A comparison of discrimination learning in touchscreen and 2-choice swim tank using an allelic series of Huntington's disease mice.

BACKGROUND: Progressive cognitive impairments are a major, debilitating symptom of neurodegenerative disorders such as Alzheimer's disease (AD) and Huntington's disease (HD). Developing treatments to slow or prevent cognitive decline is a key challenge for these fields. Unfortunately, preclinical therapeutic testing has not kept pace with molecular advances, and the methods for systematic cognitive testing in mice remain largely unchanged. Although higher throughput semi-automated systems exist, the lack of a 'positive control' (i.e. a drug or treatment that works) makes it challenging to test their sensitivity and predict usefulness for preclinical drug testing. NEW METHOD: We used an allelic series of transgenic HD mice to test the sensitivity and flexibility of two cognitive testing systems; a semi-automated touchscreen system and a traditional water-based task, the 2-choice swim tank. RESULTS: We found significant differences in performance of HD mice with different CAG repeats, with timing and severity of deficits dependent on CAG repeat length. We also found deficits in long-term memory retention that have not been reported previously. COMPARISON WITH EXISTING METHOD(S): Both systems were useful for detecting deficits, and were sensitive enough to detect small changes (10-20%) in cognitive performance. CONCLUSIONS: While the touchscreen system is more sensitive and can identify deficits up to 10 weeks earlier than the 2-choice swim tank, both tests detected similar patterns of deficit progression in HD mice, regardless of CAG repeat length. Thus, although it has its limitations, the 2-choice swim tank remains a simple, cheap and accessible system for assessing cognitive function.

"Brain training" improves cognitive performance and survival in a transgenic mouse model of Huntington's disease.

Environmental enrichment (EE) has been shown to improve neurological function and cognitive performance in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). We have shown recently that even when they are already living in an enriched environment, additional EE had beneficial effects in R6/2 mice. Here we examined the effects of three different enrichment paradigms on cognitive dysfunction in R6/2 mice in a longitudinal study. The EE consisted of either enforced physical exercise on the Rotarod (predominantly motor stimulation), training in a novel type of maze, the 'noughts and crosses' (OX) maze (mainly cognitive stimulation), or access to a playground, that gave the mice the opportunity for increased, self-motivated activity using running wheels and other toys in a social context (mixed EE). We designed the OX maze to test spatial memory in the R6/2 mouse while minimizing motor demands. Control mice remained in their home cages during the training period. Mice were given enrichment between 6 and 8 weeks of age, followed by cognitive (Lashley maze) and motor testing (Rotarod) between 8 and 10 weeks. Mice were then given a further period of enrichment between 10 and 12 weeks, and their behavior was re-tested between 12 and 14 weeks of age. We also collected body weights and age at death from all mice. The OX maze was as sensitive for detecting learning deficits in the R6/2 mice as other types of mazes (such as the Morris water maze). Interestingly, providing cognitive stimulation via training in the OX maze produced significant improvements in subsequent cognitive performance by male, but not female, R6/2 mice in the Lashley maze task. OX maze training also significantly improved loss of body weight and survival in male R6/2 mice. These effects became apparent after as little as 2 weeks of training in the OX maze. These data suggest that there is a cognitive reserve that may be exploited in neurodegenerative disease. While brain training was not beneficial for all mice, it produced no deleterious effects, and so warrants further study in rodent models of HD.

Clorgyline-mediated reversal of neurological deficits in a Complexin 2 knockout mouse.

