Tauopathies are a group of neurodegenerative diseases characterised by abnormally hyperphosphorylated and insoluble aggregates of the microtubule-associated protein tau [
Despite the varied neuropathological and clinical profiles, all tauopathies are characterised by the same tau aberrations: abnormal and hyperphosphorylation [
Numerous models of tauopathy, in both vertebrates [
Aside from the very obvious advantages afforded by its small size, inexpensive maintenance, rapid propagation, and short life span,
100 years of extensive use as an experimental genetic model (as described above), as well as a sequenced and highly annotated genome, provides
Method | Purpose | How it works | Examples |
---|---|---|---|
Gal4/UAS [ | Allows tissue-specific expression of the gene of interest, in a modular fashion. | One line of flies with Gal4 protein under a promoter expressed in the tissue of interest is crossed with another line harbouring the gene of interest downstream of GAL4-binding UAS DNA sequences and a minimal promotor. In F1 flies with copies of both transgenes, Gal4 will bind to UAS and drive expression, and thus the gene of interest will be expressed only in the tissue of interest. | The majority of |
Gal80TS (TARGET) [ | Provides temporal control of the Gal4/UAS system. | Adding another layer to the Gal4/UAS system to drive a gene of interest at the time and place of the experimenter’s choosing. Gal80TS is a temperature-dependent repressor of Gal4. Under permissive conditions (18°C), Gal80TS functions normally and blocks Gal4-mediated transactivation of | Colodner and Feany (2010) used this technique in order to cease expression of human tau in glia after NFTs had already formed (see text) [ |
EP lines [ | A large set of lines of flies with gain or loss of function in numerous genes can be screened for interaction. | Enhancer-promotor-transposable (EP) elements are inserted into the genome at random. They contain GAL4-binding UAS sequences and a promotor. When they land near a gene in the same direction, their activation by Gal4 may promote that gene’s transcription in the tissue of interest. | These lines have been screened for modifiers of rough-eye phenotype in flies expressing human tau in the retina [ |
A comprehensive set of fly lines to knockdown the expression of | Interfering double-stranded RNA for a | Individual | |
MARCM (Mosaic analysis with a repressible cell marker) [ | Generates a mosaic animal with GFP-marked clonal cells homozygous for an allele of interest, for direct comparison with non-GFP heterozygous or wildtype cells in the same animal. | MARCM relies upon Flp/FRT-mediated homologous recombination in mitotic cells to generate clonal subsets of daughter cells that are either (i) marked with GFP and homozygous for an allele of interest or (ii) heterozygous or wildtype for the allele and unmarked by GFP. Parental cells contain the allele of interest and Gal80 on homologous chromosomes distal to FRT sites as well as | Nishimura et al., (2004) used this technique to show that |
An additional highly valuable resource for tau biologists using
If for any reason a novel transgenic fly needs to be generated, this is a relatively straight forward, rapid, and inexpensive task. The transgenesis process (described in more detail in Pandey and Nichols 2011 [
Overall, the sophisticated genetic tools together with the vast number of mutant fly stocks that already exist make it relatively easy to study tauopathies in
Another outcome of the extensive use of
Despite its relative simplicity compared to other model organisms,
It is relatively simple to establish an enormous number of different crosses between flies. This, coupled with the fact that there are numerous readouts of tau-mediated neuronal toxicity and dysfunction in
Tauopathies are characterised by aggregates of abnormally phosphorylated and misfolded wild-type (wt) or mutant tau. To model tauopathies therefore, most studies have utilised the UAS-GAL4 expression system to target the expression of either wt or mutant human (and even bovine, rodent, and
Some of these models have been used in unbiased genetic enhancer/suppressor screens to identify proteins that interact with tau. Shulman and Feany were the first to report the results of a forward genetic screen looking for modifiers of the rough eye phenotype induced by the expression of V337M human tau [
These enhancer/suppressor studies are often accompanied by complimentary experiments in which the identified genes are either coexpressed with human tau [
In some studies, the
One of the critical obstacles to developing disease-modifying therapies for the treatment of tauopathies is our incomplete understanding about disease pathogenesis.
