Mical modulates Tau toxicity via cysteine oxidation in vivo

Tau accumulation is clearly linked to pathogenesis in Alzheimer’s disease and other Tauopathies. However, processes leading to Tau fibrillization and reasons for its pathogenicity remain largely elusive. Mical emerged as a novel interacting protein of human Tau expressed in Drosophila brains. Mical is characterized by the presence of a flavoprotein monooxygenase domain that generates redox potential with which it can oxidize target proteins. In the well-established Drosophila Tauopathy model, we use genetic interactions to show that Mical alters Tau interactions with microtubules and the Actin cytoskeleton and greatly affects Tau aggregation propensity and Tau-associated toxicity and dysfunction. Exploration of the mechanism was pursued using a Mical inhibitor, a mutation in Mical that selectively disrupts its monooxygenase domain, Tau transgenes mutated at cysteine residues targeted by Mical and mass spectrometry analysis to quantify cysteine oxidation. The collective evidence strongly indicates that Mical’s redox activity mediates the effects on Tau via oxidation of Cys322. Importantly, we also validate results from the fly model in human Tauopathy samples by showing that MICAL1 is up-regulated in patient brains and co-localizes with Tau in Pick bodies. Our work provides mechanistic insights into the role of the Tau cysteine residues as redox-switches regulating the process of Tau self-assembly into inclusions in vivo, its function as a cytoskeletal protein and its effect on neuronal toxicity and dysfunction.

The microtubule associated protein tau suppresses the axonal distribution of PDF neuropeptide and mitochondria in circadian clock neurons

Disrupted circadian rhythms is a prominent feature of multiple neurodegenerative diseases. Yet mechanisms linking Tau to rhythmic behavior remain unclear. Here we find that expression of a phosphomimetic human Tau mutant (TauE14) in Drosophila circadian pacemaker neurons disrupts free-running rhythmicity. While cell number and oscillations of the core clock protein PERIOD are unaffected in the small LNv (sLNv) neurons important for free running rhythms, we observe a near complete loss of the major LNv neuropeptide pigment dispersing factor (PDF) in the dorsal axonal projections of the sLNvs. This was accompanied by a ~ 50% reduction in the area of the dorsal terminals and a modest decrease in cell body PDF levels. Expression of wild-type Tau also reduced axonal PDF levels but to a lesser extent than TauE14. TauE14 also induces a complete loss of mitochondria from these sLNv projections. However, mitochondria were increased in sLNv cell bodies in TauE14 flies. These results suggest that TauE14 disrupts axonal transport of neuropeptides and mitochondria in circadian pacemaker neurons, providing a mechanism by which Tau can disrupt circadian behavior prior to cell loss.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Identification of cis-acting determinants mediating the unconventional secretion of tau

The deposition of tau aggregates throughout the brain is a pathological characteristic within a group of neurodegenerative diseases collectively termed tauopathies, which includes Alzheimer’s disease. While recent findings suggest the involvement of unconventional secretory pathways driving tau into the extracellular space and mediating the propagation of the disease-associated pathology, many of the mechanistic details governing this process remain elusive. In the current study, we provide an in-depth characterization of the unconventional secretory pathway of tau and identify novel molecular determinants that are required for this process. Here, using Drosophila models of tauopathy, we correlate the hyperphosphorylation and aggregation state of tau with the disease-related neurotoxicity. These newly established systems recapitulate all the previously identified hallmarks of tau secretion, including the contribution of tau hyperphosphorylation as well as the requirement for PI(4,5)P₂ triggering the direct translocation of tau. Using a series of cellular assays, we demonstrate that both the sulfated proteoglycans on the cell surface and the correct orientation of the protein at the inner plasma membrane leaflet are critical determinants of this process. Finally, we identify two cysteine residues within the microtubule binding repeat domain as novel cis-elements that are important for both unconventional secretion and trans-cellular propagation of tau.

