Proliferation deficits and gene expression dysregulation in Down's syndrome (Ts1Cje) neural progenitor cells cultured from neurospheres

Down-syndrome-induced senescence disrupts the nuclear architecture of neural progenitors

Down syndrome (DS) is a genetic disorder driven by the triplication of chromosome 21 (T21) and characterized by a wide range of neurodevelopmental and physical disabilities. Transcriptomic analysis of tissue samples from individuals with DS has revealed that T21 induces a genome-wide transcriptional disruption. However, the consequences of T21 on the nuclear architecture and its interplay with the transcriptome remain unknown. In this study, we find that unlike human induced pluripotent stem cells (iPSCs), iPSC-derived neural progenitor cells (NPCs) exhibit genome-wide "chromosomal introversion," disruption of lamina-associated domains, and global chromatin accessibility changes in response to T21, consistent with the transcriptional and nuclear architecture changes characteristic of senescent cells. Treatment of T21-harboring NPCs with senolytic drugs alleviates the transcriptional, molecular, and cellular dysfunctions associated with DS. Our findings provide a mechanistic link between T21 and global transcriptional disruption and indicate that senescence-associated phenotypes may play a key role in the neurodevelopmental pathogenesis of DS.

Triple play of DYRK1A kinase in cortical progenitor cells of Trisomy 21

Down syndrome (DS) also known as Trisomy 21 is a genetic disorder that occurs in ∼1 in 800 live births. The disorder is caused by the triplication of all or part of human chromosome 21 and therefore, is thought to arise from the increased dosage of genes found within chromosome 21. The manifestations of the disease include among others physical growth delays and intellectual disability. A prominent anatomical feature of DS is the microcephaly that results from altered brain development. Recent studies using mouse models of DS have shed new light on DYRK1A (dual-specificity tyrosine-phosphorylation-regulated kinase 1A), a gene located on human chromosome 21 that plays a critical role in neocortical development. The present review summarizes effects of the increased dosage of DYRK1A on the proliferative, neurogenic and astrogliogenic potentials of cortical neural progenitor cells, and relates these findings to the clinical manifestations of the disease.

Neurogenesis impairment: An early developmental defect in Down syndrome

Down syndrome (DS) is characterized by brain hypotrophy and intellectual disability starting from early life stages. Accumulating evidence shows that the phenotypic features of the DS brain can be traced back to the fetal period since the DS brain exhibits proliferation potency reduction starting from the critical time window of fetal neurogenesis. This defect is worsened by the fact that neural progenitor cells exhibit reduced acquisition of a neuronal phenotype and an increase in the acquisition of an astrocytic phenotype. Consequently, the DS brain has fewer neurons in comparison with the typical brain. Although apoptotic cell death may be increased in DS, this does not seem to be the major cause of brain hypocellularity. Evidence obtained in brains of individuals with DS, DS-derived induced pluripotent stem cells (iPSCs), and DS mouse models has provided some insight into the mechanisms underlying the developmental defects due to the trisomic condition. Although many triplicated genes may be involved, in the light of the studies reviewed here, DYRK1A, APP, RCAN1 and OLIG1/2 appear to be particularly important determinants of many neurodevelopmental alterations that characterize DS because their triplication affects both the proliferation and fate of neural precursor cells as well as apoptotic cell death. Based on the evidence reviewed here, pathways downstream to these genes may represent strategic targets, for the design of possible interventions.

A Comprehensive Diverse '-omics' Approach to Better Understanding the Molecular Pathomechanisms of Down Syndrome

Diverse ‘-omics’ technologies permit the comprehensive quantitative profiling of a variety of biological molecules. Comparative ‘-omics’ analyses, such as transcriptomics and proteomics, are powerful and useful tools for unraveling the molecular pathomechanisms of various diseases. As enhanced oxidative stress has been demonstrated in humans and mice with Down syndrome (DS), a redox proteomic analysis is useful for understanding how enhanced oxidative stress aggravates the state of individuals with oxidative stress-related disorders. In this review, ‘-omics’ analyses in humans with DS and mouse models of DS are summarized, and the molecular dissection of this syndrome is discussed.