The expression of wild-type or FTDP-17 mutant human tau in the
The cellular mechanism by which high tau expression causes neuronal death is not entirely understood. However, more than one toxic mechanism may play a role since a variety of morphological changes have been described in the dying human-tau-expressing cells. Affected neurons have been shown to exhibit signs of both necrotic and apoptotic degeneration. Williams et al., (2000) expressed various tau transgenes (human wild-type 0N3R, bovine, and rodent) in larval sensory neurons and reported degeneration characterised by abnormal axon bundling, reduced arborisation, axonal swelling, and beading; in severely affected animals there was also a clear loss of axonal projections [
Like apoptosis, oxidative stress is believed to play a role in many chronic neurodegenerative diseases. Studies in
It has traditionally been thought that the aggregates of tau (filaments and tangles) are in themselves toxic and thus are responsible for neurodegeneration in tauopathies. This view is now being challenged (Section
Another potential mechanism responsible for tau toxicity is the displacement of tau from the axon to the soma. Tau is classified as an axonal protein and its displacement into the somatodendritic compartment is often considered by some to be a pathogenic event culminating in tangle formation and degeneration. One study from a
The role played by tau phosphorylation in mediating tau toxicity has also been investigated in most of the
In addition to the mechanisms discussed above, other novel pathways and cellular processes may be involved in mediating tau toxicity. Some of these have been identified from genetic enhancer/suppressor screens and include the JAK/STAT pathway [
Though, as discussed above, there are many
Almost all the studies in which a soluble tau species is associated with neuronal dysfunction in the absence of degeneration implicate the phosphorylation state of the tau in the causative mechanism. In our studies, we found that reducing tau phosphorylation (at the PHF-1 and AT8 sites) by treatment with LiCl suppressed, whilst increasing tau phosphorylation by the coexpression of
The mechanism by which highly phosphorylated tau disrupts neuronal function is likely to involve a phosphorylation-mediated reduced ability of hyperphosphorylated tau to bind to and stabilise microtubules. Highly phosphorylated tau has been shown to have a reduced ability to bind to microtubules
An unexpected additional pathogenic effect of soluble highly phosphorylated human tau that we uncovered in our model is that it binds to the endogenous
Although misfolded protein aggregates characterise many common proteinopathies including Alzheimer’s Disease, Parkinson’s Disease, and Huntington’s Disease, the role that they play in the disease process is debatable. Emerging evidence from various models of these diseases suggests that there is a dissociation between the aggregates themselves and the underlying toxicity, and that instead the precursors of the aggregates may be the toxic species.
One might wonder why there is no insoluble tau in most fly models. In the case of models expressing tau in motor neurons with the D42 driver [
As is implicit in the preceding paragraph, one key phenomenon of tau toxicity that has been evident in fly studies is the cell-type specificity of tau’s effects on neurons. It has long been apparent from the human conditions that some cell types are more susceptible to tau pathology than others [
Where insoluble tau has been detected in fly, it does not appear to be detrimental. In Chau et al.’s study older tau-expressing flies developed insoluble tau (the precise nature of which was unclear from their report) at 22 days of age; their rough-eye phenotype was no worse than young flies without insoluble tau [
Taken together, these results from the
Protein quality control features prominently in the discussion surrounding tauopathies and other neurodegenerative diseases collectively termed “foldopathies”. Protein quality control can be broadly thought to encompass both systems that are activated when proteins misfold, (such as the chaperone family and the unfolded protein response (UPR)), as well as those that deal with the clearance of terminally misfolded or aggregated proteins (such as the autophagic/lysosomal systems and the ubiquitin proteosome pathway (UPS)).
It was suggested that tau could be cleared by the autophagic/lysosomal pathway when Berger et al., demonstrated that Rapamycin, a known inducer of autophagy, marginally suppressed the tau-mediated rough eye phenotype [
The unfolded protein response (UPR) deals with excess misfolded protein in the ER and aims to restore homeostasis by reducing entry of such proteins into the ER, stimulating the degradation of the misfolded/aggregated proteins, or upregulating heat shock proteins (HSPs) and other protein quality control genes. Loewen and Feany (2010) recently demonstrated that the UPR is activated in transgenic flies expressing human wild-type and mutant tau, and that there is a positive correlation between the extent of the UPR activation and the degree of toxicity [
Though there is a lot of evidence from rodent and cell culture models of tauopathy showing that chaperone proteins such as CHIP (C-terminus of Hsc-70 interacting protein) regulate the turnover of phosphorylated tau via the UPS [
The relationship between the tau and amyloid pathologies and the pathways that facilitate an interaction between them has been a matter of intense debate and scientific research in this field. The general consensus is that amyloid pathology lies upstream of tau pathology and thus must activate cellular processes that cause the hyperphosphorylation and aggregation of tau. The results that have come forth from the few
Torroja et al., demonstrated in 1999 that the pan-neuronal expression of either bovine tau or the
Other reports suggest that tau and A
Overall, the
Alternative splicing of one gene leads to the translation of 6 isoforms of tau in the adult human brain, which differ in the presence or absence of 1 or 2 N-terminal domains and have either three (3R) or four (4R) C-terminal microtubule binding domains. In the adult brain, the ratio of 3R : 4R isoforms is 1. Rodents have 3 isoforms of tau and
Chen et al., (2007) demonstrated that although the expression of both wild type human tau (2N4R) and
The insights about the causal role of phosphorylation in the pathogenesis of disease that have emerged from
An unanimous conclusion that one may draw from the findings of
With regard to the mechanism by which hyperphosphorylated tau causes toxicity, the findings from numerous
The
The insights that have come forth so far from