The Two Cysteines of Tau Protein Are Functionally Distinct and Contribute Differentially to Its Pathogenicity in Vivo

Although Tau accumulation is clearly linked to pathogenesis in Alzheimer's disease and other Tauopathies, the mechanism that initiates the aggregation of this highly soluble protein in vivo remains largely unanswered. Interestingly, in vitro Tau can be induced to form fibrillar filaments by oxidation of its two cysteine residues, generating an intermolecular disulfide bond that promotes dimerization and fibrillization. The recently solved structures of Tau filaments revealed that the two cysteine residues are not structurally equivalent since Cys-322 is incorporated into the core of the fibril, whereas Cys-291 projects away from the core to form the fuzzy coat. Here, we examined whether mutation of these cysteines to alanine affects differentially Tau mediated toxicity and dysfunction in the well-established Drosophila Tauopathy model. Experiments were conducted with both sexes, or with either sex. Each cysteine residue contributes differentially to Tau stability, phosphorylation status, aggregation propensity, resistance to stress, learning, and memory. Importantly, our work uncovers a critical role of Cys-322 in determining Tau toxicity and dysfunction.SIGNIFICANCE STATEMENT Cysteine-291 and Cysteine-322, the only two cysteine residues of Tau present in only 4-Repeat or all isoforms, respectively, have competing functions: as the key residues in the catalytic center, they enable Tau auto-acetylation; and as residues within the microtubule-binding repeat region are important not only for Tau function but also instrumental in the initiation of Tau aggregation. In this study, we present the first in vivo evidence that their substitution leads to differential consequences on Tau's physiological and pathophysiological functions. These differences raise the possibility that cysteine residues play a potential role in determining the functional diversity between isoforms.

Cleavage of human tau at Asp421 inhibits hyperphosphorylated tau induced pathology in a Drosophila model

Hyperphosphorylated and truncated tau variants are enriched in neuropathological aggregates in diseases known as tauopathies. However, whether the interaction of these posttranslational modifications affects tau toxicity as a whole remains unresolved. By expressing human tau with disease-related Ser/Thr residues to simulate hyperphosphorylation, we show that despite severe neurodegeneration in full-length tau, with the truncation at Asp421, the toxicity is ameliorated. Cytological and biochemical analyses reveal that hyperphosphorylated full-length tau distributes in the soma, the axon, and the axonal terminal without evident distinction, whereas the Asp421-truncated version is mostly restricted from the axonal terminal. This discrepancy is correlated with the fact that fly expressing hyperphosphorylated full-length tau, but not Asp421-cleaved one, develops axonopathy lesions, including axonal spheroids and aberrant actin accumulations. The reduced presence of hyperphosphorylated tau in the axonal terminal is corroborated with the observation that flies expressing Asp421-truncated variants showed less motor deficit, suggesting synaptic function is preserved. The Asp421 cleavage of tau is a proteolytic product commonly found in the neurofibrillary tangles. Our finding suggests the coordination of different posttranslational modifications on tau may have an unexpected impact on the protein subcellular localization and cytotoxicity, which may be valuable when considering tau for therapeutic purposes.

An assessment of the translational relevance of Drosophila in drug discovery

INTRODUCTION

Drosophila melanogaster offers a powerful expedient and economical system with facile genetics. Because of the high sequence and functional conservation with human disease-associated genes, it has been cardinal in deciphering disease mechanisms at the genetic and molecular level. Drosophila are amenable to and respond well to pharmaceutical treatment which coupled to their genetic tractability has led to discovery, repositioning, and validation of a number of compounds. Areas covered: This review summarizes the generation of fly models of human diseases, their advantages and use in elucidation of human disease mechanisms. Representative studies provide examples of the utility of this system in modeling diseases and the discovery, repositioning and testing on pharmaceuticals to ameliorate them. Expert opinion: Drosophila offers a facile and economical whole animal system with many homologous organs to humans, high functional conservation and established methods of generating and validating human disease models. Nevertheless, it remains relatively underused as a drug discovery tool probably because its relevance to mammalian systems remains under question. However, recent exciting success stories using Drosophila disease models for drug screening, repositioning and validation strongly suggest that fly models should figure prominently in the drug discovery pipeline from bench to bedside.

