylation at SP/TP sites by knockdown of Sgg/GSK3 did not reduce tau levels, suppress tau mislocalization to the cytosol, or diminish Aβ-mediated augmentation of tau toxicity. These results suggest that stabilization of microtubule-unbound tau by phosphorylation at Ser262/356 via the PAR-1/MARK may act in the initial steps of tau mismetabolism in AD pathogenesis, and that such tau species may represent a potential therapeutic target for AD (Ando, 2016).

Acetylation mimic of lysine 280 exacerbates human Tau neurotoxicity in vivo
Dysfunction and accumulation of the microtubule-associated human Tau (hTau) protein into intraneuronal aggregates is observed in many neurodegenerative disorders including Alzheimer's disease (AD). Reversible lysine acetylation has recently emerged as a post-translational modification that may play an important role in the modulation of hTau pathology. Acetylated hTau species have been observed within hTau aggregates in human AD brains and multi-acetylation of hTau in vitro regulates its propensity to aggregate. However, whether lysine acetylation at position 280 (K280) modulates hTau-induced toxicity in vivo is unknown. This study generated new Drosophila transgenic models of hTau pathology to evaluate the contribution of K280 acetylation to hTau toxicity, by analysing the respective toxicity of pseudo-acetylated (K280Q) and pseudo-de-acetylated (K280R) mutant forms of hTau. It was observed that mis-expression of pseudo-acetylated K280Q-hTau in the adult fly nervous system potently exacerbated fly locomotion defects and photoreceptor neurodegeneration. In addition, modulation of K280 influenced total hTau levels and phosphorylation without changing hTau solubility. Altogether, these results indicate that pseudo-acetylation of the single K280 residue is sufficient to exacerbate hTau neurotoxicity in vivo, suggesting that acetylated K280-hTau species contribute to the pathological events leading to neurodegeneration in AD (Gorsky, 2016).

Tau excess impairs mitosis and kinesin-5 function, leading to aneuploidy and cell death
In neurodegenerative diseases like Alzheimer's disease (AD), cell cycle defects and associated aneuploidy have been described. However, the importance of these defects in the physiopathology of AD and the underlying mechanistic processes are largely unknown in particular with respect to the microtubule-binding protein Tau, which is found in excess in the brain and cerebrospinal fluid of patients. Although it has long been known that Tau is phosphorylated during mitosis to generate a lower affinity for microtubules, there has been no indication that an excess of this protein could affect mitosis. The effect of an excess of human Tau (hTau) protein on cell mitosis was studied in vivo. Using the Drosophila developing wing disc epithelium as a model, this study shows that an excess of hTau induces a mitotic arrest, with the presence of monopolar spindles. This mitotic defect leads to aneuploidy and apoptotic cell death. The mechanism of action of hTau was studied and it was found that the MT-binding domain of hTau is responsible for these defects. hTau effects occur via the inhibition of the function of the kinesin Klp61F, the Drosophila homologue of kinesin-5 (also called Eg5 or KIF11). This deleterious effect of hTau is also found in other Drosophila cell types (neuroblasts) and tissues (the developing eye disc) as well as in human Hela cells.By demonstrating that microtubule-bound Tau inhibits the Eg5/KIF11 kinesin and cell mitosis, this work provides a new framework to consider the role of Tau in neurodegenerative diseases (Bouge, 2016).

Uncoupling neuronal death and dysfunction in Drosophila models of neurodegenerative disease

Common neurodegenerative proteinopathies, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are characterized by the misfolding and aggregation of toxic protein species, including the amyloid β (Aβ) peptide, microtubule-associated protein Tau (Tau), and alpha-synuclein (αSyn) protein. These factors also show toxicity in Drosophila. Using standardized conditions and medium-throughput assays, this study expressed human Tau, Aβ or αSyn selectively in neurons of the adult Drosophila retina and monitored age-dependent changes in both structure and function, based on tissue histology and recordings of the electroretinogram (ERG), respectively. Each protein was found to cause a unique profile of neurodegenerative pathology. Strikingly, expression of Tau leads to progressive loss of ERG responses whereas retinal architecture and neuronal numbers are largely preserved. By contrast, Aβ induces modest, age-dependent neuronal loss without degrading the retinal ERG. αSyn expression is characterized by marked retinal vacuolar change, progress photoreceptor cell death, and delayed-onset but modest ERG changes. Surprisingly, Tau and αSyn each cause prominent but distinct synaptotoxic profiles, including disorganization or enlargement of photoreceptor terminals, respectively. These findings suggest that Drosophila may be useful for revealing determinants of neuronal dysfunction that precede cell loss, including synaptic changes, in the adult nervous system (Chouhan, 2016).

