It is stating the obvious that having good animal models is critical to the success of any drug discovery program. In many more complex diseases however, good animal models are not available. The ‘gold standard’ animal models for Alzheimer’s disease, Aβ-overproducing transgenic AD mice; do not demonstrate robust tauopathy and subsequent neuronal loss without the addition of genes not linked to familial AD.
In a recent paper Cohen et al., (1) have generated transgenic rats bearing human mutant APP (amyloid precursor protein) and PS1 (presenilin 1). These animals appear to manifest the full spectrum of age-dependent Alzheimer’s disease pathologies alongside cognitive disturbances. They have age-dependent β-amyloid deposition as well as intraneuronal Aβ1-42 and soluble Aβ oligomers. Many mouse models do present with some tauopathy, however, they do not present with neurofibrillary tangles (NFT) as observed in human AD. In this rat model however, they identified striking tauopathy. As well as hyperphosphorylated Tau, structures reminiscent to NFTs were identified close to β-amyloid plaques in aged rats. In addition immunostaining revealed structures consistent with NFTs in 16 month old rats. These NFT-like structures were also frequently observed in areas without plaques, as is found in human AD.
In concert with the molecular pathology, these transgenic rats exhibited neuronal loss and neuronal degeneration that was progressive and age-dependent. There was also an inverse correlation between the neuronal numbers and Aβ1-42 abundance. TUNEL staining indicated the presence of nicked DNA and measurements of active caspase-3 suggested the neurons were apoptosing. This neuronal loss paralleled changes in behavioural characteristics such as novel object recognition (which is a hippocampal-dependent measure of working memory) that was significantly impaired in older transgenic animals. This was repeated in the Barnes maze, where there were no difference between wild-type and transgenic animals at 6 months, but after 15 months the transgenic animals made significantly more errors than wild-type.
With recent late-stage failures of treatments for Alzheimers this new animal model opens up the possibility to test novel therapeutics in a more human disease-like model.