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

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

T-complex protein 1-ring complex enhances retrograde axonal transport by modulating tau phosphorylation

The cytosolic chaperonin T-complex protein (TCP) 1-ring complex (TRiC) has been shown to exert neuroprotective effects on axonal transport through clearance of mutant Huntingtin (mHTT) in Huntington's disease. However, it is presently unknown if TRiC also has any effect on axonal transport in wild-type neurons. Here, we examined how TRiC impacted the retrograde axonal transport of brain-derived neurotrophic factor (BDNF). We found that expression of a single TRiC subunit significantly enhanced axonal transport of BDNF, leading to an increase in instantaneous velocity with a concomitant decrease in pauses for retrograde BDNF transport. The transport enhancing effect by TRiC was dependent on endogenous tau expression because no effect was seen in neurons from tau knockout mice. We showed that TRiC regulated the level of cyclin-dependent kinase 5 (CDK5)/p35 positively, contributing to TRiC-mediated tau phosphorylation (ptau). Expression of a single TRiC subunit increased the level of ptau while downregulation of the TRiC complex decreased ptau. We further demonstrated that TRiC-mediated increase in ptau induced detachment of tau from microtubules. Our study has thus revealed that TRiC-mediated increase in tau phosphorylation impacts retrograde axonal transport.

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, we took advantage of the model organism Drosophila, which genome contains only one homologue of the Tau/MAP2/MAP4 family to decipher (endogenous) Tau functions. We found that Tau depletion leads 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, we show 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.

Identification of DmTTLL5 as a Major Tubulin Glutamylase in the Drosophila Nervous System

Microtubules (MTs) play crucial roles during neuronal life. They are formed by heterodimers of alpha and beta-tubulins, which are subjected to several post-translational modifications (PTMs). Amongst them, glutamylation consists in the reversible addition of a variable number of glutamate residues to the C-terminal tails of tubulins. Glutamylation is the most abundant MT PTM in the mammalian adult brain, suggesting that it plays an important role in the nervous system (NS). Here, we show that the previously uncharacterized CG31108 gene encodes an alpha-tubulin glutamylase acting in the Drosophila NS. We show that this glutamylase, which we named DmTTLL5, initiates MT glutamylation specifically on alpha-tubulin, which are the only glutamylated tubulin in the Drosophila brain. In DmTTLL5 mutants, MT glutamylation was not detected in the NS, allowing for determining its potential function. DmTTLL5 mutants are viable and we did not find any defect in vesicular axonal transport, synapse morphology and larval locomotion. Moreover, DmTTLL5 mutant flies display normal negative geotaxis behavior and their lifespan is not altered. Thus, our work identifies DmTTLL5 as the major enzyme responsible for initiating neuronal MT glutamylation specifically on alpha-tubulin and we show that the absence of MT glutamylation is not detrimental for Drosophila NS function.

An overview of circulating cell-free microRNAs as putative biomarkers in Alzheimer's and Parkinson's Diseases

Circulating cell-free microRNAs (miRNAs) are stable in many biological fluids and their expression profiles can suffer changes under different physiological and pathological conditions. In the last few years, miRNAs have been proposed as putative noninvasive biomarkers in diagnosis, prognosis and response to treatment for several diseases, including neurodegenerative disorders as Alzheimer's disease (AD) and Parkinson's disease (PD). Cognitive and/or motor impairments are usually considered for establishing clinical diagnosis, and at this stage, the majority of the neurons may already be lost making difficult attempts of novel therapies. In this review, we intend to survey the circulating cell-free miRNAs found as dysregulated in cerebrospinal fluid, serum and plasma samples in AD and PD patients, and show how those miRNAs can be useful for early and differential diagnosis. Beyond that, we highlighted the miRNAs that are possibly related to common molecular mechanisms in the neurodegeneration process, as well those miRNAs related to specific disease pathways.

