Date Published: September 20, 2017
Publisher: BioMed Central
Author(s): Thomas Blackmore, Soraya Meftah, Tracey Karen Murray, Peter James Craig, Anthony Blockeel, Keith Phillips, Brian Eastwood, Michael J. O’Neill, Hugh Marston, Zeshan Ahmed, Gary Gilmour, Francois Gastambide.
The choice and appropriate use of animal models in drug discovery for Alzheimer’s disease (AD) is pivotal to successful clinical translation of novel therapeutics, yet true alignment of research is challenging. Current models do not fully recapitulate the human disease, and even exhibit various degrees of regional pathological burden and diverse functional alterations. Given this, relevant pathological and functional endpoints must be determined on a model-by-model basis. The present work explores the rTg4510 mouse model of tauopathy as a case study to define best practices for the selection and validation of cognitive and functional endpoints for the purposes of pre-clinical AD drug discovery.
Male rTg4510 mice were first tested at an advanced age, 12 months, in multiple behavioural assays (step 1). Severe tau pathology and neurodegeneration was associated with profound locomotor hyperactivity and spatial memory deficits. Four of these assays were then selected for longitudinal assessment, from 4 to 12 months, to investigate whether behavioural performance changes as a function of accumulation of tau pathology (step 2). Experimental suppression of tau pathology—via doxycycline administration—was also investigated for its effect on functional performance.
Progressive behavioural changes were detected where locomotor activity and rewarded alternation were found to most closely correlate with tau burden and neurodegeneration. Doxycycline initiated at 4 months led to a 50% suppression of transgene expression, which was sufficient to prevent subsequent increases in tau pathology and arrest related functional decline.
This two-step approach demonstrates the importance of selecting assays most sensitive to the phenotype of the model. A robust relationship was observed between pathological progression, development of phenotype, and their experimental manipulation—three crucial factors for assessing the translational relevance of future pre-clinical findings.
The vast societal and economic burden of Alzheimer’s disease (AD) represents a growing problem . Despite extensive effort, AD drug discovery programmes have so far lacked late-stage clinical success [2, 3]. While some of these failures have arisen from incomplete understanding of drug properties or inappropriate study design [4, 5], many may also reflect true negative effects. Given the several hundred interventions reported to mitigate pathological and behavioural alterations in AD mouse models, it is reasonable to question the predictive validity of early AD drug discovery . Pre-clinical validation forms an integral part of nearly every drug project, yet justification of disease model as well as the endpoint(s) chosen often appears ad hoc, or simply lacking. In contrast, clinical efficacy of AD therapies is currently based on pre-defined cognitive test batteries and activities of daily living questionnaires. Therefore, a major consideration for future AD pre-clinical efforts would be to better understand the benefits and limitations of existing behavioural and functional assays, their relationship with disease progression (e.g. ), and ultimately with clinical endpoints.
The present study aimed to extend on the initial phenotypic characterisation of the rTg4510 mouse model and, in doing so, to define a common experimental approach to selection and validation of the most relevant cognitive and behavioural endpoints. Out of the seven behavioural assays tested at a late pathological stage, four were sensitive to tau pathology. Performance in these assays was shown to worsen progressively with age, to correlate well with developing tau pathology, and to be responsive to experimental modulation of tau expression by doxycycline treatment. The confluence of these three characteristics further illustrated that functional measures such as behavioural performance can be a meaningful proxy for underlying pathology, without the need for invasive measures.
The two-step experimental approach used in this study provides a detailed cognitive and behavioural profiling of male rTg4510 mice, with careful matching to pathological progression. Only four of the seven behavioural assays initially tested produced robust deficits in male rTg4510 mice, highlighting the importance of careful selection and validation of functional endpoints prior to testing. This approach can easily be applied to increase our understanding of other transgenic mouse models of AD. Ultimately, better understanding of the nature of functional and pathological decline in such models of interest may lead to better study design, and in turn improve confidence in pre-clinical validation of novel, emerging therapeutic interventions. This will hopefully lead to more robust, translatable packages enhancing chances of clinical success in the future.