Animal models for drug discovery in dystonia

Deep brain stimulation by optimized stimulators in a phenotypic model of dystonia: Effects of different frequencies

Deep brain stimulation (DBS) of the globus pallidus internus (GPi, entopeduncular nucleus, EPN, in rodents) has become important for the treatment of generalized dystonia, a severe and often intractable movement disorder. It is unclear if lower frequencies of GPi-DBS or stimulations of the subthalamic nucleus (STN) are of advantage. In the present study, the main objective was to examined the effects of bilateral EPN-DBS at different frequencies (130 Hz, 40 Hz, 15 Hz) on the severity of dystonia in the dt^sz mutant hamster. In addition, STN stimulations were done at a frequency, proven to be effective by the present EPN-DBS in dystonic hamsters. In order to obtain precise bilateral electrical stimuli with magnitude of 50 μA, a pulse width of 60 μs and defined frequencies, it was necessary to develop a new optimized stimulator prior to the experiments. Since the individual highest severity of dystonic episodes is known to be reached within three hours after induction in dt^sz hamsters, the duration of DBS was 180 min. During DBS with 130 Hz the severity of dystonia was significantly lower within the third hour than without DBS in the same animals (p < 0.05). DBS with 40 Hz tended to exert antidystonic effects after three hours, while 15 Hz stimulations of the EPN and 130 Hz stimulations of the STN failed to show any effects on the severity. DBS of the EPN at 130 Hz was most effective against generalized dystonia in the dt^sz mutant. The response to EPN-DBS confirms that the dt^sz mutant is suitable to further investigate the effects of long-term DBS on severity of dystonia and neuronal network activities, important to give insights into the mechanisms of DBS.

A Novel Transgenic Mouse Model to Investigate the Cell-Autonomous Effects of torsinA(ΔE) Expression in Striatal Output Neurons

Dystonia is a disabling neurological syndrome characterized by abnormal movements and postures that result from intermittent or sustained involuntary muscle contractions; mutations of DYT1/TOR1A are the most common cause of childhood-onset, generalized, inherited dystonia. Patient and mouse model data strongly support dysregulation of the nigrostriatal dopamine neurotransmission circuit in the presence of the DYT1-causing mutation. To determine striatal medium spiny neuron (MSN) cell-autonomous and non-cell autonomous effects relevant to dopamine transmission, we created a transgenic mouse in which expression of mutant torsinA in forebrain is restricted to MSNs. We assayed electrically evoked and cocaine-enhanced dopamine release and locomotor activity, dopamine uptake, gene expression of dopamine-associated neuropeptides and receptors, and response to the muscarinic cholinergic antagonist, trihexyphenidyl. We found that over-expression of mutant torsinA in MSNs produces complex cell-autonomous and non-cell autonomous alterations in nigrostriatal dopaminergic and intrastriatal cholinergic function, similar to that found in pan-cellular DYT1 mouse models. These data introduce targets for future studies to identify which are causative and which are compensatory in DYT1 dystonia, and thereby aid in defining appropriate therapies.

Expression of metabotropic glutamate 5 receptors in the striatum and cortex and effects of modulators on the severity of dystonia in the phenotypic dtsz model

The metabotropic glutamate 5 (mGlu₅) receptor has been suggested as therapeutic target for L-Dopa-induced dyskinesia which is often associated with dystonic symptoms. Therefore, we investigated the acute effects of the non-competitive mGlu₅ receptor antagonist fenobam as well as the positive modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) on the severity of inherited dystonia in the mutant dt^sz hamster, a phenotypic model with age-dependent episodes of dystonia. Fenobam did not exert significant antidystonic effects (20-50 mg/kg intraperinoneal, i.p.). CDPPB (10, 20 mg/kg i.p.) which was expected to worsen dystonia also failed to show any effects on the severity of dystonia. Interestingly, CDPPB caused axial dyskinesia in addition to the dystonic symptoms in mutant hamsters. This adverse effect could not be observed in non-dystonic control hamsters, indicating possible changes in the expression of mGlu₅ receptors in dystonic hamsters. The mGlu₅ receptor mRNA did not differ between the dt^sz mutant and control hamsters, while immunohistochemical studies indicated that the mGlu₅ receptor expression was about 35% higher in striatum and cortex of mutant hamsters at the age of high dystonia severity scores, notably not after spontaneous remission of dystonia, compared to age-matched controls. This difference in mGlu₅ receptor protein may be due to altered protein conformation instead of protein level, as western blots revealed similar amounts of monomeric and dimeric protein in mutant hamsters versus control. Thus, the present data do not provide clear evidence for an important role of the mGlu5 receptor in the pathophysiology and as a therapeutic target for types of inherited dystonia.