Cellular and molecular modifier pathways in tauopathies: the big picture from screening invertebrate models

Abnormal tau accumulations were observed and documented in post-mortem brains of patients affected by Alzheimer's disease (AD) long before the identification of mutations in the Microtubule-associated protein tau (MAPT) gene, encoding the tau protein, in a different neurodegenerative disease called Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the MAPT gene associated with FTDP-17 highlighted that dysfunctions in tau alone are sufficient to cause neurodegeneration. Invertebrate models have been diligently utilized in investigating tauopathies, contributing to the understanding of cellular and molecular pathways involved in disease etiology. An important discovery came with the demonstration that over-expression of human tau in Drosophila leads to premature mortality and neuronal dysfunction including neurodegeneration, recapitulating some key neuropathological features of the human disease. The simplicity of handling invertebrate models combined with the availability of a diverse range of experimental resources make these models, in particular Drosophila a powerful invertebrate screening tool. Consequently, several large-scale screens have been performed using Drosophila, to identify modifiers of tau toxicity. The screens have revealed not only common cellular and molecular pathways, but in some instances the same modifier has been independently identified in two or more screens suggesting a possible role for these modifiers in regulating tau toxicity. The purpose of this review is to discuss the genetic modifier screens on tauopathies performed in Drosophila and C. elegans models, and to highlight the common cellular and molecular pathways that have emerged from these studies. Here, we summarize results of tau toxicity screens providing mechanistic insights into pathological alterations in tauopathies. Key pathways or modifiers that have been identified are associated with a broad range of processes including, but not limited to, phosphorylation, cytoskeleton organization, axonal transport, regulation of cellular proteostasis, transcription, RNA metabolism, cell cycle regulation, and apoptosis. We discuss the utility and application of invertebrate models in elucidating the cellular and molecular functions of novel and uncharacterized disease modifiers identified in large-scale screens as well as for investigating the function of genes identified as risk factors in genome-wide association studies from human patients in the post-genomic era. In this review, we combined and summarized several large-scale modifier screens performed in invertebrate models to identify modifiers of tau toxicity. A summary of the screens show that diverse cellular processes are implicated in the modification of tau toxicity. Kinases and phosphatases are the most predominant class of modifiers followed by components required for cellular proteostasis and axonal transport and cytoskeleton elements.

Behind the curtain of tauopathy: a show of multiple players orchestrating tau toxicity

tau, a microtubule-associated protein, directly binds with microtubules to dynamically regulate the organization of cellular cytoskeletons, and is especially abundant in neurons of the central nervous system. Under disease conditions such as Pick's disease, progressive supranuclear palsy, frontotemporal dementia, parkinsonism linked to chromosome 17 and Alzheimer's disease, tau proteins can self-assemble to paired helical filaments progressing to neurofibrillary tangles. In these diseases, collectively referred to as "tauopathies", alterations of diverse tau modifications including phosphorylation, metal ion binding, glycosylation, as well as structural changes of tau proteins have all been observed, indicating the complexity and variability of factors in the regulation of tau toxicity. Here, we review our current knowledge and hypotheses from relevant studies on tau toxicity, emphasizing the roles of phosphorylations, metal ions, folding and clearance control underlining tau etiology and their regulations. A summary of clinical efforts and associated findings of drug candidates under development is also presented. It is hoped that a more comprehensive understanding of tau regulation will provide us with a better blueprint of tau networking in neuronal cells and offer hints for the design of more efficient strategies to tackle tau-related diseases in the future.

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.