Molecular Mechanism Underlying Abnormal Differentiation of Neural Progenitor Cells in the Developing Down Syndrome Brain

Down syndrome (DS) is caused by trisomy for human chromosome 21. Individuals with DS commonly exhibit mental retardation, which is associated with abnormal brain development. In the neocortex of the DS brain, the density of neurons is markedly reduced, whereas that of astrocytes is increased. Similar to abnormalities seen in DS brains, mouse models of DS show deficits in brain development, and neural progenitor cells that give rise to neurons and glia show dysregulation in their differentiation. These suggest that the dysregulation of progenitor fate choices contributes to alterations in the numbers of neurons and astrocytes in the DS brain. Nevertheless, the molecular basis underlying these defects remains largely unknown. We showed that the overexpression of two human chromosome 21 genes, DYRK1A and DSCR1, contributes to suppressed neuronal differentiation of progenitors in the Ts1Cje mouse model of DS. In addition, the effect of DYRK1A and DSCR1 overexpression on neuronal differentiation is mediated by excessive attenuation of the transcription factor NFATc. Additionally, we demonstrated that an increased dosage of DYRK1A contributes to elevated potential of Ts1Cje progenitors to differentiate into astrocytes and enhanced astrogliogenesis in the Ts1Cje neocortex. Further, we linked the increased dosage of DYRK1A to dysregulation of STAT, a transcription factor critical for astrogliogenesis. Together, our studies identify critical pathways responsible for the proper differentiation of neural progenitors into neurons and astrocytes, with direct implications for the anomalies in brain development observed in DS.

DYRK1A, a Dosage-Sensitive Gene Involved in Neurodevelopmental Disorders, Is a Target for Drug Development in Down Syndrome

Down syndrome (DS) is one of the leading causes of intellectual disability, and patients with DS face various health issues, including learning and memory deficits, congenital heart disease, Alzheimer's disease (AD), leukemia, and cancer, leading to huge medical and social costs. Remarkable advances on DS research have been made in improving cognitive function in mouse models for future therapeutic approaches in patients. Among the different approaches, DYRK1A inhibitors have emerged as promising therapeutics to reduce DS cognitive deficits. DYRK1A is a dual-specificity kinase that is overexpressed in DS and plays a key role in neurogenesis, outgrowth of axons and dendrites, neuronal trafficking and aging. Its pivotal role in the DS phenotype makes it a prime target for the development of therapeutics. Recently, disruption of DYRK1A has been found in Autosomal Dominant Mental Retardation 7 (MRD7), resulting in severe mental deficiency. Recent advances in the development of kinase inhibitors are expected, in the near future, to remove DS from the list of incurable diseases, providing certain conditions such as drug dosage and correct timing for the optimum long-term treatment. In addition the exact molecular and cellular mechanisms that are targeted by the inhibition of DYRK1A are still to be discovered.

DYRK1A overexpression enhances STAT activity and astrogliogenesis in a Down syndrome mouse model

Down syndrome (DS) arises from triplication of genes on human chromosome 21 and is associated with anomalies in brain development such as reduced production of neurons and increased generation of astrocytes. Here, we show that differentiation of cortical progenitor cells into astrocytes is promoted by DYRK1A, a Ser/Thr kinase encoded on human chromosome 21. In the Ts1Cje mouse model of DS, increased dosage of DYRK1A augments the propensity of progenitors to differentiate into astrocytes. This tendency is associated with enhanced astrogliogenesis in the developing neocortex. We also find that overexpression of DYRK1A upregulates the activity of the astrogliogenic transcription factor STAT in wild-type progenitors. Ts1Cje progenitors exhibit elevated STAT activity, and depletion of DYRK1A in these cells reverses the deregulation of STAT. In sum, our findings indicate that potentiation of the DYRK1A-STAT pathway in progenitors contributes to aberrant astrogliogenesis in DS.

The fetal brain transcriptome and neonatal behavioral phenotype in the Ts1Cje mouse model of Down syndrome

Human fetuses with Down syndrome demonstrate abnormal brain growth and reduced neurogenesis. Despite the prenatal onset of the phenotype, most therapeutic trials have been conducted in adults. Here, we present evidence for fetal brain molecular and neonatal behavioral alterations in the Ts1Cje mouse model of Down syndrome. Embryonic day 15.5 brain hemisphere RNA from Ts1Cje embryos (n = 5) and wild type littermates (n = 5) was processed and hybridized to mouse gene 1.0 ST arrays. Bioinformatic analyses were implemented to identify differential gene and pathway regulation during Ts1Cje fetal brain development. In separate experiments, the Fox scale, ultrasonic vocalization and homing tests were used to investigate behavioral deficits in Ts1Cje pups (n = 29) versus WT littermates (n = 64) at postnatal days 3-21. Ts1Cje fetal brains displayed more differentially regulated genes (n = 71) than adult (n = 31) when compared to their age-matched euploid brains. Ts1Cje embryonic brains showed up-regulation of cell cycle markers and down-regulation of the solute-carrier amino acid transporters. Several cellular processes were dysregulated at both stages, including apoptosis, inflammation, Jak/Stat signaling, G-protein signaling, and oxidoreductase activity. In addition, early behavioral deficits in surface righting, cliff aversion, negative geotaxis, forelimb grasp, ultrasonic vocalization, and the homing tests were observed. The Ts1Cje mouse model exhibits abnormal gene expression during fetal brain development, and significant neonatal behavioral deficits in the pre-weaning period. In combination with human studies, this suggests that the Down syndrome phenotype manifests prenatally and provides a rationale for prenatal therapy to improve perinatal brain development and postnatal neurocognition.

