Amrit Mudher

Drug repositioning for Alzheimer's disease

Existing drugs for Alzheimers disease provide symptomatic benefit for up to 12 months, but there are no approved disease-modifying therapies. Given the recent failures of various novel disease-modifying therapies in clinical trials, a complementary strategy based on repositioning drugs that are approved for other indications could be attractive. Indeed, a substantial body of preclinical work indicates that several classes of such drugs have potentially beneficial effects on Alzheimers-like brain pathology, and for some drugs the evidence is also supported by epidemiological data or preliminary clinical trials. Here, we present a formal consensus evaluation of these opportunities, based on a systematic review of published literature. We highlight several compounds for which sufficient evidence is available to encourage further investigation to clarify an optimal dose and consider progression to clinical trials in patients with Alzheimers disease.

Are tau aggregates toxic or protective in tauopathies

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

Increased throughput assays of locomotor dysfunction in Drosophila larvae

Larval locomotion is a sensitive readout of a range of nervous system deficits in Drosophila, and has been utilised to quantify modulation of the disease phenotype in models of human disease. Single larvae are typically analysed in series using manual quantification of parameters such as contraction rate, or grouped together and studied en-masse. Here, we describe the development of tests for the analysis of several spatially isolated third instar larvae in parallel. We rapidly quantify larval turning rate and velocity during wandering behaviour in a 4 plate assay. In a second test, larvae are recorded as they race along five parallel lanes towards a yeast stimulus. This allows increased throughput analysis of comparative genotypes simultaneously, video archiving, and detection of exacerbation or rescue of defective locomotion in a Drosophila model of tauopathy, as we demonstrate genetically and through delivery of candidate therapeutic chemicals in fly food. The tests are well-suited for rapid comparison of locomotion capability in Drosophila mutants or candidate modulation screens in Drosophila models of human disease.

What is the evidence that tau pathology spreads through prion-like propagation?

Emerging experimental evidence suggests that the spread of tau pathology in the brain in Tauopathies reflects the propagation of abnormal tau species along neuroanatomically connected brain areas. This propagation could occur through a “prion-like” mechanism involving transfer of abnormal tau seeds from a “donor cell” to a “recipient cell” and recruitment of normal tau in the latter to generate new tau seeds. This review critically appraises the evidence that the spread of tau pathology occurs via such a “prion-like” mechanism and proposes a number of recommendations for directing future research. Recommendations for definitions of frequently used terms in the tau field are presented in an attempt to clarify and standardize interpretation of research findings. Molecular and cellular factors affecting tau aggregation are briefly reviewed, as are potential contributions of physiological and pathological post-translational modifications of tau. Additionally, the experimental evidence for tau seeding and “prion-like” propagation of tau aggregation that has emerged from cellular assays and in vivo models is discussed. Propagation of tau pathology using “prion-like” mechanisms is expected to incorporate several steps including cellular uptake, templated seeding, secretion and intercellular transfer through synaptic and non-synaptic pathways. The experimental findings supporting each of these steps are reviewed. The clinical validity of these experimental findings is then debated by considering the supportive or contradictory findings from patient samples. Further, the role of physiological tau release in this scenario is examined because emerging data shows that tau is secreted but the physiological function (if any) of this secretion in the context of propagation of pathological tau seeds is unclear. Bona fide prions exhibit specific properties, including transmission from cell to cell, tissue to tissue and organism to organism. The propagation of tau pathology has so far not been shown to exhibit all of these steps and how this influences the debate of whether or not abnormal tau species can propagate in a “prion-like” manner is discussed. The exact nature of tau seeds responsible for propagation of tau pathology in human tauopathies remains controversial; it might be tightly linked to the existence of tau strains stably propagating peculiar patterns of neuropathological lesions, corresponding to the different patterns seen in human tauopathies. That this is a property shared by all seed-competent tau conformers is not yet firmly established. Further investigation is also required to clarify the relationship between propagation of tau aggregates and tau-induced toxicity. Genetic variants identified as risks factors for tauopathies might play a role in propagation of tau pathology, but many more studies are needed to document this. The contribution of selective vulnerability of neuronal populations, as an alternative to prion-like mechanisms to explain spreading of tau pathology needs to be clarified. Learning from the prion field will be helpful to enhance our understanding of propagation of tau pathology. Finally, development of better models is expected to answer some of these key questions and allow for the testing of propagation-centred therapies.

