Understanding mental retardation in Down's syndrome using trisomy 16 mouse models

Working memory in the Ts65Dn mouse, a model for Down syndrome

This study used a matching-to-position schedule of reinforcement to examine working memory in Ts65Dn and littermate control mice. Initially there appeared to be a memory deficit in the Ts65Dn mice, which disappeared with extended practice. Thus, what appeared as a memory deficit may actually be the result of a delay in learning the concept of matching. These results suggest that delayed learning may be an important factor in other procedures examining working memory in Ts65Dn mice and have important implications for clinical treatment of Down syndrome patients.

Abnormal synaptic plasticity in the Ts1Cje segmental trisomy 16 mouse model of Down syndrome

Due to the homology between human chromosome 21 and mouse chromosome 16, trisomy 16 mice are considered animal models of Down syndrome (DS). Abnormal hippocampal synaptic plasticity and behavior have been reported in the segmental trisomy 16 Ts65Dn mouse. In the Ts1Cje DS mouse model, which has a shorter triplicated chromosomal segment than Ts65Dn, more subtle hippocampal behavioral deficits have been reported. In this study, we investigated CA1 hippocampal synaptic plasticity, long-term potentiation (LTP) and depression (LTD) in the Ts1Cje mouse. Field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 area of in vitro hippocampal slices from the Ts1Cje mouse and diploid controls, LTP was induced by a single tetanizing train pulse (1 s) at 100 Hz and LTD by a 900-pulse train at 1 Hz. We report for the first time that compared to diploid controls, the hippocampus from the Ts1Cje mouse had a smaller LTP and an increased LTD. The changes are less dramatic than had been reported previously for the Ts65Dn mouse. Furthermore, in the Ts1Cje mouse trains of pulses at both 20 Hz and 100 Hz produced a decrease in the evoked fEPSPs over the length of the train in comparison to diploid fEPSPs. These findings suggest that genes from Ts1Cje chromosome, including GIRK2 potassium channel, contribute to abnormal short- and long-term plasticity.

Behavioral, cognitive and biochemical responses to different environmental conditions in male Ts65Dn mice, a model of Down syndrome

Ts65Dn mouse is the most widely accepted model for Down syndrome. We previously showed that environmental enrichment improved spatial learning in female but deteriorated it in male Ts65Dn mice. This study analyzed the factors contributing to the disturbed cognition of male Ts65Dn mice after enriched housing, by allocating male control and Ts65Dn mice in four conditions after weaning: small (n = 2-3) and large group (n = 8-10) housing, and enriched housing in small (2-3) and large groups (8-10). Learning, aggressive behavior, anxiety-like behavior and biochemical correlates of stress were evaluated when Ts65Dn and control mice were 4-5 months old. Environmental enrichment in large mixed colonies of Ts65Dn and diploid littermates disturbed behavioral and learning skills of Ts65Dn mice in the Morris water maze. ACTH and testosterone levels were not modified in any group of mice. Ts65Dn and control mice subjected to enriched housing in large groups and Ts65Dn mice housed in large groups showed higher corticosterone levels. Aggressive behavior was evaluated by measuring the number of attacks performed in the presence of an intruder. Ts65Dn mice performed less attacks than controls in all conditions, especially after enriched housing, indicating subordination. In the plus maze, cognitive aspects (i.e. risk assessment) and motor components (open arm avoidance) of anxiety behavior were evaluated; no difference in any condition was found. It is suggested that an excess of social and/or physical stimulation in Ts65Dn mice may affect cognition by disturbing the emotional and behavioral components of the learning process.