Assessing apoptosis gene expression profiling with a PCR array in the hippocampus of Ts65Dn mice

It is well known that Down syndrome (DS) is a condition in which extra genetic material causes delays in the way a child develops, both mentally and physically. Intellectual disability is the foremost and most debilitating trait, which caused loss of cognitive abilities and the development of early onset Alzheimer's disease (AD). Ts65Dn mice were used in this study. We isolated the hippocampus. First, we used transmission scanning electron microscopy to directly observe the hippocampus and confirm if apoptosis had occurred. Second, we customized a PCR array with 53 genes, including several important genes related to cell apoptosis. Gene expression was detected by RT-PCR. There were varying degrees of changes characteristic of apoptosis in the hippocampus of Ts65Dn mice, which mainly included the following: nuclear membrane thinning, unevenly distributed chromosomes, the production of chromatin crescents, and pyknosis of the nuclei with some nuclear fragmentation. Meanwhile, three genes (API5, AIFM1, and NFκB1) showed changes of expression in the hippocampus of Ts65Dn mice compared with normal mice. Only NFκB1 expression was significantly increased, while the expressions of API5 and AIFM1 were notably decreased. The fold changes in the expression of API5, AIFM1, and NFκB1 were 11.55, 5.94, and 3.11, respectively. However, some well-known genes related to cell apoptosis, such as the caspase family, Bcl-2, Bad, Bid, Fas, and TNF, did not show changes in expression levels. The genes we found which were differentially expressed in the hippocampus of Ts65Dn mice may be closely related to cell apoptosis. PCR array technology can assist in the screening and identification of genes involved in apoptosis.

A case study of brain morphometry in triplets discordant for Down syndrome

Down syndrome, the most common genetic cause of intellectual disability, offers the opportunity to explore the associations between genetics and both neuroanatomic and neuropsychological phenotypes. This case report summarizes the findings of a neuroimaging and neuropsychology study of two adolescent females with Down syndrome and their same-sex discordant triplet siblings (one from each family; n = 4). Using high-resolution magnetic resonance imaging and surface based morphometric approaches, we offer the first in vivo report of cortical surface area reductions and increases in the thickness of the cortical sheet in youth with Down syndrome relative to their typically developing same-sex triplet siblings.

Analysis of adult cerebral cortex and hippocampus transcriptomes reveals unique molecular changes in the Ts1Cje mouse model of down syndrome

We investigated gene expression and functional differences between Ts1Cje mice and wild-type (WT) littermates in adult cerebral cortex and hippocampus. These two brain regions are affected in people with Down syndrome, but have not been previously molecularly characterized in Ts1Cje mice. Total RNA was prepared from the brains of 8-10-week-old Ts1Cje mice (n = 6) and WT littermates (n = 5) and hybridized to Affymetrix 1.0 ST gene mouse arrays. Differentially regulated genes were identified and used to perform in silico functional analyses to better characterize dysregulated pathways in both brain regions. Hippocampus had more significantly differentially expressed genes compared with cortex (30 vs. 7 at a Benjamini-Hochberg false discovery rate of 20%). We identified novel genes that were differentially regulated in adult brains, including Cyb5r1, Fsbp, Vmn2r110, Snd1 and Zhx2. Functional analyses in Ts1Cje mice highlighted the importance of NFAT signaling, oxidative stress, neuroinflammation and olfactory perception via G-protein signaling. In a comparison of adult Ts1Cje and WT brains, we identified new genes and pathway differences in the cortex and hippocampus. Our analyses identified physiologically relevant pathways that can serve as targets for the development of future treatments to improve neurocognition in Down syndrome.

