Mouse Models of Cognitive Disorders in Trisomy 21: A Review

Overexpression of miR-99a in hippocampus leads to impairment of reversal learning in mice

As one of the most common human genetic disorders, Down syndrome (DS) is characterized by a mild-to-moderate cognitive disability, which mainly results from genes overexpression on chromosome 21. The expression of miR-99a, a gene harboring on chromosome 21, is increased by 50 folds in DS brain samples. This study aims to investigate the effect of miR-99a overexpression in the hippocampus on mouse behaviors and explore the underlying mechanisms. Lentivirus vectors were delivered into the hippocampus for focal miR-99a overexpression in mice. Then behaviors were observed by an open field, elevated plus maze, rotarod motor test, and Morris water maze. The genes affected by miR-99a were identified by RNA sequencing (RNA-seq) and confirmed by quantitative RT-PCR (qRT-PCR) in samples isolated from the hippocampus injected with lentivirus-GFP-miR-99a or lentivirus-GFP vectors. It was found that the expression of miR-99a with intrahippocampal delivery of lentivirus-GFP-miR-99a resulted in reversal learning impairment in mice although it had no influence on motor function and anxiety. Meanwhile, RNA-seq results showed that 92 genes including mRNAs and microRNAs were significantly regulated by miR-99a, consistent with qRT-PCR consequence. Moreover, dual-luciferase reporter assay showed that miR-99a could directly bind to the 3'-untranslated regions (3'UTR) of target genes (Clic6 and Kcnj13) with an inhibitory effect on their activity. Furthermore, we also found that miR-99a overexpression affected different biological processes by bioinformatic analyses. Our study showed that miR-99a overexpression in the hippocampus leads to cognitive impairment through regulating the expressions of various genes, which reveals a novel function of miR-99a and provides new insights into understanding the pathophysiologic process of DS.

Long-term voluntary running modifies the levels of proteins of the excitatory/inhibitory system and reduces reactive astrogliosis in the brain of Ts65Dn mouse model for Down syndrome

We showed previously that voluntary long-term running improved cognition and motor skills, but in an age-dependent manner, in the Ts65Dn mouse model for Down syndrome (DS). Presently, we investigated the effect of running on the levels of some key proteins of the excitatory/inhibitory system, which is impaired in the trisomic brain, and on astroglia, a vital component of this system. Ts65Dn mice had free access to a running wheel for 9-13 months either from weaning or from the age of 7 months. Sedentary Ts65Dn mice served as controls. We found that running modified the levels of four of the seven proteins we tested that are associated with the glutamatergic/GABA-ergic system. Thus, Ts65Dn runners demonstrated increased levels of glutamine synthetase and metabotropic glutamate receptor 1 and decreased levels of glutamate transporter 1 and glutamic acid decarboxylase 65 (GAD65) versus sedentary mice, but of metabotropic glutamate receptor 1 and GAD65 only in the post-weaning cohort. GAD67, ionotropic N-methyl-D-aspartate type receptor subunit 1, and GABA_Aα5 receptors' levels were similar in runners and sedentary animals. The number of glial fibrillary acidic protein (GFAP)-positive astrocytes and the levels of GFAP were significantly reduced in runners relative to sedentary mice. Our study provides new insight into the mechanisms underlying the beneficial effect of voluntary, sustained running on function of the trisomic brain by identifying the involvement of proteins associated with glutamatergic and GABAergic systems and reduction in reactive astrogliosis.

Functional Interactions Between Sleep and Circadian Rhythms in Learning and Learning Disabilities

The propensity for sleep is timed by the circadian system. Many studies have shown that learning and memory performance is affected by circadian phase. And, of course it is well established that critical processes of memory consolidation occur during and depend on sleep. This chapter presents evidence that sleep and circadian rhythms do not just have separate influences on learning and memory that happen to coincide because of the circadian timing of sleep, but rather sleep and circadian systems have a critical functional interaction in the processes of memory consolidation. The evidence comes primarily from research on two models of learning disability: Down's syndrome model mice and Siberian hamsters. The Down's syndrome model mouse (Ts65Dn) has severe learning disability that has been shown to be due to GABAergic over-inhibition. Short-term, chronic therapies with GABA_A antagonists restore learning ability in these mice long-term, but only if the antagonist treatments are given during the dark or sleep phase of the daily rhythm. The Siberian hamster is a model circadian animal except for the fact that a light treatment that gives the animal a phase advance on one day and a phase delay on the next day can result in total circadian arrhythmia for life. Once arrhythmic, the hamsters cannot learn. Learning, but not rhythmicity, is restored by short-term chronic treatment with GABA antagonists. Like many other species, if these hamsters are made arrhythmic by SCN lesion, their learning is unaffected. However, if made arrhythmic and learning disabled by the light treatment, subsequent lesions of their SCNs restore learning. SCN lesions also appear to restore learning in the Ts65Dn mice. The collective work on these two animal models of learning disability suggests that the circadian system modulates neuroplasticity. Our hypothesis is that a previously unrecognized function of the circadian system is to dampen neuroplasticity during the sleep phase to stabilize memory transcripts during their transfer to long-term memory. Thus, sleep and circadian systems have integrated roles to play in memory consolidation and do not just have separate but coincident influences on that process.

Lifespan analysis of brain development, gene expression and behavioral phenotypes in the Ts1Cje, Ts65Dn and Dp(16)1/Yey mouse models of Down syndrome

Down syndrome (DS) results from triplication of human chromosome 21. Neuropathological hallmarks of DS include atypical central nervous system development that manifests prenatally and extends throughout life. As a result, individuals with DS exhibit cognitive and motor deficits, and have delays in achieving developmental milestones. To determine whether different mouse models of DS recapitulate the human prenatal and postnatal phenotypes, here, we directly compared brain histogenesis, gene expression and behavior over the lifespan of three cytogenetically distinct mouse models of DS: Ts1Cje, Ts65Dn and Dp(16)1/Yey. Histological data indicated that Ts65Dn mice were the most consistently affected with respect to somatic growth, neurogenesis and brain morphogenesis. Embryonic and adult gene expression results showed that Ts1Cje and Ts65Dn brains had considerably more differentially expressed (DEX) genes compared with Dp(16)1/Yey mice, despite the larger number of triplicated genes in the latter model. In addition, DEX genes showed little overlap in identity and chromosomal distribution in the three models, leading to dissimilarities in affected functional pathways. Perinatal and adult behavioral testing also highlighted differences among the models in their abilities to achieve various developmental milestones and perform hippocampal- and motor-based tasks. Interestingly, Dp(16)1/Yey mice showed no abnormalities in prenatal brain phenotypes, yet they manifested behavioral deficits starting at postnatal day 15 that continued through adulthood. In contrast, Ts1Cje mice showed mildly abnormal embryonic brain phenotypes, but only select behavioral deficits as neonates and adults. Altogether, our data showed widespread and unexpected fundamental differences in behavioral, gene expression and brain development phenotypes between these three mouse models. Our findings illustrate unique limitations of each model when studying aspects of brain development and function in DS. This work helps to inform model selection in future studies investigating how observed neurodevelopmental abnormalities arise, how they contribute to cognitive impairment, and when testing therapeutic molecules to ameliorate the intellectual disability associated with DS.This article has an associated First Person interview with the first author of the paper.