Loss of axonal mitochondria promotes tau-mediated neurodegeneration and Alzheimer's disease-related tau phosphorylation via PAR-1

Abnormal phosphorylation and toxicity of a microtubule-associated protein tau are involved in the pathogenesis of Alzheimer's disease (AD); however, what pathological conditions trigger tau abnormality in AD is not fully understood. A reduction in the number of mitochondria in the axon has been implicated in AD. This study investigated whether and how loss of axonal mitochondria promotes tau phosphorylation and toxicity in vivo. Using transgenic Drosophila expressing human tau, it was found that RNAi-mediated knockdown of milton or Miro, an adaptor protein essential for axonal transport of mitochondria, enhanced human tau-induced neurodegeneration. Tau phosphorylation at an AD-related site Ser262 increased with knockdown of milton or Miro; and partitioning defective-1 (PAR-1), the Drosophila homolog of mammalian microtubule affinity-regulating kinase, mediated this increase of tau phosphorylation. Tau phosphorylation at Ser262 has been reported to promote tau detachment from microtubules, and this study found that the levels of microtubule-unbound free tau increased by milton knockdown. Blocking tau phosphorylation at Ser262 site by PAR-1 knockdown or by mutating the Ser262 site to unphosphorylatable alanine suppressed the enhancement of tau-induced neurodegeneration caused by milton knockdown. Furthermore, knockdown of milton or Miro increased the levels of active PAR-1. These results suggest that an increase in tau phosphorylation at Ser262 through PAR-1 contributes to tau-mediated neurodegeneration under a pathological condition in which axonal mitochondria is depleted. Intriguingly, this study found that knockdown of milton or Miro alone caused late-onset neurodegeneration in the fly brain, and this neurodegeneration could be suppressed by knockdown of Drosophila tau or PAR-1. These results suggest that loss of axonal mitochondria may play an important role in tau phosphorylation and toxicity in the pathogenesis of AD (Iijima-Ando, 2012).

Inhibition of GSK-3 ameliorates Aβ pathology in an adult-onset Drosophila model of Alzheimer's disease

Aβ peptide accumulation is thought to be the primary event in the pathogenesis of Alzheimer's disease (AD), with downstream neurotoxic effects including the hyperphosphorylation of tau protein. Glycogen synthase kinase-3 (GSK-3) is increasingly implicated as playing a pivotal role in this amyloid cascade. This study developed an adult-onset Drosophila model of AD, using an inducible gene expression system to express Arctic mutant Aβ42 specifically in adult neurons, to avoid developmental effects. Aβ42 accumulated with age in these flies and they displayed increased mortality together with progressive neuronal dysfunction, but in the apparent absence of neuronal loss. This fly model can thus be used to examine the role of events during adulthood and early AD aetiology. Expression of Aβ42 in adult neurons increased GSK-3 activity, and inhibition of GSK-3 (either genetically or pharmacologically by lithium treatment) rescued Aβ42 toxicity. Aβ42 pathogenesis was also reduced by removal of endogenous fly tau; but, within the limits of detection of available methods, tau phosphorylation did not appear to be altered in flies expressing Aβ42. The GSK-3-mediated effects on Aβ42 toxicity appear to be at least in part mediated by tau-independent mechanisms, because the protective effect of lithium alone was greater than that of the removal of tau alone. Finally, Aβ42 levels were reduced upon GSK-3 inhibition, pointing to a direct role of GSK-3 in the regulation of Aβ42 peptide level, in the absence of APP processing. This study points to the need both to identify the mechanisms by which GSK-3 modulates Aβ42 levels in the fly and to determine if similar mechanisms are present in mammals, and it supports the potential therapeutic use of GSK-3 inhibitors in AD (Sofola, 2010).


REFERENCES

Search PubMed for articles about Drosophila Tau

Alves-Silva, J., Sanchez-Soriano, N., Beaven, R., Klein, M., Parkin, J., Millard, T. H., Bellen, H. J., Venken, K. J., Ballestrem, C., Kammerer, R. A. and Prokop, A. (2012). Spectraplakins promote microtubule-mediated axonal growth by functioning as structural microtubule-associated proteins and EB1-dependent +TIPs (tip interacting proteins). J Neurosci 32(27): 9143-9158. PubMed ID: 22764224

Ando, K., Maruko-Otake, A., Ohtake, Y., Hayashishita, M., Sekiya, M. and Iijima, K. M. (2016). Stabilization of microtubule-unbound Tau via Tau phosphorylation at Ser262/356 by Par-1/MARK contributes to augmentation of AD-related phosphorylation and Aβ42-induced Tau toxicity. PLoS Genet 12: e1005917. PubMed ID: 27023670

Bardai, F. H., Wang, L., Mutreja, Y., Yenjerla, M., Gamblin, T. C. and Feany, M. B. (2017). A conserved cytoskeletal signaling cascade mediates neurotoxicity of FTDP-17 tau mutations in vivo. J Neurosci. PubMed ID: 29138281