Complexin 2 is a protein modulator of neurotransmitter release that is downregulated in humans suffering from depression, animal models of depression and neurological disorders such as Huntington's disease in which depression is a major symptom. Although complexin 2 knockout (Cplx2-/-) mice are overtly normal, they show significant abnormalities in cognitive function and synaptic plasticity. Here we show that Cplx2-/- mice also have disturbances in emotional behaviours that include abnormal social interactions and depressive-like behaviour. Since neurotransmitter deficiencies are thought to underlie depression, we examined neurotransmitter levels in Cplx2-/- mice and found a significant decrease in levels of noradrenaline and the serotonin metabolite 5-hydroxyindoleacetic acid in the hippocampus. Chronic treatment with clorgyline, an irreversible inhibitor of monoamine oxidase A, restored hippocampal noradrenaline to normal levels (from 60 to 97% of vehicle-treated Cplx2+/+ mice, P<0.001), and reversed the behavioural deficits seen in Cplx2-/- mice. For example, clorgyline-treated Cplx2-/- mice spent significantly more time interacting with a novel visitor mouse compared with vehicle-treated Cplx2-/- mice in the social recognition test (34 compared with 13%, P<0.01). We were also able to reverse the selective deficit seen in mossy fibre-long-term potentiation (MF-LTP) in Cplx2-/- mice using the noradrenergic agonist isoprenaline. Pre-treatment with isoprenaline in vitro increased MF-LTP by 125% (P<0.001), thus restoring it to control levels. Our data strongly support the idea that complexin 2 is a key player in normal neurological function, and that downregulation of complexin 2 could lead to changes in neurotransmitter release sufficient to cause significant behavioural abnormalities such as depression.

Paradoxical delay in the onset of disease caused by super-long CAG repeat expansions in R6/2 mice.

Huntington's disease (HD) is caused by an expanded CAG repeat in the HD gene. The pathological threshold for expansion in HD is around 36 CAG repeats, although 'super-long' expansions are found in brains of HD patients. We examined the effect of varying the CAG repeat length (from 170 to 450) on behavior and neuropathology of R6/2 mice. Unexpectedly, we found that increasing the repeat length delayed onset of disease and prolonged survival, from around 4 months to over 18 months in mice with the longest repeats. The delay in onset correlated with a delayed appearance of neuronal intranuclear inclusions (NIIs). However, super-long CAG repeats are not neuroprotective. Mice carrying 2 copies of the mutant transgene die earlier than those carrying a single copy. Furthermore, neurodegeneration is present in super-long repeat length mice at mid-stage disease, whereas little neurodegeneration is seen in mice with shorter CAG repeats until end stage. Expanding the CAG repeat beyond the range where NII formation is the dominant pathology has unmasked a slowly progressing neurological phenotype in R6/2 mice with brain pathology, including the identification of a novel form of inclusion, that more closely resembles that seen in adult onset cases of HD. This mouse may represent a better model for adult-onset HD than R6/2 mice with shorter repeats.

Depletion of Complexin II does not affect disease progression in a mouse model of Huntington's disease (HD); support for role for complexin II in behavioural pathology in a mouse model of HD.

Huntington's disease (HD) is a progressive, inherited, neurological disorder with a complicated phenotype that is characterised by movement abnormalities, cognitive impairments and psychiatric symptoms. Although HD is a neurodegenerative disease, recent evidence indicates that neurological dysfunction, rather than frank neurodegeneration contributes to, and may even cause early symptoms in the absence of neurodegeneration. One protein that may contribute to neurological dysfunction in HD is complexin II. Complexins are presynaptic proteins that are believed to modulate neurotransmitter release. Complexin II levels are reduced in human HD striatum and cortex, and a progressive depletion of complexin II mRNA and protein has also been shown in the R6/2 mouse model of HD. Interestingly, complexin II knockout mice share behavioural deficits in reversal learning in common with R6/2 mice. Further, the two strains both show abnormalities in long-term potentiation. This evidence led us to wonder whether or not loss of complexin II underlies some of the behavioural deficits seen in R6/2 mice. To investigate this, we crossbred complexin II knockout mice with R6/2 mice to generate a double mutant mouse. The behavioural phenotype of R6/2 mice on a null complexin II background was characterised and was compared to that seen in control mice. Complete knockout of complexin II did not significantly affect the phenotype of R6/2 mice. This indicates that loss of complexin II is part of the mechanism underlying the R6/2 phenotype. Whether it is causal or compensatory remains to be determined.