The medial temporal memory system in Down syndrome: Translating animal models of hippocampal compromise

Recent studies have highlighted the dentate gyrus as a region of increased vulnerability in mouse models of Down syndrome (DS). It is unclear to what extent these findings are reflected in the memory profile of people with the condition. We developed a series of novel tasks to probe distinct medial temporal functions in children and young adults with DS, including object, spatial, and temporal order memory. Relative to mental age-matched controls (n = 45), individuals with DS (n = 28) were unimpaired on subtests involving short-term object or configural recall that was divorced from spatial or temporal contexts. By contrast, the DS group had difficulty recalling spatial locations when contextual information was salient and recalling the order in which objects were serially presented. Results are consistent with dysfunction of spatial and temporal contextual pattern separation abilities in individuals with DS, mediated by the hippocampus, including the dentate gyrus. Amidst increasing calls to bridge human and animal work, the memory profile demonstrated here in humans with DS is strikingly similar to that of the Ts65Dn mouse model of DS. The study highlights the trisynaptic circuit as a potentially fruitful intervention target to mitigate cognitive impairments associated with DS.

Differential Brain, Cognitive and Motor Profiles Associated with Partial Trisomy. Modeling Down Syndrome in Mice

We hypothesize that the trisomy 21 (Down syndrome) is the additive and interactive outcome of the triple copy of different regions of HSA21. Because of the small number of patients with partial trisomy 21, we addressed the question in the Mouse in which three chromosomal regions located on MMU10, MMU17 and MMU16 carries almost all the HSA21 homologs. Male mice from four segmental trisomic strains covering the D21S17-ETS2 (syntenic to MMU16) were examined with an exhaustive battery of cognitive tests, motor tasks and MRI and compared with TS65Dn that encompasses D21S17-ETS2. None of the four strains gather all the impairments (measured by the effect size) of TS65Dn strain. The 152F7 strain was close to TS65Dn for motor behavior and reference memory and the three other strains 230E8, 141G6 and 285E6 for working memory. Episodic memory was impaired only in strain 285E6. The hippocampus and cerebellum reduced sizes that were seen in all the strains indicate that trisomy 21 is not only a hippocampus syndrome but that it results from abnormal interactions between the two structures.

Absence of Prenatal Forebrain Defects in the Dp(16)1Yey/+ Mouse Model of Down Syndrome

Studies in humans with Down syndrome (DS) show that alterations in fetal brain development are followed by postnatal deficits in neuronal numbers, synaptic plasticity, and cognitive and motor function. This same progression is replicated in several mouse models of DS. Dp(16)1Yey/+ (hereafter called Dp16) is a recently developed mouse model of DS in which the entire region of mouse chromosome 16 that is homologous to human chromosome 21 has been triplicated. As such, Dp16 mice may more closely reproduce neurodevelopmental changes occurring in humans with DS. Here, we present the first comprehensive cellular and behavioral study of the Dp16 forebrain from embryonic to adult stages. Unexpectedly, our results demonstrate that Dp16 mice do not have prenatal brain defects previously reported in human fetal neocortex and in the developing forebrains of other mouse models, including microcephaly, reduced neurogenesis, and abnormal cell proliferation. Nevertheless, we found impairments in postnatal developmental milestones, fewer inhibitory forebrain neurons, and deficits in motor and cognitive performance in Dp16 mice. Therefore, although this new model does not express prenatal morphological phenotypes associated with DS, abnormalities in the postnatal period appear sufficient to produce significant cognitive deficits in Dp16.

Dementia in Down's syndrome

Down's syndrome is the most common genetic cause of learning difficulties, and individuals with this condition represent the largest group of people with dementia under the age of 50 years. Genetic drivers result in a high frequency of Alzheimer's pathology in these individuals, evident from neuroimaging, biomarker, and neuropathological findings, and a high incidence of cognitive decline and dementia. However, cognitive assessment is challenging, and diagnostic methods have not been fully validated for use in these patients; hence, early diagnosis remains difficult. Evidence regarding the benefits of cholinesterase inhibitors and other therapeutic options to treat or delay progressive cognitive decline or dementia is very scarce. Despite close similarities with late-onset Alzheimer's disease, individuals with Down's syndrome respond differently to treatment, and a targeted approach to drug development is thus necessary. Genetic and preclinical studies offer opportunities for treatment development, and potential therapies have been identified using these approaches.

Longitudinal measures of cognition in the Ts65Dn mouse: Refining windows and defining modalities for therapeutic intervention in Down syndrome

Mouse models have provided insights into adult changes in learning and memory in Down syndrome, but an in-depth assessment of how these abnormalities develop over time has never been conducted. To address this shortcoming, we conducted a longitudinal behavioral study from birth until late adulthood in the Ts65Dn mouse model to measure the emergence and continuity of learning and memory deficits in individuals with a broad array of tests. Our results demonstrate for the first time that the pace at which neonatal and perinatal milestones are acquired is correlated with later cognitive performance as an adult. In addition, we find that life-long behavioral indexing stratifies mice within each genotype. Our expanded assessment reveals that diminished cognitive flexibility, as measured by reversal learning, is the most robust learning and memory impairment in both young and old Ts65Dn mice. Moreover, we find that reversal learning degrades with age and is therefore a useful biomarker for studying age-related decline in cognitive ability. Altogether, our results indicate that preclinical studies aiming to restore cognitive function in Ts65Dn should target both neonatal milestones and reversal learning in adulthood. Here we provide the quantitative framework for this type of approach.

Evidence of altered age-related brain cytoarchitecture in mouse models of down syndrome: a diffusional kurtosis imaging study

Mouse models of Down syndrome (DS) exhibit abnormal brain developmental and neurodegenerative changes similar to those seen in individuals with DS. Although DS mice have been well characterized cognitively and morphologically there are no prior reports utilizing diffusion MRI. In this study we investigated the ability of diffusional kurtosis imaging (DKI) to detect the progressive developmental and neurodegenerative changes in the Ts65Dn (TS) DS mouse model. TS mice displayed higher diffusional kurtosis (DK) in the frontal cortex (FC) compared to normal mice at 2months of age. At 5months of age, TS mice had lower radial kurtosis in the striatum (ST), which persisted in the 8-month-old mice. The TS mice exhibited lower DK metrics values in the dorsal hippocampus (HD) at all ages, and the group difference in this region was larger at 8-months. Regression analysis showed that normal mice had a significant age-related increase in DK metrics in FC, ST and HD. On the contrary, the TS mice lacked significant age-related increase in DK metrics in FC and ST. Although preliminary, these results demonstrate that DK metrics can detect TS brain developmental and neurodegenerative abnormalities.