Atypical, non-standard functions of the microtubule associated Tau protein

Since the discovery of the microtubule-associated protein Tau (MAPT) over 40 years ago, most studies have focused on Tau’s role in microtubule stability and regulation, as well as on the neuropathological consequences of Tau hyperphosphorylation and aggregation in Alzheimer’s disease (AD) brains. In recent years, however, research efforts identified new interaction partners and different sub-cellular localizations for Tau suggesting additional roles beyond its standard function as microtubule regulating protein. Moreover, despite the increasing research focus on AD over the last decades, Tau was only recently considered as a promising therapeutic target for the treatment and prevention of AD as well as for neurological pathologies beyond AD e.g. epilepsy, excitotoxicity, and environmental stress. This review will focus on atypical, non-standard roles of Tau on neuronal function and dysfunction in AD and other neurological pathologies providing novel insights about neuroplastic and neuropathological implications of Tau in both the central and the peripheral nervous system.

Low Endogenous and Chemical Induced Heat Shock Protein Induction in a 0N3Rtau-Expressing Drosophila Larval Model of Alzheimer's Disease

Reduction of tau phosphorylation and aggregation by manipulation of heat shock protein (HSP) molecular chaperones has received much attention in attempts to further understand and treat tauopathies such as Alzheimers disease. We examined whether endogenous HSPs are induced in Drosophila larvae expressing human tau (3R-tau) in motor neurons, and screened several chemical compounds that target the HSP system using medium-throughput behavioral analysis to assay their effects on tau-induced neuronal dysfunction in vivo. Tau-expressing larvae did not show a significant endogenous HSP induction response, whereas robust induction of hsp70 was detectable in a similar larval model of polyglutamine disease. Although pan-neuronal tau expression augmented the induction of hsp70 following heat shock, several candidate HSP inducing compounds induced hsp70 protein in mammalian cells in vitro but did not detectably induce hsp70 mRNA or protein in tau expressing larvae. The hsp90 inhibitors 17-AAG and radicicol nevertheless caused a dose-dependent reduction in total human tau levels in transgenic larvae without specifically altering tau hyperphosphorylated at S396/S404. These and several other HSP modulating compounds also failed to rescue the tau-induced larval locomotion deficit in this model. Tau pathology in tau-expressing larvae, therefore, induces weak de novo HSP expression relative to other neurodegenerative disease models, and unlike these disease models, pharmacological manipulation of the hsp90 pathway does not lead to further induction of the heat shock response. Forthcoming studies investigating the effects of HSP induction on tau-mediated dysfunction/toxicity in such models will require more robust, non-pharmacological (perhaps genetic) means of manipulating the hsp90 pathway.

Using Drosophila models of neurodegenerative diseases for drug discovery.

Introduction: Neurodegenerative diseases such as Alzheimers disease, Parkinsons disease and Huntingtons disease are increasing in prevalence as our aging population increases in size. Despite this, currently there are no disease-modifying drugs available for the treatment of these conditions. Drosophila melanogaster is a highly tractable model organism that has been successfully used to emulate various aspects of these diseases in vivo. These Drosophila models have not been fully exploited in drug discovery and design strategies. Areas covered: This review explores how Drosophila models can be used to facilitate drug discovery. Specifically, we review their uses as a physiologically-relevant medium to high-throughput screening tool for the identification of therapeutic compounds and discuss how they can aid drug discovery by highlighting disease mechanisms that may serve as druggable targets in the future. The reader will appreciate how the various attributes of Drosophila make it an unsurpassed model organism and how Drosophila models of neurodegeneration can contribute to drug discovery in a variety of ways. Expert opinion: Drosophila models of human neurodegenerative diseases can make a significant contribution to the unmet need of disease-modifying therapeutic intervention for the treatment of these increasingly common neurodegenerative conditions.

Potential mechanisms and implications for the formation of tau oligomeric strains

Abstract The culmination of many years of increasing research into the toxicity of tau aggregation in neurodegenerative disease has led to the consensus that soluble, oligomeric forms of tau are likely the most toxic entities in disease. While tauopathies overlap in the presence of tau pathology, each disease has a unique combination of symptoms and pathological features; however, most study into tau has grouped tau oligomers and studied them as a homogenous population. Established evidence from the prion field combined with the most recent tau and amyloidogenic protein research suggests that tau is a prion-like protein, capable of seeding the spread of pathology throughout the brain. Thus, it is likely that tau may also form prion-like strains or diverse conformational structures that may differ by disease and underlie some of the differences in symptoms and pathology in neurodegenerative tauopathies. The development of techniques and new technology for the detection of tau oligomeric strains may, therefore, lead to more efficacious diagnostic and treatment strategies for neurodegenerative disease. Graphical Abstract

The use of human neurons for novel drug discovery in dementia research.