Recessive Mutations in RTN4IP1 Cause Isolated and Syndromic Optic Neuropathies

Autosomal-recessive optic neuropathies are rare blinding conditions related to retinal ganglion cell (RGC) and optic-nerve degeneration, for which only mutations in TMEM126A and ACO2 are known. In four families with early-onset recessive optic neuropathy, we identified mutations in RTN4IP1, which encodes a mitochondrial ubiquinol oxydo-reductase. RTN4IP1 is a partner of RTN4 (also known as NOGO), and its ortholog Rad8 in C. elegans is involved in UV light response. Analysis of fibroblasts from affected individuals with a RTN4IP1 mutation showed loss of the altered protein, a deficit of mitochondrial respiratory complex I and IV activities, and increased susceptibility to UV light. Silencing of RTN4IP1 altered the number and morphogenesis of mouse RGC dendrites in vitro and the eye size, neuro-retinal development, and swimming behavior in zebrafish in vivo. Altogether, these data point to a pathophysiological mechanism responsible for RGC early degeneration and optic neuropathy and linking RTN4IP1 functions to mitochondrial physiology, response to UV light, and dendrite growth during eye maturation.

Starvation and inhibition of lysosomal function increased tau secretion by primary cortical neurons

Recent studies have demonstrated that human tau can be secreted by neurons and non-neuronal cells, an event linked to the propagation of tau pathology in the brain. In the present study, we confirmed that under physiological conditions, one tau-positive band was detected in the culture medium with an anti-tau antibody recognizing total tau and the Tau-1 antibody directed against unphosphorylated tau. We then examined whether tau secretion was modified upon insults. Tau secretion was increased by starvation [Earle's Balanced Salt Solution (EBSS)], inhibition of lysosomal function (leupeptin) and when both of these conditions were superimposed, this combined treatment having the most important effects on tau secretion. Interestingly, the pattern of tau secretion was distinct from that of control neurons when neurons were treated either with EBSS alone or EBSS + leupeptin. In these conditions, three tau-positive bands were detected in the culture medium. Two of these three bands were immunoreactive to Tau-1 antibody revealing that at least two tau species were released upon these treatments. Collectively, our results indicate that insults such as nutrient deprivation and lysosomal dysfunction observed in neurodegenerative diseases could result in an increase of tau secretion and propagation of tau pathology in the brain.

Modification of the Drosophila model of in vivo Tau toxicity reveals protective phosphorylation by GSK3β

Hyperphosphorylation of the microtubule associated protein, Tau, is the hallmark of a group of neurodegenerative disorders known as the tauopathies which includes Alzheimer's disease. Precisely how and why Tau phosphorylation is increased in disease is not fully understood, nor how individual sites modify Tau function. Several groups have used the Drosophila visual system as an in vivo model to examine how the toxicity of Tau varies with phosphorylation status. This system relies on overexpression of Tau from transgenes but is susceptible to position effects altering expression and activity of the transgenes. We have refined the system by eliminating position effects through the use of site-specific integration. By standardising Tau expression levels we have been able to compare directly the toxicity of different isoforms of Tau and Tau point mutants that abolish important phosphorylation events. We have also examined the importance of human kinases in modulating Tau toxicity in vivo. We were able to confirm that human GSK3β phosphorylates Tau and increases toxicity but, unexpectedly, we identified that preventing phosphorylation of Ser404 is a protective event. When phosphorylation at this site is prevented, Tau toxicity in the Drosophila visual system is increased in the presence of GSK3β. Our data suggest that not all phosphorylation events on Tau are associated with toxicity.

Interactions between Tau and α-synuclein augment neurotoxicity in a Drosophila model of Parkinson's disease