Nest building is impaired in the Ts65Dn mouse model of Down syndrome and rescued by blocking 5HT2a receptors

Down syndrome (DS) has an incidence of about 1/700 births, and is therefore the most common cause of cognitive and behavioral impairments in children. Recent studies on mouse models of DS indicate that a number of pharmacotherapies could be beneficial for restoring cognitive abilities in individuals with DS. Attention deficits that are present in DS account in part for learning and memory deficiencies yet have been scarcely studied in corresponding models. Investigations of this relevant group of behaviors is more difficult in mouse models because of the difficulty in homologizing mouse and human behaviors and because standard laboratory environments do not always elicit behaviors of interest. Here we characterize nest building as a goal-directed behavior that is seriously impaired in young Ts65Dn mice, a genetic model of DS. We believe this impairment may reflect in part attention deficits, and we investigate the physiological, genetic, and pharmacological factors influencing its expression. Nesting behavior in young Ts65Dn mice was severely impaired when the animals were placed in a novel environment. But this context-dependent impairment was transient and reversible. The genetic determinants of this deficiency are restricted to a ∼100 gene segment on the murine chromosome 16. Nest building behavior is a highly integrated phenotypic trait that relies in part on limbic circuitry and on the frontal cortex in relation to cognitive and attention processes. We show that both serotonin content and 5HT2a receptors are increased in the frontal cortex of Ts65Dn mice and that pharmacological blockage of 5HT2a receptors in Ts65Dn mice rescues their context dependent nest building impairment. We propose that the nest-building trait could represent a marker of attention related deficits in DS models and could be of value in designing pharmacotherapies for this specific aspect of DS. 5HT2a modulation may improve goal-directed behavior in DS.

DYRK1A-mediated phosphorylation of GluN2A at Ser(1048) regulates the surface expression and channel activity of GluN1/GluN2A receptors

N-methyl-D-aspartate glutamate receptors (NMDARs) play a pivotal role in neural development and synaptic plasticity, as well as in neurological disease. Since NMDARs exert their function at the cell surface, their density in the plasma membrane is finely tuned by a plethora of molecules that regulate their production, trafficking, docking and internalization in response to external stimuli. In addition to transcriptional regulation, the density of NMDARs is also influenced by post-translational mechanisms like phosphorylation, a modification that also affects their biophysical properties. We previously described the increased surface expression of GluN1/GluN2A receptors in transgenic mice overexpressing the Dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A), suggesting that DYRK1A regulates NMDARs. Here we have further investigated whether the density and activity of NMDARs were modulated by DYRK1A phosphorylation. Accordingly, we show that endogenous DYRK1A is recruited to GluN2A-containing NMDARs in the adult mouse brain, and we identify a DYRK1A phosphorylation site at Ser¹⁰⁴⁸ of GluN2A, within its intracellular C-terminal domain. Mechanistically, the DYRK1A-dependent phosphorylation of GluN2A at Ser¹⁰⁴⁸ hinders the internalization of GluN1/GluN2A, causing an increase of surface GluN1/GluN2A in heterologous systems, as well as in primary cortical neurons. Furthermore, GluN2A phosphorylation at Ser¹⁰⁴⁸ increases the current density and potentiates the gating of GluN1/GluN2A receptors. We conclude that DYRK1A is a direct regulator of NMDA receptors and we propose a novel mechanism for the control of NMDAR activity in neurons.

Dentate gyrus mediates cognitive function in the Ts65Dn/DnJ mouse model of Down syndrome

In the Ts65Dn/DnJ mouse model of Down syndrome (DS), hippocampal deficits of learning and memory are the most robust features supporting this mouse as a valid cognitive model of DS. Although dentate gyrus (DG) dysfunction is suggested by excessive GABAergic inhibition, its role in perturbing DG functions in DS is unknown. We hypothesize that in the Ts65Dn/DnJ mouse, the specific role of the DG is disturbed in its support of contextual and spatial information. Support for this hypothesis comes from rats with DG lesions that show similar deficits. In order to test this hypothesis, we have developed a novel series of spontaneous exploratory tasks that emphasize the importance of recognizing spatial and contextual cues and that involve DG function. The results with this exploratory battery show that Ts65Dn/DnJ mice are impaired in DG-dependent short-term recognition tests involving object recognition with contextual cues, in place recognition and in metric distance recognition relative to wild type littermate controls. Further, whereas Ts65Dn/DnJ mice can recognize object novelty in the absence of contextual cues after a 5-min delay, they cannot do so after a delay of 24 h, suggesting a problem with CA1-mediated consolidation. The results also show that Ts65Dn/DnJ mice are not impaired in tasks (object recognition and configural object recognition) that are mediated by the perirhinal cortex (PRh). These results implicate the DG as a specific therapeutic target and the PRh as a potential therapeutic strength for future research to ameliorate learning and memory in DS.

Fluoxetine in adulthood normalizes GABA release and rescues hippocampal synaptic plasticity and spatial memory in a mouse model of Down syndrome

Down syndrome (DS) is the most common genetic disorder associated with mental retardation. It has been repeatedly shown that Ts65Dn mice, the major animal model for DS, have severe cognitive and synaptic plasticity dysfunctions caused by excessive inhibition in their temporal lobe structures. Here we employed a multidisciplinary approach spanning from the behavioral to the electrophysiological and molecular level to investigate the effects elicited by fluoxetine on cognitive abilities, hippocampal synaptic plasticity and GABA release in adult Ts65Dn mice. We report that a chronic treatment with fluoxetine administered in the drinking water normalizes GABA release and promotes recovery of spatial memory abilities, spatial working memory for alternation, and hippocampal synaptic plasticity in adult Ts65Dn mice. Our findings might encourage new experimental attempts aimed at investigating the potential of fluoxetine for application in the treatment of major functional deficits in adult people with DS.

Normalization of Dyrk1A expression by AAV2/1-shDyrk1A attenuates hippocampal-dependent defects in the Ts65Dn mouse model of Down syndrome

The cognitive dysfunctions of Down Syndrome (DS) individuals are the most disabling alterations caused by the trisomy of human chromosome 21 (HSA21). In trisomic Ts65Dn mice, a genetic model for DS, the overexpression of HSA21 homologous genes has been associated with strong visuo-spatial cognitive alterations, ascribed to hippocampal dysfunction. In the present study, we evaluated whether the normalization of the expression levels of Dyrk1A (Dual specificity tyrosine-phosphorylation-regulated kinase 1A), a candidate gene for DS, might correct hippocampal defects in Ts65Dn mice. In the hippocampus of 2 month-old Ts65Dn mice, such normalization was achieved through the stereotaxical injection of adeno-associated viruses containing a short hairpin RNA against Dyrk1A (AAV2/1-shDyrk1A) and a luciferase reporter gene. The injected hippocampi were efficiently transduced, as shown by bioluminescence in vivo imaging, luciferase activity quantification and immunohistochemical analysis. At the molecular level, viral infusion allowed the normalization of the targeted Dyrk1A expression, as well as of the key players of the MAPK/CREB pathway. The electrophysiological recordings of hippocampal slices from Ts65Dn mice injected with AAV2/1-shDyrk1A displayed attenuation of the synaptic plasticity defects of trisomic mice. In contrast, contralateral hippocampal injection with an AAV2/1 control virus containing a scrambled sequence, showed neither the normalization of Dyrk1A levels nor changes of synaptic plasticity. In the Morris water maze task, although long-term consolidation of the task was not achieved, treated Ts65Dn mice displayed initially a normalized thigmotactic behavior, similar to euploid littermates, indicating the partial improvement in their hippocampal-dependent search strategy. Taken together, these results show Dyrk1A as a critical player in the pathophysiology of DS and define Dyrk1A as a therapeutic target in adult trisomic mice.

