Behavioural and pharmacological examinations in a transgenic mouse model of early-onset torsion dystonia

Abstract

Early-onset torsion dystonia is an autosomal dominant movement disorder associated with the DYT1 gene (TOR1A) defect which results in a deletion of a glutamic acid residue in the protein torsinA. The pathophysiology of dystonia is poorly understood. Well characterized animal models can help to give insights into the underlying mechanisms and thereby to develop new therapeutics. In the present study, we further characterized transgenic DYT1 mice, which were initially described to exhibit “dystonia-like” postures. In the present study, several behavioural tests in untreated animals did not show strong differences between transgenic and control mice, but nearly all transgenic mice showed “dystonia-like” postures. However, these movements, also observed in control mice, have to be regarded as a clasping reflex. Since dystonia is thought to be related to dopaminergic dysfunctions, pharmacological investigations have been performed to clarify if dopaminergic substances alter motor behaviour in transgenic mice. Chronic treatment with L-DOPA (combined with carbidopa) enhanced the hindlimb claspings only in transgenic mice, while acute applications of drugs, which exert more selective effects on the dopaminergic system, caused similar reactions in transgenic mice and control mice. Therefore, these data do not provide clear evidence for dysfunctions of the dopaminergic system in this mouse model.

Introduction

Dystonia is a movement disorder characterized by involuntary, sustained, patterned, and often repetitive muscle contractions of opposing muscles, frequently causing twisting movements or abnormal postures (Fahn et al., 1998, Jankovic, 2006). The pathogenesis of idiopathic types is poorly understood and probably heterogeneous in the different phenotypic and genotypic subtypes (Breakefield et al., 2008). Thus, there is a need for animal models which are clearly defined for various types of dystonia, which can be helpful for giving insights into the underlying mechanisms and for the development of more effective therapeutics (Richter and Löscher, 2000).

Early-onset torsion dystonia is the most common form of primary generalized dystonia. This type of dystonia usually occurs between the 5th and 28th year of age and has a penetrance of only 30–40% in gene carriers (Bandmann and Müler, 2002, Bressman, 2006). Most cases are caused by a 3 bp (GAG) deletion in the DYT1 gene (TOR1A) on chromosome 9q34, resulting in the loss of a glutamic acid residue in the torsinA protein (ΔE-torsinA) (Ozelius et al., 1997). Protein torsinA is a member of AAA+ proteins which are involved in many cellular functions, e.g., vesicle fusion and protein folding (Ogura and Wilkinson, 2001). However, the mechanisms by which the gene defect and the ΔE-torsinA cause dystonia are still unknown, but dopaminergic dysfunctions have been suggested to be involved. TorsinA is expressed in dopaminergic neurons of the substantia nigra pars compacta but also in other non-dopaminergic brain regions (Augood et al., 2003, Rostasy et al., 2003). Furthermore, significant increases in the turnover of dopamine and a reduction in dopamine D₁ and D₂ receptor binding have been reported in the striatum of DYT1 patients (Asanuma et al., 2005, Augood et al., 2002).

Different mouse lines which carry the human gene defect are recommended as models for the early-onset torsion dystonia (Grundmann et al., 2007, Sharma et al., 2005, Shashidharan et al., 2005). The initial descriptions by Shashidharan et al. (2005) led to the conclusion that these transgenic mice show “dystonic-like” postures, whereas other DYT1 mouse models have failed to exhibit such symptoms (Grundmann et al., 2007, Sharma et al., 2005). Since a dystonic phenotype is important for preclinical drug testing, the mouse model generated by Shashidharan et al. (2005) was chosen for further characterization by behavioural and pharmacological experiments in the present study. Initially, several tests were performed in untreated transgenic mice for the evaluation of motor behaviour. Heiman et al. (2004) described anxiety-related behaviour in DYT1 patients. Therefore, the elevated plus maze test was included in the present study. These investigations were carried out in mice at different ages (3, 6, 9 and 12 months) to recognize a possible progression of the disease, as known in DYT1 patients. In order to determine the possible functional role of dopaminergic dysfunctions in transgenic mice, different dopaminergic drugs were acutely administered in the present study. Thereby, attention was paid to the effects on the initially described “dystonic-like” postures (Shashidharan et al., 2005). Since these postures were also observed in control mice, we considered these postures as clasping reflexes. We also determined if possible changes of the clasping reflex could be provoked by long-lasting manipulations of the dopaminergic system.