Rho GTPases, dendritic structure, and mental retardation

A consistent feature of neurons in patients with mental retardation is abnormal dendritic structure and/or alterations in dendritic spine morphology. Deficits in the regulation of the dendritic cytoskeleton affect both the structure and function of dendrites and synapses and are believed to underlie mental retardation in some instances. In support of this, there is good evidence that alterations in signaling pathways involving the Rho family of small GTPases, key regulators of the actin and microtubule cytoskeletons, contribute to both syndromic and nonsyndromic mental retardation disorders. Because the Rho GTPases have been shown to play increasingly well-defined roles in determining dendrite and dendritic spine development and morphology, Rho signaling has been suggested to be important for normal cognition. The purpose of this review is to summarize recent data on the Rho GTPases pertaining to dendrite and dendritic spine morphogenesis, as well as to highlight their involvement in mental retardation resulting from a variety of genetic mutations within regulators and effectors of these molecules.

Endosome function and dysfunction in Alzheimer's disease and other neurodegenerative diseases

Endocytosis is universally important in cell function. In the brain, the roles of endosomes are relatively more complex due to the unique polar morphology of neurons and specialized needs for inter-cellular communication. New evidence shows that endosome function is altered in a surprising range of neurodegenerative disorders, including in several inherited neurologic disorders where the causative mutations occur in genes that regulate endosome function. In Alzheimer's disease (AD), endosome abnormalities are among the earliest neuropathologic features to develop and have now been closely linked to genetic risk factors for AD, including APP triplication in Trisomy 21 (Down syndrome, DS) and ApoE4 genotype in sporadic AD. Recent findings on endosome regulation and developmental and late-onset neurodegenerative disease disorders are beginning to reveal how endocytic pathway impairment may lead to neuronal dysfunction and cell death in these disorders and may also promote amyloidogenesis in AD.

Sleep-wake architecture in mouse models for Down syndrome

Sleep-wake homeostasis is crucial for behavioral performances and memory both in the general population and in patients with learning disability, among whom were Down syndrome (DS) patients. We investigated, in mouse models of DS, cortical EEG and sleep-wake architecture under baseline conditions and after a 4-h sleep deprivation (SD). Young hemizygous mice (hSODwt/+) transgenic for the human CuZn superoxide dismutase (hSOD1) or for the human amyloid precursor protein (HuAPP(695); hAPPwt/+) were obtained on the same FVB/N inbred background. Baseline records for slow wave sleep (SWS) and wake (W) parameters were unchanged, whereas paradoxical sleep (PS) episode numbers were decreased and PS latency increased after lights off in hSODwt/+ mice versus controls. hSODwt/+ mice did not experience SWS or PS rebounds after SD but EEG activity in the delta-SWS activity (SWA) was enhanced. hAPPwt/+ mice exhibited no change in PS but an increase in W and a decrease in SWS before light transition as well as an increase in theta-power in PS and W. After SD, hAPPwt/+ mice exhibited SWS and PS rebounds as well as enhancement of SWA. We investigated also the nitrite/nitrate levels in all mice and found an increase in the brainstem of hSODwt/+ mice only versus control ones. These preliminary data provide useful results to investigate other genetically manipulated mice and to better understand the biochemical basis of sleep disorders in DS patients.

Motor phenotypic alterations in TgDyrk1a transgenic mice implicate DYRK1A in Down syndrome motor dysfunction

Motor deficits are among the most frequent impairments in Down syndrome (DS), but their neuropathological and molecular bases remain elusive. Here we investigate the motor profile of transgenic mice overexpressing Dyrk1a, Tg(Dyrk1a)1Cff (hereafter TgDyrk1a), a candidate gene hypothesized to cause some of the neurological defects associated with DS. We have previously shown DYRK1A expression in the cerebellum and functionally related structures, most brainstem motor nuclei and spinal cord, supporting a role for Dyrk1a in controlling motor function. Here we demonstrate that TgDyrk1a mice present DYRK1A overexpression in these areas along with specific motor dysfunction. The main finding that emerged was impairment of motor learning and alteration of the organization of locomotor behavior, which agrees with reported clinical observations in subjects with DS. These results confirm and extend previous data and provide further insight to the functional domains that might be altered in TgDyrk1a mice and underlying molecular mechanisms of DS motor dysfunction.