Human and mouse model cognitive phenotypes in Down syndrome: implications for assessment

The study of cognitive function in Down syndrome (DS) has advanced rapidly in the past decade. Mouse models have generated data regarding the neurological basis for the specific cognitive profile of DS (i.e., deficits in aspects of hippocampal, prefrontal, and cerebellar function) and have uncovered pharmacological treatments with the potential to affect this phenotype. Given this progress, the field is at a juncture in which we require assessments that may effectively translate the findings acquired in mouse models to humans with DS. In this chapter, we describe the cognitive profile of humans with DS and associated mouse models, discussing the ways in which we may merge these findings so as to more fully understand cognitive strengths and weaknesses in this population. New directions for approaches to cognitive assessment in mice and humans are discussed.

Trisomy of the G protein-coupled K+ channel gene, Kcnj6, affects reward mechanisms, cognitive functions, and synaptic plasticity in mice

G protein-activated inwardly rectifying K+ channels (GIRK) generate slow inhibitory postsynaptic potentials in the brain via G(i/o) protein-coupled receptors. GIRK2, a GIRK subunit, is widely abundant in the brain and has been implicated in various functions and pathologies, such as learning and memory, reward, motor coordination, and Down syndrome. Down syndrome, the most prevalent cause of mental retardation, results from the presence of an extra maternal chromosome 21 (trisomy 21), which comprises the Kcnj6 gene (GIRK2). The present study examined the behaviors and cellular physiology properties in mice harboring a single trisomy of the Kcnj6 gene. Kcnj6 triploid mice exhibit deficits in hippocampal-dependent learning and memory, altered responses to rewards, hampered depotentiation, a form of excitatory synaptic plasticity, and have accentuated long-term synaptic depression. Collectively the findings suggest that triplication of Kcnj6 gene may play an active role in some of the abnormal neurological phenotypes found in Down syndrome.

Role of phosphoinositides at the neuronal synapse

Synaptic transmission is amongst the most sophisticated and tightly controlled biological phenomena in higher eukaryotes. In the past few decades, tremendous progress has been made in our understanding of the molecular mechanisms underlying multiple facets of neurotransmission, both pre- and postsynaptically. Brought under the spotlight by pioneer studies in the areas of secretion and signal transduction, phosphoinositides and their metabolizing enzymes have been increasingly recognized as key protagonists in fundamental aspects of neurotransmission. Not surprisingly, dysregulation of phosphoinositide metabolism has also been implicated in synaptic malfunction associated with a variety of brain disorders. In the present chapter, we summarize current knowledge on the role of phosphoinositides at the neuronal synapse and highlight some of the outstanding questions in this research field.

Environmental enrichment decreases GABAergic inhibition and improves cognitive abilities, synaptic plasticity, and visual functions in a mouse model of Down syndrome

Down syndrome (DS) is the most common genetic disorder associated with mental retardation. It has been repeatedly shown that Ts65Dn mice, the prime animal model for DS, have severe cognitive and neural plasticity defects due to excessive inhibition. We report that increasing sensory-motor stimulation in adulthood through environmental enrichment (EE) reduces brain inhibition levels and promotes recovery of spatial memory abilities, hippocampal synaptic plasticity, and visual functions in adult Ts65Dn mice.

Galantamine improves olfactory learning in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS) is the most common form of congenital intellectual disability. Although DS involves multiple disturbances in various tissues, there is little doubt that in terms of quality of life cognitive impairment is the most serious facet and there is no effective treatment for this aspect of the syndrome. The Ts65Dn mouse model of DS recapitulates multiple aspects of DS including cognitive impairment. Here the Ts65Dn mouse model of DS was evaluated in an associative learning paradigm based on olfactory cues. In contrast to disomic controls, trisomic mice exhibited significant deficits in olfactory learning. Treatment of trisomic mice with the acetylcholinesterase inhibitor galantamine resulted in a significant improvement in olfactory learning. Collectively, our study indicates that olfactory learning can be a sensitive tool for evaluating deficits in associative learning in mouse models of DS and that galantamine has therapeutic potential for improving cognitive abilities.

Chronic Treatment with a Promnesiant GABA-A α5-Selective Inverse Agonist Increases Immediate Early Genes Expression during Memory Processing in Mice and Rectifies Their Expression Levels in a Down Syndrome Mouse Model

Decrease of GABAergic transmission has been proposed to improve memory functions. Indeed, inverse agonists selective for α5 GABA-A-benzodiazepine receptors (α5IA) have promnesiant activity. Interestingly, we have recently shown that α5IA can rescue cognitive deficits in Ts65Dn mice, a Down syndrome mouse model with altered GABAergic transmission. Here, we studied the impact of chronic treatment with α5IA on gene expression in the hippocampus of Ts65Dn and control euploid mice after being trained in the Morris water maze task. In euploid mice, chronic treatment with α5IA increased IEGs expression, particularly of c-Fos and Arc genes. In Ts65Dn mice, deficits of IEGs activation were completely rescued after treatment with α5IA. In addition, normalization of Sod1 overexpression in Ts65Dn mice after α5IA treatment was observed. IEG expression regulation after α5IA treatment following behavioral stimulation could be a contributing factor for both the general promnesiant activity of α5IA and its rescuing effect in Ts65Dn mice alongside signaling cascades that are critical for memory consolidation and cognition.

On the promise of pharmacotherapies targeted at cognitive and neurodegenerative components of Down syndrome

Down syndrome (DS) is the phenotypic consequence of trisomy 21 and is the most common genetically defined cause of intellectual disability. The most complete, widely available, and well-studied animal model of DS is the Ts65Dn mouse. Recent preclinical successes in rescuing learning and memory deficits in Ts65Dn mice are legitimate causes for optimism that pharmacotherapies for cognitive deficits in DS might be within reach. This article provides a snapshot of potential pharmacotherapies for DS, with emphasis on our recent results showing that the N-methyl-D-aspartate receptor antagonist memantine can reverse learning and memory deficits in Ts65Dn mice. Because memantine has already been approved for the therapy of Alzheimer's dementia, we have been able to very rapidly translate these results into human research and are currently conducting a 16-week, randomized, double-blind, placebo-controlled evaluation of the efficacy, tolerability and safety of memantine hydrochloride on enhancing the cognitive abilities of young adults with DS. The design and current status of this clinical trial will be discussed, which will be followed by some speculation on the potential impact of this and future clinical trials in the field of DS.

Brain plasticity and disease: a matter of inhibition

One major goal in Neuroscience is the development of strategies promoting neural plasticity in the adult central nervous system, when functional recovery from brain disease and injury is limited. New evidence has underscored a pivotal role for cortical inhibitory circuitries in regulating plasticity both during development and in adulthood. This paper summarizes recent findings showing that the inhibition-excitation balance controls adult brain plasticity and is at the core of the pathogenesis of neurodevelopmental disorders like autism, Down syndrome, and Rett syndrome.