Alterations of M1 and M4 acetylcholine receptors in the genetically dystonic (dtsz) hamster and moderate antidystonic efficacy of M1 and M4 anticholinergics

Striatal cholinergic dysfunction has been suggested to play a critical role in the pathophysiology of dystonia. In the dt^sz hamster, a phenotypic model of paroxysmal dystonia, M1 antagonists exerted moderate antidystonic efficacy after acute systemic administration. In the present study, we examined the effects of the M4 preferring antagonist tropicamid and whether long-term systemic or acute intrastriatal injections of the M1 preferring antagonist trihexyphenidyl are more effective in mutant hamsters. Furthermore, M1 and M4 receptors were analyzed by autoradiography and immunohistochemistry. Tropicamide retarded the onset of dystonic attacks, as previously observed after acute systemic administration of trihexyphenidyl. Combined systemic administration of trihexyphenidyl (30mg/kg) and tropicamide (15mg/kg) reduced the severity in acute trials and delayed the onset of dystonia during long-term treatment. In contrast, acute striatal microinjections of trihexyphenidyl, tropicamid or the positive allosteric M4 receptor modulator VU0152100 did not exert significant effects. Receptor analyses revealed changes of M1 receptors in the dorsomedial striatum, suggesting that the cholinergic system is involved in abnormal striatal plasticity in dt^sz hamsters, but the pharmacological data argue against a crucial role on the phenotype in this animal model. However, antidystonic effects of tropicamide after systemic administration point to a novel therapeutic potential of M4 preferring anticholinergics for the treatment of dystonia.

Sensorimotor tests unmask a phenotype in the DYT1 knock-in mouse model of dystonia

Hereditary generalized dystonia is often caused by a GAG deletion in TOR1A (DYT1) that encodes for the protein torsinA. Although mutation carriers show alterations in neuronal connectivity and sensorimotor deficits, only 30% develop dystonia. Uncovering the factors triggering the dystonic symptoms and underlying pathophysiology would greatly benefit the development of more effective therapies. In DYT1 knock-in (KI) mice, the expression of torsinA mutant alters the connectivity of neurons and the function of striatal cholinergic interneurons. We aimed to determine if heterozygous DYT1 KI mice exhibit deficits in behavioural tests that explore the connectivity of the sensory and motor system. DYT1 KI mice were tested in cognitive tests and challenging motor paradigms, followed by the adhesive removal test and the adaptive rotating beam test which both require sensorimotor integration. DYT1 KI mice did not exhibit cognitive deficits and were able to perform similarly to wild type mice even in challenging motor tests with relatively stable sensory input. Conversely, DYT1 KI mice spent more time on sensing and removing an adhesive sticker from the back of the nose; they exhibited difficulty to traverse rotating rods, especially if the surface was smooth and the diameter small. Our observations further support a role of sensorimotor integration in manifestation of this movement disorder. Future studies in DYT1 KI mice will explore the involved neurocircuitry and underlying molecular mechanisms.