Clinical and pathological studies have suggested considerable overlap between tauopathies and synucleinopathies. Several genome-wide association studies have identified alpha-Synuclein (SNCA) and Tau (MAPT) polymorphisms as common risk factors for sporadic Parkinson's disease (PD). However, the mechanisms by which subtle variations in the expression of wild-type SNCA and MAPT influence risk for PD and the underlying cellular events that effect neurotoxicity remain unclear. To examine causes of neurotoxicity associated with the α-Syn/Tau interaction, we used the fruit fly as a model. We utilized misexpression paradigms in three different tissues to probe the α-Syn/Tau interaction: the retina, dopaminergic neurons and the larval neuromuscular junction. Misexpression of Tau and α-Syn enhanced a rough eye phenotype and loss of dopaminergic neurons in fly tauopathy and synucleinopathy models, respectively. Our findings suggest that interactions between α-Syn and Tau at the cellular level cause disruption of cytoskeletal organization, axonal transport defects and aberrant synaptic organization that contribute to neuronal dysfunction and death associated with sporadic PD. α-Syn did not alter levels of Tau phosphorylated at the AT8 epitope. However, α-Syn and Tau colocalized in ubiquitin-positive aggregates in eye imaginal discs. The presence of Tau also led to an increase in urea soluble α-Syn. Our findings have important implications in understanding the cellular and molecular mechanisms underlying α-Syn/Tau-mediated synaptic dysfunction, which likely arise in the early asymptomatic phase of sporadic PD.

A huntingtin peptide inhibits polyQ-huntingtin associated defects

BACKGROUND

Huntington's disease (HD) is caused by the abnormal expansion of the polyglutamine tract in the human Huntingtin protein (polyQ-hHtt). Although this mutation behaves dominantly, huntingtin loss of function also contributes to HD pathogenesis. Indeed, wild-type Huntingtin plays a protective role with respect to polyQ-hHtt induced defects.

METHODOLOGY/PRINCIPAL FINDINGS

The question that we addressed here is what part of the wild-type Huntingtin is responsible for these protective properties. We first screened peptides from the Huntingtin protein in HeLa cells and identified a 23 aa peptide (P42) that inhibits polyQ-hHtt aggregation. P42 is part of the endogenous Huntingtin protein and lies within a region rich in proteolytic sites that plays a critical role in the pathogenesis process. Using a Drosophila model of HD, we tested the protective properties of this peptide on aggregation, as well as on different polyQ-hHtt induced neuronal phenotypes: eye degeneration (an indicator of cell death), impairment of vesicular axonal trafficking, and physiological behaviors such as larval locomotion and adult survival. Together, our results demonstrate high protective properties for P42 in vivo, in whole animals. These data also demonstrate a specific role of P42 on Huntington's disease model, since it has no effect on other models of polyQ-induced diseases, such as spinocerebellar ataxias.

CONCLUSIONS/SIGNIFICANCE

Altogether our data show that P42, a 23 aa-long hHtt peptide, plays a protective role with respect to polyQ-hHtt aggregation as well as cellular and behavioral dysfunctions induced by polyQ-hHtt in vivo. Our study also confirms the correlation between polyQ-hHtt aggregation and neuronal defects. Finally, these results strongly suggest a therapeutic potential for P42, specific of Huntington's disease.

Loss of vesicular dopamine release precedes tauopathy in degenerative dopaminergic neurons in a Drosophila model expressing human tau

While a number of genome-wide association studies have identified microtubule-associated protein tau as a strong risk factor for Parkinson’s disease (PD), little is known about the mechanism through which human tau can predispose an individual to this disease. Here, we demonstrate that expression of human wild-type tau is sufficient to disrupt the survival of dopaminergic neurons in a Drosophila model. Tau triggers a synaptic pathology visualized by vesicular monoamine transporter-pHGFP that precedes both the age-dependent formation of tau-containing neurofibrillary tangle-like pathology and the progressive loss of DA neurons, thereby recapitulating the pathological hallmarks of PD. Flies overexpressing tau also exhibit progressive impairments of both motor and learning behaviors. Surprisingly, contrary to common belief that hyperphosphorylated tau could aggravate toxicity, DA neuron degeneration is alleviated by expressing the modified, hyperphosphorylated tau^E14. Together, these results show that impairment of VMAT-containing synaptic vesicle, released to synapses before overt tauopathy may be the underlying mechanism of tau-associated PD and suggest that correction or prevention of this deficit may be appropriate targets for early therapeutic intervention.