Meta-analysis of heterogeneous Down Syndrome data reveals consistent genome-wide dosage effects related to neurological processes

Background

Down syndrome (DS; trisomy 21) is the most common genetic cause of mental retardation in the human population and key molecular networks dysregulated in DS are still unknown. Many different experimental techniques have been applied to analyse the effects of dosage imbalance at the molecular and phenotypical level, however, currently no integrative approach exists that attempts to extract the common information.

Results

We have performed a statistical meta-analysis from 45 heterogeneous publicly available DS data sets in order to identify consistent dosage effects from these studies. We identified 324 genes with significant genome-wide dosage effects, including well investigated genes like SOD1, APP, RUNX1 and DYRK1A as well as a large proportion of novel genes (N = 62). Furthermore, we characterized these genes using gene ontology, molecular interactions and promoter sequence analysis. In order to judge relevance of the 324 genes for more general cerebral pathologies we used independent publicly available microarry data from brain studies not related with DS and identified a subset of 79 genes with potential impact for neurocognitive processes. All results have been made available through a web server under http://ds-geneminer.molgen.mpg.de/.

Conclusions

Our study represents a comprehensive integrative analysis of heterogeneous data including genome-wide transcript levels in the domain of trisomy 21. The detected dosage effects build a resource for further studies of DS pathology and the development of new therapies.

From molecules to behavior: lessons from the study of rare genetic disorders

Rare diseases are defined as conditions with a prevalence of less than 1/2,000. To date between 6,000 and 7,000 rare diseases have been identified and many of those have manifestations that include intellectual disability, developmental disorders or other behavioural phenotypes. In this special issue we bring together a range of papers where rare diseases were used as models to delineate specific aspects of learning and memory, or behaviour. In this introductory paper we summarize some of the lessons we can learn from rare diseases. Firstly, we learn that, collectively, rare diseases are not at all rare. As many as 1 in 20 individuals may be affected by a rare disease at some point in their life. Secondly, we learn that rare diseases may share common pathophysiological mechanisms. A discovery in one can therefore have direct relevance to many others. A third lesson is that the study of rare diseases can lead to an understanding of common disorders, as exemplified by the relationship between Trisomy 21 (Down syndrome) and Alzheimer's disease. A fourth lesson from rare diseases is that the 'one gene-one functional consequence' assumption is not correct. Finally, rare diseases have shed new light on the strengths and weaknesses of animal models in the study of behavioural phenotypes.

Gene expression profiling in a mouse model identifies fetal liver- and placenta-derived potential biomarkers for Down Syndrome screening

Background

As a first step to identify novel potential biomarkers for prenatal Down Syndrome screening, we analyzed gene expression in embryos of wild type mice and the Down Syndrome model Ts1Cje. Since current Down Syndrome screening markers are derived from placenta and fetal liver, these tissues were chosen as target.

Methodology/Principal Findings

Placenta and fetal liver at 15.5 days gestation were analyzed by microarray profiling. We confirmed increased expression of genes located at the trisomic chromosomal region. Overall, between the two genotypes more differentially expressed genes were found in fetal liver than in placenta. Furthermore, the fetal liver data are in line with the hematological aberrations found in humans with Down Syndrome as well as Ts1Cje mice. Together, we found 25 targets that are predicted (by Gene Ontology, UniProt, or the Human Plasma Proteome project) to be detectable in human serum.

Conclusions/Significance

Fetal liver might harbor more promising targets for Down Syndrome screening studies. We expect these new targets will help focus further experimental studies on identifying and validating human maternal serum biomarkers for Down Syndrome screening.

Transcript catalogs of human chromosome 21 and orthologous chimpanzee and mouse regions

A comprehensive representation of the gene content of the long arm of human chromosome 21 (Hsa21q) remains of interest for the study of Down syndrome, its associated phenotypic features, and mouse models. Here we compare transcript catalogs for Hsa21q, chimpanzee chromosome 21 (Ptr21q), and orthologous regions of mouse chromosomes 16, 17, and 10 for open reading frame (ORF) characteristics and conservation. The Hsa21q and mouse catalogs contain 552 and 444 gene models, respectively, of which only 162 are highly conserved. Hsa21q transcripts were used to identify orthologous exons in Ptr21q and assemble 533 putative transcripts. Transcript catalogs for all three organisms are searchable for nucleotide and amino acid sequence features of ORF length, repeat content, experimental support, gene structure, and conservation. For human and mouse comparisons, three additional summaries are provided: (1) the chromosomal distribution of novel ORF transcripts versus potential functional RNAs, (2) the distribution of species-specific transcripts within Hsa21q and mouse models of Down syndrome, and (3) the organization of sense-antisense and putative sense-antisense structures defining potential regulatory mechanisms. Catalogs, summaries, and nucleotide and amino acid sequences of all composite transcripts are available and searchable at http://gfuncpathdb.ucdenver.edu/iddrc/chr21/home.php. These data sets provide comprehensive information useful for evaluation of candidate genes and mouse models of Down syndrome and for identification of potential functional RNA genes and novel regulatory mechanisms involving Hsa21q genes. These catalogs and search tools complement and extend information available from other gene annotation projects.

Animal models of intellectual disability: towards a translational approach

Intellectual disability is a prevalent form of cognitive impairment, affecting 2-3% of the general population. It is a daunting societal problem characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social and practical adaptive skills. Intellectual disability is a clinically important disorder for which the etiology and pathogenesis are still poorly understood. Moreover, although tremendous progress has been made, pharmacological intervention is still currently non-existent and therapeutic strategies remain limited. Studies in humans have a very limited capacity to explain basic mechanisms of this condition. In this sense, animal models have been invaluable in intellectual disability investigation. Certainly, a great deal of the knowledge that has improved our understanding of several pathologies has derived from appropriate animal models. Moreover, to improve human health, scientific discoveries must be translated into practical applications. Translational research specifically aims at taking basic scientific discoveries and best practices to benefit the lives of people in our communities. In this context, the challenge that basic science research needs to meet is to make use of a comparative approach to benefit the most from what each animal model can tell us. Intellectual disability results from many different genetic and environmental insults. Taken together, the present review will describe several animal models of potential intellectual disability risk factors.

The mouse model of Down syndrome Ts65Dn presents visual deficits as assessed by pattern visual evoked potentials

PURPOSE

The Ts65Dn mouse is the most complete widely available animal model of Down syndrome (DS). Quantitative information was generated about visual function in the Ts65Dn mouse by investigating their visual capabilities by means of electroretinography (ERG) and patterned visual evoked potentials (pVEPs).

METHODS

pVEPs were recorded directly from specific regions of the binocular visual cortex of anesthetized mice in response to horizontal sinusoidal gratings of different spatial frequency, contrast, and luminance generated by a specialized video card and presented on a 21-in. computer display suitably linearized by gamma correction.

RESULTS

ERG assessments indicated no significant deficit in retinal physiology in Ts65Dn mice compared with euploid control mice. The Ts65Dn mice were found to exhibit deficits in luminance threshold, spatial resolution, and contrast threshold, compared with the euploid control mice. The behavioral counterparts of these parameters are luminance sensitivity, visual acuity, and the inverse of contrast sensitivity, respectively.