Functional transcriptome analysis of the postnatal brain of the Ts1Cje mouse model for Down syndrome reveals global disruption of interferon-related molecular networks

Background

The Ts1Cje mouse model of Down syndrome (DS) has partial triplication of mouse chromosome 16 (MMU16), which is partially homologous to human chromosome 21. These mice develop various neuropathological features identified in DS individuals. We analysed the effect of partial triplication of the MMU16 segment on global gene expression in the cerebral cortex, cerebellum and hippocampus of Ts1Cje mice at 4 time-points: postnatal day (P)1, P15, P30 and P84.

Results

Gene expression profiling identified a total of 317 differentially expressed genes (DEGs), selected from various spatiotemporal comparisons, between Ts1Cje and disomic mice. A total of 201 DEGs were identified from the cerebellum, 129 from the hippocampus and 40 from the cerebral cortex. Of these, only 18 DEGs were identified as common to all three brain regions and 15 were located in the triplicated segment. We validated 8 selected DEGs from the cerebral cortex (Brwd1, Donson, Erdr1, Ifnar1, Itgb8, Itsn1, Mrps6 and Tmem50b), 18 DEGs from the cerebellum (Atp5o, Brwd1, Donson, Dopey2, Erdr1, Hmgn1, Ifnar1, Ifnar2, Ifngr2, Itgb8, Itsn1, Mrps6, Paxbp1, Son, Stat1, Tbata, Tmem50b and Wrb) and 11 DEGs from the hippocampus (Atp5o, Brwd1, Cbr1, Donson, Erdr1, Itgb8, Itsn1, Morc3, Son, Tmem50b and Wrb). Functional clustering analysis of the 317 DEGs identified interferon-related signal transduction as the most significantly dysregulated pathway in Ts1Cje postnatal brain development. RT-qPCR and western blotting analysis showed both Ifnar1 and Stat1 were over-expressed in P84 Ts1Cje cerebral cortex and cerebellum as compared to wild type littermates.

Conclusions

These findings suggest over-expression of interferon receptor may lead to over-stimulation of Jak-Stat signaling pathway which may contribute to the neuropathology in Ts1Cje or DS brain. The role of interferon mediated activation or inhibition of signal transduction including Jak-Stat signaling pathway has been well characterized in various biological processes and disease models including DS but information pertaining to the role of this pathway in the development and function of the Ts1Cje or DS brain remains scarce and warrants further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-624) contains supplementary material, which is available to authorized users.

Increased dosage of DYRK1A and DSCR1 delays neuronal differentiation in neocortical progenitor cells

Down's syndrome (DS), a major genetic cause of mental retardation, arises from triplication of genes on human chromosome 21. Here we show that DYRK1A (dual-specificity tyrosine-phosphorylated and -regulated kinase 1A) and DSCR1 (DS critical region 1), two genes lying within human chromosome 21 and encoding for a serine/threonine kinase and calcineurin regulator, respectively, are expressed in neural progenitors in the mouse developing neocortex. Increasing the dosage of both proteins in neural progenitors leads to a delay in neuronal differentiation, resulting ultimately in alteration of their laminar fate. This defect is mediated by the cooperative actions of DYRK1A and DSCR1 in suppressing the activity of the transcription factor NFATc. In Ts1Cje mice, a DS mouse model, dysregulation of NFATc in conjunction with increased levels of DYRK1A and DSCR1 was observed. Furthermore, counteracting the dysregulated pathway ameliorates the delayed neuronal differentiation observed in Ts1Cje mice. In sum, our findings suggest that dosage of DYRK1A and DSCR1 is critical for proper neurogenesis through NFATc and provide a potential mechanism to explain the neurodevelopmental defects in DS.

Cognitive and pharmacological insights from the Ts65Dn mouse model of Down syndrome

Down syndrome (DS) is a multi-faceted condition resulting in the most common genetic form of intellectual disability. Mouse models of DS, especially the Ts65Dn model, have been pivotal in furthering our understanding of the genetic, molecular and neurobiological mechanisms that underlie learning and memory impairments in DS. Cognitive and pharmacological insights from the Ts65Dn mouse model have led to remarkable translational progress in the development of therapeutic targets and in the emergence of DS clinical trials. Unravelling the pathogenic role of trisomic genes on human chromosome 21 and the genotype-phenotype relationship still remains a pertinent goal for tackling cognitive deficits in DS.