Drosophila models of tauopathies: what have we learned?

Aggregates of the microtubule-associated protein Tau are neuropathological hallmark lesions in Alzheimer's disease (AD) and related primary tauopathies. In addition, Tau is genetically implicated in a number of human neurodegenerative disorders including frontotemporal dementia (FTD) and Parkinson's disease (PD). The exact mechanism by which Tau exerts its neurotoxicity is incompletely understood. Here, we give an overview of how studies using the genetic model organism Drosophila over the past decade have contributed to the molecular understanding of Tau neurotoxicity. We compare the different available readouts for Tau neurotoxicity in flies and review the molecular pathways in which Tau has been implicated. Finally, we emphasize that the integration of genome-wide approaches in human or mice with high-throughput genetic validation in Drosophila is a fruitful approach.

Inhibition of tau aggregation in a novel Caenorhabditis elegans model of tauopathy mitigates proteotoxicity

Increased Tau protein amyloidogenicity has been causatively implicated in several neurodegenerative diseases, collectively called tauopathies. In pathological conditions, Tau becomes hyperphosphorylated and forms intracellular aggregates. The deletion of K280, which is a mutation that commonly appears in patients with frontotemporal dementia with Parkinsonism linked to chromosome 17, enhances Tau aggregation propensity (pro-aggregation). In contrast, introduction of the I277P and I308P mutations prevents β-sheet formation and subsequent aggregation (anti-aggregation). In this study, we created a tauopathy model by expressing pro- or anti-aggregant Tau species in the nervous system of Caenorhabditis elegans. Animals expressing the highly amyloidogenic Tau species showed accelerated Tau aggregation and pathology manifested by severely impaired motility and evident neuronal dysfunction. In addition, we observed that the axonal transport of mitochondria was perturbed in these animals. Control animals expressing the anti-aggregant combination had rather mild phenotype. We subsequently tested several Tau aggregation inhibitor compounds and observed a mitigation of Tau proteotoxicity. In particular, a novel compound that crosses the blood-brain barrier of mammals proved effective in ameliorating the motility as well as delaying the accumulation of neuronal defects. Our study establishes a new C. elegans model of Tau aggregation-mediated toxicity and supports the emerging notion that inhibiting the nucleation of Tau aggregation can be neuroprotective.

Tau Phosphorylation by GSK3 in Different Conditions

Almost a 20% of the residues of tau protein are phosphorylatable amino acids: serine, threonine, and tyrosine. In this paper we comment on the consequences for tau of being a phosphoprotein. We will focus on serine/threonine phosphorylation. It will be discussed that, depending on the modified residue in tau molecule, phosphorylation could be protective, in processes like hibernation, or toxic like in development of those diseases known as tauopathies, which are characterized by an hyperphosphorylation and aggregation of tau.

Identification of oligomers at early stages of tau aggregation in Alzheimer's disease

Neurofibrillary tangles (NFTs) are a pathological hallmark of Alzheimer's disease (AD); however, the relationship between NFTs and disease progression remains controversial. Analyses of tau animal models suggest that phenotypes coincide with accumulation of soluble aggregated tau species but not the accumulation of NFTs. The pathological role of prefilamentous tau aggregates, e.g., tau oligomeric intermediates, is poorly understood, in part because of methodological challenges. Here, we engineered a novel tau oligomer-specific antibody, T22, and used it to elucidate the temporal course and biochemical features of oligomers during NFT development in AD brain. We found that tau oligomers in human AD brain samples were 4-fold higher than those in the controls. We also revealed the role of oligomeric tau conformers in pretangles, neuritic plaques, and neuropil threads in the frontal cortex tissue from AD brains; this analysis uncovers a consistent code that governs tau oligomerization with regard to degree of neuronal cytopathology. These data are the first to characterize the role of tau oligomers in the natural history of NFTs, and they highlight the suitability of tau oligomers as therapeutic targets in AD and related tauopathies.