CONCLUSIONS

DS includes various phenotypes associated with the visual system, including deficits in visual acuity, accommodation, and contrast sensitivity. The present study provides electrophysiological evidence of visual deficits in Ts65Dn mice that are similar to those reported in persons with DS. These findings strengthen the role of the Ts65Dn mouse as a model for DS. Also, given the historical assumption of integrity of the visual system in most behavioral assessments of Ts65Dn mice, such as the hidden-platform component of the Morris water maze, the visual deficits described herein may represent a significant confounding factor in the interpretation of results from such experiments.

Molecular basis of pharmacotherapies for cognition in Down syndrome

Intellectual disability in Down syndrome (DS) ranges from low normal to severely impaired and has a significant impact on the quality-of-life of the individuals affected and their families. Because the incidence of DS remains at approximately 1 in 700 live births and the lifespan is now >50 years, development of pharmacotherapies for cognitive deficits is an important goal. DS is due to an extra copy of human chromosome 21 and has often been considered too complex a genetic abnormality to be amenable to intervention. However, recent successes in rescuing learning/memory impairments in a mouse model of DS suggest that this negative outlook may not be justified. In this contribution, we first review the DS phenotype, chromosome 21 gene content and mouse models. We then discuss recent successes and the remaining challenges in the identification of targets for and preclinical evaluation of potential therapeutics.

Communication breaks-Down: from neurodevelopment defects to cognitive disabilities in Down syndrome

Down syndrome (DS) is the leading cause of genetically-defined intellectual disability and congenital birth defects. Despite being one of the first genetic diseases identified, only recently, thanks to the phenotypic analysis of DS mouse genetic models, we have begun to understand how trisomy may impact cognitive function. Cognitive disabilities in DS appear to result mainly from two pathological processes: neurogenesis impairment and Alzheimer-like degeneration. In DS brain, suboptimal network architecture and altered synaptic communication arising from neurodevelopmental impairment are key determinants of cognitive defects. Hypocellularity and hypoplasia start at early developmental stages and likely depend upon impaired proliferation of neuronal precursors, resulting in reduction of numbers of neurons and synaptic contacts. The impairment of neuronal precursor proliferation extends to adult neurogenesis and may affect learning and memory. Neurodegenerative mechanisms also contribute to DS cognitive impairment. Early onset Alzheimer disease occurs with extremely high incidence in DS patients and is causally-related to overexpression of beta-amyloid precursor protein (betaAPP), which is one of the triplicated genes in DS. In this review, we will survey the available findings on neurodevelopmental and neurodegenerative changes occurring in DS throughout life. Moreover, we will discuss the potential mechanisms by which defects in neurogenesis and neurodegenerative processes lead to altered formation of neural circuits and impair cognitive function, in connection with findings on pharmacological treatments of potential benefit for DS.

Genetics and Cognition

How genes contribute to cognition is a perennial question for psychologists and geneticists. In the early 21st century, familial studies, including twin studies, supported the theory that genetic variations contribute to differences in cognition, but have been of little practical use to clinical and educational practitioners as no individual predictions can be made using such data; heritability cannot predict the impact of environmental factors or intervention programs. With the sequencing of animal genomes and the development of molecular genetics, new methodologies have been developed: gene targeting (replacing a functional gene with a neutral gene by homologous recombination), transgenesis (overexpressing one gene or a set of genes from one species in another species), and genome-wide scans and quantitative trait loci mapping (a strategy for identifying chromosomal regions involved in complex traits). Association studies can be performed to find associations between allelic forms and variations in IQ. Genes linked to “normal” variations in cognition have been detected but for the moment such discoveries have had no direct applications in a clinical setting; the number of genes identified as being linked to intellectual impairment has increased rapidly. Links have been reported between chromosomal deletions and triplications and behavioral phenotypes. The identification of mechanisms involved in genetic diseases should have long-term consequences on educational and/or psychological support programs as well as on health care. Psychologists need to keep up to date on advances in research establishing relationships between genetics and intellectual disability and will thus be able to refer children with cognitive impairments to specialized care services.

Nurturing brain plasticity: impact of environmental enrichment

Environmental enrichment (EE) is known to profoundly affect the central nervous system (CNS) at the functional, anatomical and molecular level, both during the critical period and during adulthood. Recent studies focusing on the visual system have shown that these effects are associated with the recruitment of previously unsuspected neural plasticity processes. At early stages of brain development, EE triggers a marked acceleration in the maturation of the visual system, with maternal behaviour acting as a fundamental mediator of the enriched experience in both the foetus and the newborn. In adult brain, EE enhances plasticity in the cerebral cortex, allowing the recovery of visual functions in amblyopic animals. The molecular substrate of the effects of EE on brain plasticity is multi-factorial, with reduced intracerebral inhibition, enhanced neurotrophin expression and epigenetic changes at the level of chromatin structure. These findings shed new light on the potential of EE as a non-invasive strategy to ameliorate deficits in the development of the CNS and to treat neurological disorders.

A new mouse model for the trisomy of the Abcg1-U2af1 region reveals the complexity of the combinatorial genetic code of down syndrome

Mental retardation in Down syndrome (DS), the most frequent trisomy in humans, varies from moderate to severe. Several studies both in human and based on mouse models identified some regions of human chromosome 21 (Hsa21) as linked to cognitive deficits. However, other intervals such as the telomeric region of Hsa21 may contribute to the DS phenotype but their role has not yet been investigated in detail. Here we show that the trisomy of the 12 genes, found in the 0.59 Mb (Abcg1–U2af1) Hsa21 sub-telomeric region, in mice (Ts1Yah) produced defects in novel object recognition, open-field and Y-maze tests, similar to other DS models, but induces an improvement of the hippocampal-dependent spatial memory in the Morris water maze along with enhanced and longer lasting long-term potentiation in vivo in the hippocampus. Overall, we demonstrate the contribution of the Abcg1–U2af1 genetic region to cognitive defect in working and short-term recognition memory in DS models. Increase in copy number of the Abcg1–U2af1 interval leads to an unexpected gain of cognitive function in spatial learning. Expression analysis pinpoints several genes, such as Ndufv3, Wdr4, Pknox1 and Cbs, as candidates whose overexpression in the hippocampus might facilitate learning and memory in Ts1Yah mice. Our work unravels the complexity of combinatorial genetic code modulating different aspect of mental retardation in DS patients. It establishes definitely the contribution of the Abcg1–U2af1 orthologous region to the DS etiology and suggests new modulatory pathways for learning and memory.

