The scope and limitations of rodent models of AD

The plethora o f transgenic mice available is symptomatic o f the complexity and lack of understanding of AD. For a mouse model to be valuable it should demonstrate an age- dependent decline in complex cognitive behaviour and show important pathological features of the disease. None o f the mouse models discussed completely reproduces the histopathological, biochemical and cognitive impairments characteristic o f AD. The major omission in all the current models is the neurofibrillary tangles of the microtubule associated protein, tau. One o f the APP transgenic mouse models demonstrated subtle

changes in tau phosphorylation (Sturchler-Pierrat et a l, 1997) and excitotoxic brain

insults in rodents have been shown to induce changes in tau (Stein-Behrens et a l, 1994)

but the stable twisted tau filaments characteristic of AD are not seen. Although the transgenic models described have been invaluable in dissecting some of the pathogenic mechanisms o f AD, there is some evidence to suggest that rodents may be inherently unsuitable for modelling this multifactorial human disease.

Aged rodents do not naturally develop AD-like disorders, which is in contrast to

a number o f other species including polar bears, primates, dogs and sheep (Selkoe et a l,

1987; Nelson et a l, 1994). Furthermore, rodent Ap does not aggregate in vitro and

transgenic mice overexpressing murine AP do not develop plaques (De Strooper et al,

1995). More importantly, considering the major omission from all the currently available transgenic models o f AD, rodents only contain a subset of the tau isoforms which have been implicated in the human disease.

Tau is expressed from a single gene on chromosome 17. In the adult human brain, six isoforms of tau ranging between 352 and 441 amino acids in length are produced as a result o f alternate splicing. A schematic representation of these isoforms is shown in figure 1.3.5.1. The incorporation or exclusion o f exon 2 or exons 2 and 3 results in proteins with 0 (ON), 29 (IN ) or 58 (2N) amino acid inserts near the N- terminus. Furthermore, exon 10 can be alternatively spliced to give products with either three (3R) or four (4R) microtubule binding repeats. Additional alternate splicing and inclusion of exon 4A can yield a group o f higher molecular weight tau, known as “big tau”. In adult human brain, 3R tau is slightly more abundant than 4R tau and there are

more ON and IN isoforms than 2N isoforms (reviewed in Spillantini and Goedert, 1998). Abnormally aggregated tau isolated from human AD brains contains all six tau

isoforms (Goedert et a l, 1992). In the context o f developing transgenic models o f AD it

is therefore o f paramount importance to note that rodents only express 4R tau (Gotz et

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Figure 1.3.5.1 Tau isoforms of the human and rodent brains

A schematic representation of the tau isoforms present in adult human and adult rodent brain is shown. The alternatively spliced amino terminal inserts are shown in grey (exon 2) and blue (exon 3). The microtubule binding repeats are shown in black with the alternatively spliced repeat (exon 10) in yellow. All six isoforms are expressed in adult human brain whereas only the four repeat isoforms are expressed in rodent brains. The paired helical filaments seen in post mortem AD brains contain hyperphosphorylated forms of all six isoforms. Figure adapted from Spillantini and Goedert, 1998.

Following the identification of the FAD-associated mutations in APP and PSl and the realisation that these all acted to increase Ap deposition, neurofibrillary tau tangles were often considered to be a downstream or non-specific aspect o f AD pathology. However, a non-Alzheimer’s dementia called fronto-temporal dementia with Parkinsonism related to chromosome 17 (FTDP-17) has recently been found to be caused by mutations in the

tau gene (Hutton et a l, 1998). Four exonic and four intronic mutatons have been

described in thirteen families. The exonic mutations are located in the microtubule binding repeat region, suggesting that they may serve to reduce the ability o f tau to interact with microtubules. The intronic mutations are located close to the splice site of the intron following exon 10 (shown in yellow in figure 1.3.5.1). These intronic mutations were shown to increase the ratio o f tau mRNAs containing exon 10 and therefore the proportion of tau isoforms with 4 microtubule binding repeat domains

(4R) (Grover et a l, 1999). This alteration in the ratio of 4R relative to 3R tau is

sufficient to lead to the development of a filamentous tau pathology and dementia. Despite the fact that no cases o f AD have been associated with mutations in tau, this important discovery has suggested that in at least some AD cases, tau dysfunction is likely to be a cause, not just a by-product, o f the neuropathology.

The response of the AD field to the identification o f these tau mutations is the proposal to generate transgenic mice expressing one o f the mutant tau isoforms and cross this line with the double mutant APP/PSl mice (Selkoe, 1999). Although this will almost undoubtedly produce the long-sought pathology of both amyloid plaques and hyperphosphorylated tau tangles, the relevance of a mouse model containing extremely rare mutations in three different genes is questionable. A more pertinent model might be one which utilised the Y AC technology described in section 1.3.1 to express the entire genomic clone of wild type human tau on the background of a tau knockout mouse expressing wild type human APP and mutant human PSl or vice versa. Whilst such a model might address the issue of species specific tau isoforms, it is possible that there are many other aspects to this complex multifactorial disorder which cannot be fully recreated in a species which does not naturally develop AD or related disorders. It is apparent from the foregoing discussion that it might be preferable to model AD in a

species with a longer life span and in which the aged population naturally became more susceptible to neurodegeneration.

Due to the current limits o f transgenesis, there has been very little work reported on attempts to model AD in species other than rodents. However, it has been found that Ap fibrils directly injected into the cerebral cortex of old but not young rhesus monkeys

can cause neurodegeneration and changes in tau phosphorylation (Geula et a l, 1998).

Notably, similar Ap injections in the brains of young or old rats produced neither of

these effects (Games et a l, 1992; Geula et a l, 1998).

A secondary limitation o f transgenic technology is that the introduction of genes into the germline is a somewhat uncertain and time-consuming procedure. Importantly in the study of a complex disorder such as AD, the study o f gene combinations then requires the breeding and subsequent crossing o f multiple transgenic lines.