Early motor development is abnormal in complexin 1 knockout mice.

Complexin I expression is dysregulated in a number of neurological diseases including schizophrenia and depression. Adult complexin 1 knockout (Cplx1(-/-)) mice are severely ataxic and show deficits in exploration and emotional reactivity. Here, we evaluated early behavioural development of Cplx1(-/-) mice. Cplx1(-/-) mice showed marked abnormalities. They develop ataxia by post-natal day 7 (P7), and by P21 show marked deficits in tasks requiring postural skills and complex movement. These deficits are consistent with abnormalities in sensory and motor development found in infants that develop schizophrenia in later life. A role for complexin I depletion should be considered in diseases where deficits in early sensory and motor development exist, such as autism and schizophrenia.

Profound ataxia in complexin I knockout mice masks a complex phenotype that includes exploratory and habituation deficits.

Complexins are presynaptic proteins that bind to the SNARE complex where they modulate neurotransmitter release. A number of studies report changes in complexins in psychiatric (schizophrenia and depression) and neurodegenerative disorders (Huntington's disease, Wernicke's encephalopathy and Parkinson's disease). Here, we characterize the behavioural phenotype of Cplx1 knockout (Cplx1-/-) mice. Cplx1-/- mice develop a strong ataxia in the absence of cerebellar degeneration. Although originally reported to die within 2-4 months after birth, when reared using an enhanced feeding regime, these mice survive normally (i.e. >2 years). Cplx1-/- mice show pronounced deficits in motor coordination and locomotion including abnormal gait, inability to run or swim, impaired rotarod performance, reduced neuromuscular strength, dystonia and resting tremor. Although the abnormal motor phenotype dominates their overt symptoms, Cplx1-/- mice also show other behavioural deficits, particularly in complex behaviours. They have deficits in grooming and rearing behaviour and show reduced exploration in several different paradigms. They also show deficits in tasks reflecting emotional reactivity. They fail to habituate to confinement and show a 'panic' response when exposed to water. The abnormalities seen in the behaviour of Cplx1-/- mice reflect those predicted from the distribution of complexin I in the brain. Our data show that complexin I is essential not only for normal motor function in mice, but also for normal performance of other complex behaviours. These results support the idea that altered expression of complexins in disease states may contribute to the symptomatology of disorders in which they are dysregulated.

Complexin II is essential for normal neurological function in mice.

Complexins (CPLXs) are modulators of synaptic vesicle release. At 1 year of age, CPLXII knockout (KO) mice appear normal. However, behavioral testing reveals underlying deficits of motor and cognitive function in these mice. We found motor deficits on the rotarod, and learning deficits in the Morris water maze (both acquisition and reversal) and the two-choice swim tank (reversal). The reversal learning deficits are particularly noticeable, being present from the earliest time of testing, when most other behaviors are normal. CPLXII KO mice also fail to develop adult patterns of exploratory behavior in the open field and show deficits in interactive grooming behaviors. The behavioral deficits worsen with age. For example, while rotarod performance is normal until 10 weeks, it is impaired from 24 weeks onwards. Similarly, deficits in spatial learning in the Morris water maze are mild at 8 weeks, but pronounced by 1 year of age. The deficits seen in CPLXII KO mice are not due to physical weakness, since their ability to run, swim and grip is unimpaired. Rather, the mice appear to have deficits of higher function. The deficits seen in CPLXII KO mice are strikingly similar to those seen in the R6/2 model of Huntington's disease (HD) where a progressive depletion of CPLXII is seen. This suggests that depletion of CPLXII contributes to cognitive abnormalities in R6/2 mice. Given that decreased expression of CPLXII is seen in HD and schizophrenic patients, a role for CPLXII depletion should be considered in other diseases where motor, cognitive and psychiatric symptoms co-exist.