Behavioral validation of the Ts65Dn mouse model for Down syndrome of a genetic background free of the retinal degeneration mutation Pde6b(rd1)

The Ts65Dn mouse is the most studied and complete aneuploid model of Down syndrome (DS) widely available. As a model for human trisomy 21, these mice display many attractive features, including performance deficits in different behavioral tasks, alterations in synaptic plasticity and adult neurogenesis, motor dysfunction, and age-dependent cholinergic neurodegeneration. Currently, Ts65Dn mice are maintained on a genetic background that leads to blindness in about 25% of their offspring, because it segregates for the retinal degeneration 1 (Pde6b(rd1)) mutation of C3H/HeSnJ. This means that 25% of the mice have to be discarded in most experiments involving these animals, which is particularly problematic because the Ts65Dn stock has low reproductive performance. To circumvent this problem, we have bred the Ts65Dn extra chromosome many generations into a closely related genetic background that does not carry the Pde6b(rd1) mutation. Although the new genetic background is expected to be nearly identical to the original, differences in genetic background have the potential to alter mouse performance in certain behavioral tests. Therefore, we designed the present study primarily as a behavioral validation of Ts65Dn mice of the new background. We compared side-by-side their performance with that of Ts65Dn mice of the original background on the following set of assessments: (1) body length and weight; (2) 24-h locomotor activity; (3) the Morris water maze; (4) fear conditioning; and (5) grip strength. Except for very subtle differences on water maze performance, we found no significant differences between Ts65Dn mice on the two backgrounds in the measures assessed.

Widespread impairment of cell proliferation in the neonate Ts65Dn mouse, a model for Down syndrome

OBJECTIVES

Among the many pathological aspects of Down syndrome, brain hypoplasia and mental retardation have been recently ascribed to defective proliferation of neural precursors during central nervous system development. By analogy, other features of Down syndrome, such as heart defects, gastrointestinal abnormalities, craniofacial dystrophy and reduced growth rate could be related, at least in theory, to similar proliferation impairment in peripheral tissues.

MATERIALS AND METHODS

In order to test this hypothesis, we evaluated cell proliferation in peripheral tissues of the Ts65Dn mouse, one of the animal models most commonly used to investigate Down syndrome.

RESULTS

In fibroblast cultures from neonatal Ts65Dn mice, we found that cell proliferation was notably impaired. While length of the cell cycle was similar in fibroblasts from Ts65Dn and control mice, the number of actively proliferating cells was significantly smaller in Ts65Dn mice. Moreover, fibroblasts from Ts65Dn animals exhibited limited population-doubling capacity, decreased proliferative lifespan and premature senescence. Analysis of cell proliferation in the skin of neonates, in vivo, showed that in Ts65Dn mice, cell proliferation was significantly reduced compared to control mice.

CONCLUSIONS

Our results suggest that defective proliferation may be a generalized feature of trisomic mice. In view of the genetic and phenotypic similarities between Ts65Dn mice and individuals with Down syndrome, proliferation impairment in various organs may also occur in subjects with Down syndrome. Thus, perturbation of a basic developmental function, cell proliferation, may be a critical determinant that contributes to the many aspects of pathology of this condition.

Molecular responses of the Ts65Dn and Ts1Cje mouse models of Down syndrome to MK-801

Down syndrome (DS), caused by trisomy of human chromosome 21 (chr21), is the most common genetic cause of intellectual disability. The Ts65Dn mouse model of DS is trisomic for orthologs of 94 chr21-encoded, confirmed protein-coding genes and displays a number of behavioral deficits. Recently, Ts65Dn mice were shown to be hypersensitive to the locomotor stimulatory effects of the high-affinity N-methyl-d-aspartate (NMDA) receptor (NMDAR) channel blocker, MK-801. This is consistent with the functions of several chr21 proteins that are predicted directly or indirectly to impact NMDAR function or NMDAR-mediated signaling. In this study, we show that a second mouse model of DS, the Ts1Cje, which is trisomic for 70 protein-coding genes, is also hypersensitive to MK-801. To investigate the molecular basis for the responses to MK-801, we have measured levels of a subset of chr21 and phosphorylated non-chr21 proteins, in the cortex and hippocampus of Ts65Dn and Ts1Cje mice and euploid controls, with and without treatment with MK-801. We show that in euploid mice, the chr21-encoded proteins, TIAM1 and DYRK1A, and phosphorylation of AKT, ERK1/2 and the transcription factor ELK are involved in the MK-801 response. However, in both Ts65Dn and Ts1Cje mice, levels of phosphorylation are constitutively elevated in naïve, unstimulated mice, and the MK-801-induced changes in TIAM1 and DYRK1A and in phosphorylation are either absent or abnormal, with both genotype and brain-region-specific patterns. These results emphasize the complexities of the pathway perturbations that arise with segmental trisomy.

Abnormal expression of synaptic proteins and neurotrophin-3 in the Down syndrome mouse model Ts65Dn

Down syndrome (DS) results from triplication of the whole or distal part of human chromosome 21. Persons with DS suffer from deficits in learning and memory and cognitive functions in general, and, starting from early development, their brains show dendritic and spine structural alterations and cell loss. These defects concern many cortical brain regions as well as the hippocampus, which is known to play a critical role in memory and cognition. Most of these abnormalities are reproduced in the mouse model Ts65Dn, which is partially trisomic for the mouse chromosome 16 that is homologous to a portion of human chromosome 21. Thus, Ts65Dn is widely utilized as an animal model of DS. To better understand the molecular defects underlying the cognitive and particularly the memory impairments of DS, we investigated whether the expression of several molecules known to play critical roles in long-term synaptic plasticity and long-term memory in a variety of species is dysregulated in either the neonatal brain or adult hippocampus of Ts65Dn mice. We found abnormal expression of the synaptic proteins synaptophysin, microtubule-associated protein 2 (MAP2) and cyclin-dependent kinase 5 (CDK5) and of the neurotrophin-3 (NT-3). Both the neonatal brain and adult hippocampus revealed significant abnormalities. These results suggest that a dysregulation in the expression of neurotrophins as well as proteins involved in synaptic development and plasticity may play a potential role in the neural pathology of DS in humans.

Synaptojanin 1-linked phosphoinositide dyshomeostasis and cognitive deficits in mouse models of Down's syndrome

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] is a signaling phospholipid implicated in a wide variety of cellular functions. At synapses, where normal PtdIns(4,5)P(2) balance is required for proper neurotransmission, the phosphoinositide phosphatase synaptojanin 1 is a key regulator of its metabolism. The underlying gene, SYNJ1, maps to human chromosome 21 and is thus a candidate for involvement in Down's syndrome (DS), a complex disorder resulting from the overexpression of trisomic genes. Here, we show that PtdIns(4,5)P(2) metabolism is altered in the brain of Ts65Dn mice, the most commonly used model of DS. This defect is rescued by restoring Synj1 to disomy in Ts65Dn mice and is recapitulated in transgenic mice overexpressing Synj1 from BAC constructs. These transgenic mice also exhibit deficits in performance of the Morris water maze task, suggesting that PtdIns(4,5)P(2) dyshomeostasis caused by gene dosage imbalance for Synj1 may contribute to brain dysfunction and cognitive disabilities in DS.

Mice transgenic for reduced folate carrier: an animal model of Down syndrome?

