Research reportAtaxic gait analysis in a mouse model of the olivocerebellar degeneration
Introduction
Lurcher mutant mice represent a natural model of genetically-determined olivocerebellar degeneration [22]. As a model of functional cerebellar decortication, they are used for the investigation of cerebellar functions, symptoms of cerebellar degeneration and the methods of therapeutic means of influencing the neurodegenerative process or its consequences.
Lurchers are heterozygotes (+/Lc) carrying a mutation in the glutamate receptor delta2-subunit gene [31], which is expressed predominantly by cerebellar Purkinje cells [1]. Lurcher mice suffer from complete postnatal loss of Purkinje cells and a secondary retrograde degeneration of the cerebellar granule cells and inferior olive neurons [28], [29]. The degenerative process starts at postnatal day 8 (P8). At P60 there are almost no Purkinje cells in the cerebellum of Lurcher mutants. The degeneration is complete at P90, when loss of Purkinje cells is virtually complete and only 10% of the granule cells and 30% of the inferior olive neurons remain [2], [3].
Since Purkinje cells axons are the only efferent pathway of the cerebellar cortex, the loss of these cells leads to a complete functional cerebellar decortication. Functional disorders related to deep cerebellar nuclei or their marked morphological changes were not discovered [3], [14]. Therefore Lurcher mutant mice are a suitable model of cerebellar decortication with almost normal deep cerebellar nuclei.
Lurcher mice suffer from cerebellar ataxia [19], [20], a detectable deterioration of spatial learning or orientation [21], [6], [23], [5], show an impaired execution of conditioned eyelid reflexes [23], and changed reactivity to painful stimuli [27].
Homozygous mutants (Lc/Lc) are not viable due to a massive loss of brainstem neurons during prenatal development and die at birth [8], [24]. Homozygous wild type mice (+/+) are completely healthy and serve as excellent controls, because they come from the same strain and colony.
Because cerebellar ataxia and namely gait changes are the dominant signs of olivocerebellar degeneration in Lurcher mice, a quantification of the gait changes would be important for evaluation of the effects of various types of experimental treatment of this type of neurodegeneration. One of the tools used for gait analysis in rodents is the CatWalk, a computer-assisted automated quantitative over-ground locomotion gait analysis system [13]. It provides a measurement of numerous gait parameters. This system has already been used to assess gait in a mouse model of Refsum disease which is manifested not only with cerebellar ataxia but also with polyneuropahy. The latter is the main source of the gait changes in this disease [11]. The CatWalk has never been used to analyse gait manifestation of cerebellar ataxia alone and there is no experimental evidence about the suitability of the CatWalk system for this particular application.
The aim of the work was to perform exploratory analysis of gait parameters in order to identify those that are changed in Lurcher mice compared to wild type animals due to cerebellar ataxia. The next aim was to asses the correlation between the identified parameters and the walking speed to reveal potential speed dependency. Since we expected different walking speed in Lurcher and wild type mice, we decided to make a correction of the different walking speed to asses whether the differences of gait parameters between these two types of mice are related to speed differences or independent of them. The last aim was to asses the correlation between gait parameters and performance in the standard rotarod test and with body weight, an indicator of the size of the animal. Our work is the first study ever evaluating gait affected entirely by cerebellar ataxia using the CatWalk system and providing thorough exploratory analysis of gait parameters influenced by the cerebellar ataxia.
Section snippets
Materials and methods
Exploratory analysis was performed to identify parameters which are changed due to the cerebellar ataxia. The CatWalk system provides assessment of a high number of parameters and there is no previous evidence as to which of them should be those of interest for evaluation of cerebellar ataxia so there is a high risk of the statistical type I error. Exploratory analysis is a statistical method that prevents one from highlighting random differences in a comparison of multiple parameters. The
Results
We found significant differences between Lurcher and wild type mice in most of the evaluated gait parameters (p-levels of the differences between Lc1 and WT1 and between Lc2 and WT2 groups of mice are shown in Table 1).
Walking speed was significantly lower in Lurchers than in wild type mice (Fig. 1(A)). However, there was a wide range of values in both types of mice. In Lurchers, the speed of the slowest run was 61.3 mm/s and the fastest 405.2 mm/s. In wild type mice, minimum observed speed was
Discussion
In this work, the gait parameters that are changed due to cerebellar ataxia in Lurcher mutant mice were identified. There are marked differences in gait parameters between the Lurcher mice with cerebellar disorder and the wild type mice when walking speed was not taken into account. However, all these differences were strongly dependent on walking speed (discussed below) and disappeared when the influence of the different walking speed was eliminated.
To reduce the number of evaluated parameters
Acknowledgements
The work was supported by the research projects VZ MSM 021620816 and VZ FNM 006423-6505 and by the Grant COST B 30/2007 0C 152 of the Ministry of Education, Youth and Sport of the Czech Republic.
The authors would like to thank to Mr. Victor Alexander Thompson for language corrections.
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