Dystonia and Paroxysmal Dyskinesias: Under-Recognized Movement Disorders in Domestic Animals? A Comparison with Human Dystonia/Paroxysmal Dyskinesias

Dystonia is defined as a neurological syndrome characterized by involuntary sustained or intermittent muscle contractions causing twisting, often repetitive movements, and postures. Paroxysmal dyskinesias are episodic movement disorders encompassing dystonia, chorea, athetosis, and ballism in conscious individuals. Several decades of research have enhanced the understanding of the etiology of human dystonia and dyskinesias that are associated with dystonia, but the pathophysiology remains largely unknown. The spontaneous occurrence of hereditary dystonia and paroxysmal dyskinesia is well documented in rodents used as animal models in basic dystonia research. Several hyperkinetic movement disorders, described in dogs, horses and cattle, show similarities to these human movement disorders. Although dystonia is regarded as the third most common movement disorder in humans, it is often misdiagnosed because of the heterogeneity of etiology and clinical presentation. Since these conditions are poorly known in veterinary practice, their prevalence may be underestimated in veterinary medicine. In order to attract attention to these movement disorders, i.e., dystonia and paroxysmal dyskinesias associated with dystonia, and to enhance interest in translational research, this review gives a brief overview of the current literature regarding dystonia/paroxysmal dyskinesia in humans and summarizes similar hereditary movement disorders reported in domestic animals.

Genetic animal models of dystonia: common features and diversities

Animal models are pivotal for studies of pathogenesis and treatment of disorders of the central nervous system which in its complexity cannot yet be modeled in vitro or using computer simulations. The choice of a specific model to test novel therapeutic strategies for a human disease should be based on validity of the model for the approach: does the model reflect symptoms, pathogenesis and treatment response present in human patients? In the movement disorder dystonia, prior to the availability of genetically engineered mice, spontaneous mutants were chosen based on expression of dystonic features, including abnormal muscle contraction, movements and postures. Recent discovery of a number of genes and gene products involved in dystonia initiated research on pathogenesis of the disorder, and the creation of novel models based on gene mutations. Here we present a review of current models of dystonia, with a focus on genetic rodent models, which will likely be first choice in the future either for pathophysiological or for preclinical drug testing or both. In order to help selection of a model depending on expression of a specific feature of dystonia, this review is organized by symptoms and current knowledge of pathogenesis of dystonia. We conclude that albeit there is increasing need for research on pathogenesis of the disease and development of improved models, current models do replicate features of dystonia and are useful tools to develop urgently demanded treatment for this debilitating disorder.

Dystonia as a network disorder: what is the role of the cerebellum?

The dystonias are a group of disorders defined by sustained or intermittent muscle contractions that result in involuntary posturing or repetitive movements. There are many different clinical manifestations and causes. Although they traditionally have been ascribed to dysfunction of the basal ganglia, recent evidence has suggested dysfunction may originate from other regions, particularly the cerebellum. This recent evidence has led to an emerging view that dystonia is a network disorder that involves multiple brain regions. The new network model for the pathogenesis of dystonia has raised many questions, particularly regarding the role of the cerebellum. For example, if dystonia may arise from cerebellar dysfunction, then why are there no cerebellar signs in dystonia? Why are focal cerebellar lesions or degenerative cerebellar disorders more commonly associated with ataxia rather than dystonia? Why is dystonia more commonly associated with basal ganglia lesions rather than cerebellar lesions? Can answers obtained from animals be extrapolated to humans? Is there any evidence that the cerebellum is not involved? Finally, what is the practical value of this new model of pathogenesis for the neuroscientist and clinician? This article explores potential answers to these questions.

The focal dystonias: current views and challenges for future research

The most common forms of dystonia are those that develop in adults and affect a relatively isolated region of the body. Although these adult-onset focal dystonias are most prevalent, knowledge of their etiologies and pathogenesis has lagged behind some of the rarer generalized dystonias, in which the identification of genetic defects has facilitated both basic and clinical research. This summary provides a brief review of the clinical manifestations of the adult-onset focal dystonias, focusing attention on less well understood clinical manifestations that need further study. It also provides a simple conceptual model for the similarities and differences among the different adult-onset focal dystonias as a rationale for lumping them together as a class of disorders while at the same time splitting them into subtypes. The concluding section outlines some of the most important research questions for the future. Answers to these questions are critical for advancing our understanding of this group of disorders and for developing novel therapeutics.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