In a previous publication we observed aberrant levels of the human reduced folate carrier (hRFC) in cortex from fetal Down syndrome (DS) subjects. Immunoreactivity for hRFC was increased as the only chromosome 21 gene product studied. We, therefore, analyzed mice transgenic for hRFC (TghRFC1) and wild-type (WT) mice for cognitive functions, behavior and in an observational neurological battery (FOB). Cognitive functions were evaluated by the Morris water maze (MWM), the open field (OF) was used for exploratory behavior, locomotor activity and anxiety-related behavior. The elevated plus maze (EPM) was used to confirm findings in the OF testing anxiety-related behavior and the rota rod (RR) to evaluate motor function. In the MWM TghRFC1 mice performed significantly worse (P < 0.0003) on the probe trial than WT mice. In the FOB visual placing was significantly reduced inTghRFC1 mice. In the OF TghRFC1 mice crossed twice as often (P < 0.029) and in the EPM individuals from this group showed a reduced number of exits from the closed arm (P < 0.044) compared to WT mice. TghRFC1 mice showed impaired performance on the RR, spending one-fourth of the time of WT on the revolving rod (P < 0.0003). Cognitive impairment is an obligatory symptom of DS and this deficiency corresponds to findings in the MWM of mice transgenic for hRFC. Findings of visual placing and failure on the RR may reflect impaired motor performance including muscular hypotonia in DS subjects. Increased crossings in the OF may indicate modulated anxiety-related behavior observed in patients with DS.

Defects in embryonic neurogenesis and initial synapse formation in the forebrain of the Ts65Dn mouse model of Down syndrome

Trisomy 21, one of the most prevalent congenital birth defects, results in a constellation of phenotypes collectively termed Down syndrome (DS). Mental retardation and motor and sensory deficits are among the many debilitating symptoms of DS. Alterations in brain growth and synaptic development are thought to underlie the cognitive impairments in DS, but the role of early brain development has not been studied because of the lack of embryonic human tissue and because of breeding difficulties in mouse models of DS. We generated a breeding colony of the Ts65Dn mouse model of DS to test the hypothesis that early defects in embryonic brain development are a component of brain dysfunction in DS. We found substantial delays in prenatal growth of the Ts65Dn cerebral cortex and hippocampus because of longer cell cycle duration and reduced neurogenesis from the ventricular zone neural precursor population. In addition, the Ts65Dn neocortex remains hypocellular after birth and there is a lasting decrease in synaptic development beginning in the first postnatal week. These results demonstrate that specific abnormalities in embryonic forebrain precursor cells precede early deficits in synaptogenesis and may underlie the postnatal disabilities in Ts65Dn and DS. The early prenatal period is therefore an important new window for possible therapeutic amelioration of the cognitive symptoms in DS.

Over-inhibition: a model for developmental intellectual disability

Developmental intellectual disability (DID) is a daunting societal problem. Although tremendous progress has been made in defining the genetic causes of DID, therapeutic strategies remain limited. In particular, there is a marked absence of a unified approach to treating cognitive impairments associated with DID. Here, we suggest that the brain in many DID-related disorders is subject to a basic imbalance in neuronal activity, with an increased contribution of inhibition to neural circuits. This over-inhibition, in turn, is predicted to lead to deficits in synaptic plasticity and learning and memory. We further discuss possibilities for pharmacological intervention in DID, focusing on the concept of drug-induced 'therapeutic neuroadaptation' as a means of stably enhancing constitutive circuit excitability and cognition over time.

Age-related changes in memory and in acetylcholine functions in the hippocampus in the Ts65Dn mouse, a model of Down syndrome

Spatial working memory and the ability of a cholinesterase inhibitor to enhance memory were assessed at 4, 10, and 16 months of ages in control and Ts65Dn mice, a partial trisomy model of Down syndrome, with possibly significant relationships to Alzheimer's disease as well. In addition, ACh release during memory testing was measured in samples collected from the hippocampus using in vivo microdialysis at 4, 10, and 22-25 months of age. When tested on a four-arm spontaneous alternation task, the Ts65Dn mice exhibited impaired memory scores at both 4 and 10 months. At 16 months, control performance had declined toward that of the Ts65Dn mice and the difference in scores across genotypes was not significant. Physostigmine (50 microg/kg) fully reversed memory deficits in the Ts65Dn mice in the 4-month-old group but not in older mice. Ts65Dn and control mice exhibited comparable baseline levels of ACh release at all ages tested; these levels did not decline significantly across age in either genotype. ACh release increased significantly during alternation testing only in the young Ts65Dn and control mice. However, the increase in ACh release during alternation testing was significantly greater in control than Ts65Dn mice at this age. The controls exhibited a significant age-related decline in the testing-related increase in ACh release. With only a small increase during testing in young Ts65Dn mice, the age-related decline in responsiveness of ACh release to testing was not significant in these mice. Overall, these results suggest that diminished responsiveness of ACh release in the hippocampus to behavioral testing may contribute memory impairments in Ts65Dn mice.

Down syndrome gene dosage imbalance on cerebellum development

Down syndrome (DS) is a chromosomal disorder whereby genes on chromosome 21 are present in three copies. This gene copy imbalance is thought to be responsible for a number of debilitating conditions experienced by individuals with DS. Amongst these is a reduced cerebellar volume, or cerebellar hypoplasia, which is believed to contribute to the perturbation of fine motor control. Mouse models of DS (such as Ts65Dn, Ts1Cje, Tc1) exhibit a cerebellar phenotype similar to that of individuals with DS and which primarily manifests as a disruption of the density of the granule cell layer. Dissecting which of the three-copy genes are responsible for this phenotype (the primary gene dosage effect) has been a task undertaken by researchers working with various segmental trisomies and transgenic mice. It is generally agreed that, when expressed, three-copy genes of trisomic mice are expressed at around 1.5 times that of the same genes in euploid (wild-type) mice. However, amongst these studies there does not appear to be a consensus on the nature and extent of differential expression of two-copy genes in trisomic mice-the secondary dosage effect. Much of this variation may have to do with the stage of development investigated and the nature and complexity of the tissue (i.e. whole brain versus the cerebellum). The recent discovery that trisomic granule cell precursors are less sensitive to sonic hedgehog-induced proliferation has opened up another avenue for the identification of three-copy genes responsible for the cerebellar phenotype. It is hoped that further investigation of this phenomenon, together with new mouse segmental trisomies and transgenics, will reveal the cause of the proliferation deficit and allow for potential treatment.

Object recognition memory is conserved in Ts1Cje, a mouse model of Down syndrome

Ts1Cje and Ts65Dn are genetic mouse models of Down syndrome (DS). Like individuals with DS, these mice exhibit various hallmarks of hippocampal pathology, and deficits in hippocampal-based, declarative learning and memory tasks. Both spatial navigation and novel object recognition, two prototypical domains of declarative memory function, have been strongly characterized in the Ts65Dn DS model. Indeed, Ts65Dn mice show navigation problems in the Morris water maze, impaired alternation in a T-maze, and deficient working and reference memory in the radial arm maze task. They, likewise, show an inability to detect object novelty over time. In contrast to the Ts65Dn model, hippocampal-dependent cognition has been less well characterized in Ts1Cje. Although Ts1Cje mice have been found to exhibit spatial difficulties in the Morris water maze and reduced spontaneous alternation, their ability to process object-based information has never been examined. Here, we report that Ts1Cje mice perform normally in short-term and long-term novel object recognition tasks. The ability of Ts1Cje mice to detect object novelty, unlike Ts65Dn, may point to differences in the extent of hippocampal pathology in the two DS mouse models.