Bolkan, B. J. and Kretzschmar, D. (2014). Loss of Tau results in defects in photoreceptor development and progressive neuronal degeneration in Drosophila. Dev Neurobiol 74(12): 1210-1225. PubMed ID: 24909306

Bouge, A. L. and Parmentier, M. L. (2016). Tau excess impairs mitosis and kinesin-5 function, leading to aneuploidy and cell death. Dis Model Mech [Epub ahead of print]. PubMed ID: 26822478

Burnouf, S., Grönke, S., Augustin, H., Dols, J., Gorsky, M.K., Werner, J., Kerr, F., Alic, N., Martinez, P. and Partridge, L. (2016). Deletion of endogenous Tau proteins is not detrimental in Drosophila. Sci Rep 6: 23102. PubMed ID: 26976084

Chouhan, A.K., Guo, C., Hsieh, Y.C., Ye, H., Senturk, M., Zuo, Z., Li, Y., Chatterjee, S., Botas, J., Jackson, G.R., Bellen, H.J. and Shulman, J.M. (2016). Uncoupling neuronal death and dysfunction in Drosophila models of neurodegenerative disease. Acta Neuropathol Commun 4: 62. PubMed ID: 27338814

Cornelison, G. L., Levy, S. A., Jenson, T. and Frost, B. (2018). Tau-induced nuclear envelope invagination causes a toxic accumulation of mRNA in Drosophila. Aging Cell: e12847. PubMed ID: 30411463

Drewes, G., Ebneth, A., Preuss, U., Mandelkow, E. M. and Mandelkow, E. (1997). MARK, a novel family of protein kinases that phosphorylate microtubule-associated proteins and trigger microtubule disruption. Cell 89(2): 297-308. PubMed ID: 9108484

Frenkel-Pinter, M., Stempler, S., Tal-Mazaki, S., Losev, Y., Singh-Anand, A., Escobar-Alvarez, D., Lezmy, J., Gazit, E., Ruppin, E. and Segal, D. S. Altered protein glycosylation predicts Alzheimer's disease and modulates its pathology in disease model Drosophila. Neurobiol Aging. PubMed ID: 28552182

Galasso, A., Cameron, C. S., Frenguelli, B. G. and Moffat, K. G. (2017). An AMPK-dependent regulatory pathway in tau-mediated toxicity. Biol Open [Epub ahead of print]. PubMed ID: 28808138

Gorsky, M. K., Burnouf, S., Dols, J., Mandelkow, E. and Partridge, L. (2016). Acetylation mimic of lysine 280 exacerbates human Tau neurotoxicity in vivo. Sci Rep 6: 22685. PubMed ID: 26940749

Heidary, G. and Fortini, M. E. (2001). Identification and characterization of the Drosophila tau homolog. Mech Dev 108(1-2): 171-178. PubMed ID: 11578871

Herzmann, S., Krumkamp, R., Rode, S., Kintrup, C. and Rumpf, S. (2017). PAR-1 promotes microtubule breakdown during dendrite pruning in Drosophila. EMBO J 36(13): 1981-1991. PubMed ID: 28554895

Herzmann, S., Gotzelmann, I., Reekers, L. F. and Rumpf, S. (2018). Spatial regulation of microtubule disruption during dendrite pruning in Drosophila. Development [Epub ahead of print]. PubMed ID: 29712642

Iijima-Ando, K., Sekiya, M., Maruko-Otake, A., Ohtake, Y., Suzuki, E., Lu, B. and Iijima, K. M. (2012). Loss of axonal mitochondria promotes tau-mediated neurodegeneration and Alzheimer's disease-related tau phosphorylation via PAR-1. PLoS Genet 8(8): e1002918. PubMed ID: 22952452

Kilian, J.G., Hsu, H.W., Mata, K., Wolf, F.W. and Kitazawa, M. (2017). Astrocyte transport of glutamate and neuronal activity reciprocally modulate tau pathology in Drosophila. Neuroscience [Epub ahead of print]. PubMed ID: 28215745

Kovacs, G. G. (2015). Invited review: Neuropathology of tauopathies: principles and practice. Neuropathol Appl Neurobiol 41(1): 3-23. PubMed ID: 25495175

Lee, H. H., Jan, L. Y. and Jan, Y. N. (2009). Drosophila IKK-related kinase Ik2 and Katanin p60-like 1 regulate dendrite pruning of sensory neuron during metamorphosis. Proc Natl Acad Sci U S A 106(15): 6363-6368. PubMed ID: 19329489