Lesch-Nyhan disease: from mechanism to model and back again

Lesch-Nyhan disease (LND) is a rare inherited disorder caused by mutations in the gene encoding hypoxanthine-guanine phosphoribosyltransferase (HPRT). LND is characterized by overproduction of uric acid, leading to gouty arthritis and nephrolithiasis. Affected patients also have characteristic neurological and behavioral anomalies. Multiple cell models have been developed to study the molecular and metabolic aspects of LND, and several animal models have been developed to elucidate the basis for the neurobehavioral syndrome. The models have different strengths and weaknesses rendering them suitable for studying different aspects of the disease. The extensive modeling efforts in LND have questioned the concept that an 'ideal' disease model is one that replicates all of its features because the pathogenesis of different elements of the disease involves different mechanisms. Instead, the modeling efforts have suggested a more fruitful approach that involves developing specific models, each tailored for addressing specific experimental questions.

Transcriptional and proteomic profiling in a cellular model of DYT1 dystonia

DYT1, the most common inherited dystonia, is caused by a common dominant mutation in the TOR1A gene that leads to a glutamic acid deletion in the protein torsinA. Wild-type torsinA locates preferentially in the endoplasmic reticulum while the disease-linked mutant accumulates in the nuclear envelope. As a result, it has been proposed that DYT1 pathogenesis could result either from transcriptional dysregulation caused by abnormal interactions of mutant torsinA with nuclear envelope proteins, or from a loss of torsinA function in the endoplasmic reticulum that would impair specific neurobiological pathways. Aiming to determine whether one or both of these potential mechanisms are implicated in DYT1 pathogenesis, we completed unbiased transcriptional and proteomic profiling in well-characterized neural cell lines that inducibly express wild-type or mutant torsinA. These experiments demonstrated that the accumulation of mutant torsinA in the nuclear envelope is not sufficient to cause transcriptional dysregulation. However, we detected expression changes at the protein level that, together with other reports, suggest a potential implication of torsinA on energy metabolism and regulation of the redox state. Furthermore, several proteins identified in this study have been previously linked to other forms of dystonia. In conclusion, our results argue against the hypothesis of transcriptional dysregulation in DYT1 dystonia, suggesting potential alternative pathogenic pathways.

Experimental therapeutics for dystonia

Dystonia is a neurological syndrome characterized by excessive involuntary muscle contractions leading to twisting movements and unnatural postures. It has many different clinical manifestations, and many different causes. More than 3 million people worldwide suffer from dystonia, yet there are few broadly effective treatments. In the past decade, progress in research has advanced our understanding of the pathogenesis of dystonia to a point where drug discovery efforts are now feasible. Several strategies can be used to develop novel therapeutics for dystonia. Existing therapies have only modest efficacy, but may be refined and improved to increase benefits while reducing side effects. Identifying rational targets for drug intervention based on the pathogenesis of dystonia is another strategy. The surge in both basic and clinical research discoveries has provided insights at all levels, including etiological, physiological and nosological, to enable such a targeted approach. The empirical approach to drug discovery, whereby compounds are identified using a nonmechanistic strategy, is complementary to the rational approach. With the recent development of multiple animal models of dystonia, it is now possible to develop assays and perform drug screens on vast numbers of compounds. This multifaceted approach to drug discovery in dystonia will likely provide lead compounds that can then be translated for clinical use.

The pathophysiological basis of dystonias

Dystonias comprise a group of movement disorders that are characterized by involuntary movements and postures. Insight into the nature of neuronal dysfunction has been provided by the identification of genes responsible for primary dystonias, the characterization of animal models and functional evaluations and in vivo brain imaging of patients with dystonia. The data suggest that alterations in neuronal development and communication within the brain create a susceptible substratum for dystonia. Although there is no overt neurodegeneration in most forms of dystonia, there are functional and microstructural brain alterations. Dystonia offers a window into the mechanisms whereby subtle changes in neuronal function, particularly in sensorimotor circuits that are associated with motor learning and memory, can corrupt normal coordination and lead to a disabling motor disorder.