ABSTRACT Introduction: Although many disease models exist for neurodegenerative disease, the translation of basic research findings to clinic is very limited. Studies using freshly resected human brain tissue, commonly discarded from neurosurgical procedures, should complement on-going work using stem cell-derived human neurons and glia thus increasing the likelihood of success in clinical trials. Areas covered: Herein, the authors discuss key issues in the lack of translation from basic research to clinic. They also review the evidence that human neurons, both freshly resected brain tissue and stem cell-derived neurons, such as induced pluripotent stem cells (iPSCs), can be used for analysis of physiological and molecular mechanisms in health and disease. Furthermore, the authors compare and contrast studies using live human brain tissue and studies using induced human stem cell-derived neuron models. Using an example from the area of neurodegeneration, the authors suggest that replicating elements of research findings from animals and stem cell models in resected human brain tissue would strengthen our understanding of disease mechanisms and the therapeutic strategies and aid translation. Expert opinion: The use of human brain tissue alongside iPSC-derived neural models can validate molecular mechanisms identified in rodent disease models and strengthen their relevance to humans. If drug target engagement and mechanism of cellular action can be validated in human brain tissue, this will increase the success rate in clinical research. The combined use of resected human brain tissue, alongside iPSC-derived neural models, could be considered a standard step in pre-clinical research and help to bridge the gap to clinical trials.

EuroTau: towing scientists to tau without tautology

What a change in situation... A few years ago, at the height of the amyloid cascade, tau biologists, (so called Tauists) were virtually invisible in Alzheimer’s disease conferences, which were occupied by amyloid biologists (so called baptists). Currently, sessions dedicated to tau and Tauopathies are increasing in several congresses on neurodegenerative diseases, including AAIC and AD/PD. Interest in tau biology is so great that a Tau consortium, set up especially to provide a forum for this area of research, has been created in the US. In Europe, tau biologists have gathered in tau-focused meetings organized in Cambridge, UK (2010, 2012), Madrid, Spain (2013) and more recently in Lille, France (2017). The microtubule-associated tau protein is not a new protein, it was discovered in 1975, and has featured in the game of neurodegenerative disorders since 1985. Tau is now the “Figura”, and the renewed interest in this protein leads one to ask “Why such interest in tau proteins and why now?”. There are many reasons for this burgeoning interest. Firstly the fact that most amyloid-centred therapies for Alzheimer’s disease (AD) and related disorders have demonstrated very modest, symptomatic efficacy, leaving an unmet medical need for new, more effective therapies. While drug development efforts in the last two decades have primarily focused on the amyloid cascade hypothesis, with disappointing results so far, tau-based strategies have, until recently, received little attention. This is despite the presence of extensive tau pathology, which is central not just to AD but is a key component of several other neurodegenerative diseases collectively called “Tauopathies”. Thus, focusing on tau as a drug target can have a profound bearing on diseasemodification for several neurodegenerative conditions facing our ageing society today. Secondly, multiple facets of tau biology, and therefore manifold potential implications for its role in Tauopathies, have emerged recently. Several laboratories world-wide made the seminal discovery that tau is the main component of the neurofibrillary tangles (NFT) found in AD patients more than thirty years ago, but since then, evidence has accumulated showing that posttranslational modifications such as acetylation, glycosylation, phosphorylation and truncation, among others [10, 14, 18] are pivotal in regulating tau functions. Thirdly, the discovery of some families with highly penetrant, dominant mutations within the tau gene causing fronto-temporal lobar degeneration [8] demonstrated that tau dysfunction, including its alternative splicing is sufficient to cause neurodegeneration and clinical dementia [1, 8, 14, 15]. Whilst it is still not clear how the mutations in the tau gene cause neurodegeneration, the overall effect of these mutations is predicted to be an increase in the rate of tau aggregation and eventually the formation of insoluble tau inclusions. As a result of this growing interest in tau biology, new hypotheses on the physiological and pathological role of tau are growing. It is no longer believed to be simply a microtubule-associated protein (MAP) [10] with recent advances in our understanding of tau’s cellular functions revealing functions beyond its classical role as a MAP. This has provided novel insights into its causative role in neurodegeneration. Such functions include neuronal polarization, axonogenesis, interactions with the plasma membrane and scaffold proteins, signal transduction, cell cycle, DNA/RNA protection, determination of dendritic spine density, and regulation of normal synaptic function [4, 11, 17]. Some of these are actively being pursued at present [12], thus broadening our range of potential therapeutic tools to treat AD and other tauopathies. Collectively, the recognition of tau as a key player in the pathobiology of human neurodegenerative diseases has driven substantial efforts to understand its biological and pathological functions. The spread of tau pathology through the brain of tauopathy patients has been the subject of recent research * Correspondence: luc.buee@inserm.fr Univ. Lille, Inserm, CHU-Lille, UMR-S 1172, LabEx DISTALZ, 59000 Lille, France Inserm U1172, Univ. Lille, Fac. Medecine, Place de Verdun, 59045 Lille cedex, France Full list of author information is available at the end of the article