Ma, Q. L., Zuo, X., Yang, F., Ubeda, O. J., Gant, D. J., Alaverdyan, M., Kiosea, N. C., Nazari, S., Chen, P. P., Nothias, F., Chan, P., Teng, E., Frautschy, S. A. and Cole, G. M. (2014). Loss of MAP function leads to hippocampal synapse loss and deficits in the Morris Water Maze with aging. J Neurosci 34(21): 7124-7136. PubMed ID: 24849348

Malmanche, N., Dourlen, P., Gistelinck, M., Demiautte, F., Link, N., Dupont, C., Vanden Broeck, L., Werkmeister, E., Amouyel, P., Bongiovanni, A., Bauderlique, H., Moechars, D., Royou, A., Bellen, H. J., Lafont, F., Callaerts, P., Lambert, J. C. and Dermaut, B. (2017). Developmental Expression of 4-Repeat-Tau Induces Neuronal Aneuploidy in Drosophila Tauopathy Models. Sci Rep 7: 40764. PubMed ID: 28112163

Maor-Nof, M., Homma, N., Raanan, C., Nof, A., Hirokawa, N. and Yaron, A. (2013). Axonal pruning is actively regulated by the microtubule-destabilizing protein kinesin superfamily protein 2A. Cell Rep 3(4): 971-977. PubMed ID: 23562155

Morris, M., Hamto, P., Adame, A., Devidze, N., Masliah, E. and Mucke, L. (2013). Age-appropriate cognition and subtle dopamine-independent motor deficits in aged tau knockout mice. Neurobiol Aging 34(6): 1523-1529. PubMed ID: 23332171

Papanikolopoulou, K., Grammenoudi, S., Samiotaki, M. and Skoulakis, E. M. C. (2018). Differential effects of 14-3-3 dimers on Tau phosphorylation, stability and toxicity in vivo. Hum Mol Genet. PubMed ID: 29659825

Passarella, D. and Goedert, M. (2018). Beta-sheet assembly of Tau and neurodegeneration in Drosophila melanogaster. Neurobiol Aging 72: 98-105. PubMed ID: 30240946

Prokop, A. (2013). The intricate relationship between microtubules and their associated motor proteins during axon growth and maintenance. Neural Dev 8: 17. PubMed ID: 24010872

Qiang, L., Yu, W., Andreadis, A., Luo, M. and Baas, P. W. (2006). Tau protects microtubules in the axon from severing by katanin. J Neurosci 26(12): 3120-3129. PubMed ID: 16554463

Sapir, T., Frotscher, M., Levy, T., Mandelkow, E. M. and Reiner, O. (2012). Tau's role in the developing brain: implications for intellectual disability. Hum Mol Genet 21(8): 1681-1692. PubMed ID: 22194194

Sofola, O., Kerr, F., Rogers, I., Killick, R., Augustin, H., Gandy, C., Allen, M. J., Hardy, J., Lovestone, S. and Partridge, L. (2010). Inhibition of GSK-3 ameliorates Aβ pathology in an adult-onset Drosophila model of Alzheimer's disease. PLoS Genet 6(9): e1001087. PubMed ID: 20824130

Stewart, A., Tsubouchi, A., Rolls, M. M., Tracey, W. D. and Sherwood, N. T. (2012). Katanin p60-like1 promotes microtubule growth and terminal dendrite stability in the larval class IV sensory neurons of Drosophila. J Neurosci 32(34): 11631-11642. PubMed ID: 22915107

Talmat-Amar, Y., Arribat, Y. and Parmentier, M. L. (2018). Vesicular Axonal Transport is Modified In Vivo by Tau Deletion or Overexpression in Drosophila. Int J Mol Sci 19(3). PubMed ID: 29509687

Voelzmann, A., Okenve-Ramos, P., Qu, Y., Chojnowska-Monga, M., Del Caño-Espinel, M., Prokop, A. and Sanchez-Soriano, N. (2016). Tau and spectraplakins promote synapse formation and maintenance through Jun kinase and neuronal trafficking. Elife 5. PubMed ID: 27501441

Zhang, B., Li, Q., Chu, X., Sun, S. and Chen, S. (2016). Salidroside reduces tau hyperphosphorylation via up-regulating GSK-3β phosphorylation in a tau transgenic Drosophila model of Alzheimer's disease. Transl Neurodegener 5: 21. PubMed ID: 27933142

Zhukareva, V., Vogelsberg-Ragaglia, V., Van Deerlin, V. M., Bruce, J., Shuck, T., Grossman, M., Clark, C. M., Arnold, S. E., Masliah, E., Galasko, D., Trojanowski, J. Q. and Lee, V. M. (2001). Loss of brain tau defines novel sporadic and familial tauopathies with frontotemporal dementia. Ann Neurol 49(2): 165-175. PubMed ID: 11220736


Biological Overview

date revised: 14 December 2018

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.