Longitudinal analysis of the behavioural phenotype in YAC128 (C57BL/6J) Huntington's disease transgenic mice

Abstract

To determine the suitability of mouse models of disease for therapeutic trials, the models must be characterised to determine their similarity to the human condition, and utility for specific therapeutic approaches. The YAC128 mouse model of HD has been bred on to C57BL/6J background in order to provide a mouse model of the disease better suited to behavioural testing, than the visually impaired original line on the FVB background. In the present study, the C57BL/6J YAC128 mice were assessed on several behavioural tasks bi-monthly between 4 and 24 months of age. On the rotarod early and stable deficits were demonstrated in the YAC128 mice from 4 months of age indicating an early abnormality in motor coordination. Early and stable deficits were also found on the balance beam measures of latency to orientate towards the beam and time to traverse it. Measures of fore and hind limb footslips on the balance beam demonstrated early and progressive limb use deficits in the YAC128 mice. On a 3-stage Morris water maze protocol, the YAC128 mice took longer and travelled further to find the hidden platform in each of the 3 locations, indicative of a spatial learning deficit. The YAC128 mice were also less reactive to the primary startle stimuli and the effects of the prepulse which may suggest striatal dysfunction. As a measure of general well being, the body weights of the mice were recorded and demonstrated increased weight in the YAC128 mice until 14 months of age, when they became comparable to that of their wildtype littermates. The YAC128 mouse on the C57BL/6J background has an early, robust and severe behavioural phenotype that shares some similarity to human HD symptomatology.

Introduction

Huntington's disease is a genetically inherited neurodegenerative disorder with 100% penetrance, caused by a mutation in the HTT gene [10], the gene responsible for the protein huntingtin (Htt). The mutation causes the pathological expansion of the polyQ region in exon 1 of the gene resulting in widespread atrophy and cell death in the brain through unknown mechanisms [8]. Cell death is largely concentrated on the medium spiny neurons (MSNs) of the striatum where there is a relative sparing of the large cholinergic inter-neurons, but cortical areas also demonstrate marked cell death with cortical thinning and sulcal widening being a feature of the cortical atrophy [2], [14], [25], [26], [32]. The most prominent marker of cellular pathology is the formation of protein aggregates created from cleaved, insoluble, N-terminal fragments of the mutant protein and other sequestered proteins. Aggregates may ultimately form dense neuronal intra-nuclear inclusions (NIIs) [4]. At present the role of the aggregation process and the NIIs in neurodegeneration is unknown. In functional terms, the neuropathology induces a broad spectrum of behavioural changes beginning with subtle cognitive and emotional changes [5], [11], [31] and including highly specific learning deficits in for example, implicit learning [12] and set-shifting tasks [15], [16], [17] prior to the manifestation of motor abnormalities and choreic movements that characterise the disorder.

Since the disease is caused by a single mutation it lends itself readily to recapitulation in the form of mouse models of the disease, of which several have been created [7], [18], [19], [22], [24], [28], [37]. The YAC128 mouse is a transgenic model of HD where the transgene carrying a construct with 128 CAG repeats [30] was introduced via a yeast artificial chromosome. The original model, which was bred on an FVB/N background strain, recapitulates several aspects of disease including the neuropathological features of striatal and cortical cell loss, protein aggregation and NIIs [9], [30], [34], as well as motor and cognitive deficits [30], [33], [34], [35], [36].

The motor deficits are apparent from an early age in the FVB variant of the YAC128. Motor abnormalities include deficits in motor learning and coordination as measured by the accelerating rotarod [33], [34], [35], [36] and initial hyperactivity is followed by a general reduction in movement in open field activity assessments [30], [35]. The hyperactivity appears at 3 months of age, with the rotarod acquisition deficits appearing around 2 months of age [36], and performance on a fixed speed rotarod correlated highly with the striatal neuronal loss at 6, 9 and 12 months of age [30]. Several cognitive deficits have been described. The YAC128 mice were slower to habituate to an open field arena at 8 months of age, and learning deficits where the time taken to swim, and the number of errors incurred, in free-swimming water T-maze tests were greater for the YAC128s than their wildtype littermates at 8.5 months of age [36]. A decreased prepulse inhibition was present by 12 months [36].

Whilst the YAC128 on the FVB background is essentially a good recapitulation of the human condition, the major drawback is that the background strain confers a pronounced retinal degeneration to the model which limits the range of tasks that the animals can perform. However, the YAC128 mouse has been crossed on a C57BL/6J making it ideally suited for behavioural testing [21], [33], although on this background strain the disease has been found to be more resistant to the disease than the original FVB line [33]. Nevertheless, motor deficits are present from an early age (4 months), and age where striatal volume is reduced but no cell loss is present [33]. To date relatively little work has been conducted on the C57BL/6J variant of the YAC128 mouse.

The aim of the present paper was to characterise the nature and evolution of behavioural deficits in the C57BL/6J YAC128 mouse line to determine whether this model is a good representation of the human condition on which to base therapeutic trials. The study was run through the longitudinal application (2–24 months of age) of a number of assessments (bodyweight, rotarod, balance beam, prepulse inhibition and acoustic startle, and a variation of the Morris water maze test), with which to gauge the relative contributions to the disease of specific motor and cognitive deficits.