The in vivo Down syndrome genomic library in mouse

Mouse models are key elements to better understand the genotype-phenotype relationship and the physiopathology of Down syndrome (DS). Even though the mouse will never recapitulate the whole spectrum of intellectual disabilities observed in the DS, mouse models have been developed over the recent decades and have been used extensively to identify homologous genes or entire regions homologous to the human chromosome 21 (Hsa21) that are necessary or sufficient to induce DS cognitive features. In this chapter, we review the principal mouse DS models which have been selected and engineered over the years either for large genomic regions or for a few or a single gene of interest. Their analyses highlight the complexity of the genetic interactions that are involved in DS cognitive phenotypes and also strengthen the hypothesis on the multigenic nature of DS. This review also addresses future research challenges relative to the making of new models and their combination to go further in the characterization of candidates and modifier of the DS features.

Transcriptional consequences of aneuploidy

Aneuploidy, or an aberrant karyotype, results in developmental disabilities and has been implicated in tumorigenesis. However, the causes of aneuploidy-induced phenotypes and the consequences of aneuploidy on cell physiology remain poorly understood. We have performed a metaanalysis on gene expression data from aneuploid cells in diverse organisms, including yeast, plants, mice, and humans. We found highly related gene expression patterns that are conserved between species: genes that were involved in the response to stress were consistently upregulated, and genes associated with the cell cycle and cell proliferation were downregulated in aneuploid cells. Within species, different aneuploidies induced similar changes in gene expression, independent of the specific chromosomal aberrations. Taken together, our results demonstrate that aneuploidies of different chromosomes and in different organisms impact similar cellular pathways and cause a stereotypical antiproliferative response that must be overcome before transformation.

Molecular and cellular alterations in Down syndrome: toward the identification of targets for therapeutics

Down syndrome is a complex disease that has challenged molecular and cellular research for more than 50 years. Understanding the molecular bases of morphological, cellular, and functional alterations resulting from the presence of an additional complete chromosome 21 would aid in targeting specific genes and pathways for rescuing some phenotypes. Recently, progress has been made by characterization of brain alterations in mouse models of Down syndrome. This review will highlight the main molecular and cellular findings recently described for these models, particularly with respect to their relationship to Down syndrome phenotypes.

Mouse models of Down syndrome as a tool to unravel the causes of mental disabilities

Down syndrome (DS) is the most common genetic cause of mental disability. Based on the homology of Hsa21 and the murine chromosomes Mmu16, Mmu17 and Mmu10, several mouse models of DS have been developed. The most commonly used model, the Ts65Dn mouse, has been widely used to investigate the neural mechanisms underlying the mental disabilities seen in DS individuals. A wide array of neuromorphological alterations appears to compromise cognitive performance in trisomic mice. Enhanced inhibition due to alterations in GABA(A)-mediated transmission and disturbances in the glutamatergic, noradrenergic and cholinergic systems, among others, has also been demonstrated. DS cognitive dysfunction caused by neurodevelopmental alterations is worsened in later life stages by neurodegenerative processes. A number of pharmacological therapies have been shown to partially restore morphological anomalies concomitantly with cognition in these mice. In conclusion, the use of mouse models is enormously effective in the study of the neurobiological substrates of mental disabilities in DS and in the testing of therapies that rescue these alterations. These studies provide the basis for developing clinical trials in DS individuals and sustain the hope that some of these drugs will be useful in rescuing mental disabilities in DS individuals.

Antioxidants in Down syndrome

Individuals with Down syndrome (DS) have high levels of oxidative stress throughout the lifespan. Mouse models of DS share some structural and functional abnormalities that parallel findings seen in the human phenotype. Several of the mouse models show evidence of cellular oxidative stress and have provided a platform for antioxidant intervention. Genes that are overexpressed on chromosome 21 are associated with oxidative stress and neuronal apoptosis. The lack of balance in the metabolism of free radicals generated during processes related to oxidative stress may have a direct role in producing the neuropathology of DS including the tendency to Alzheimer disease (AD). Mitochondria are often a target for oxidative stress and are considered to be a trigger for the onset of the AD process in DS. Biomarkers for oxidative stress have been described in DS and in AD in the general population. However, intervention trials using standard antioxidant supplements or diets have failed to produce uniform therapeutic effect. This chapter will examine the biological role of oxidative stress in DS and its relationship to abnormalities in both development and aging within the disorder. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

The Ts65Dn mouse model of Down syndrome shows reduced expression of the Bcl-X(L) antiapoptotic protein in the hippocampus not accompanied by changes in molecular or cellular markers of cell death

The Ts65Dn (TS) mouse, the most widely used model of Down syndrome (DS), has a partial trisomy of a segment of chromosome 16 that is homologous to the distal part of human chromosome 21. This mouse shares many phenotypic characteristics with people with DS including neuromorphological, neurochemical, and cognitive disturbances. Both TS and DS brains show earlier aging and neurodegeneration. Since fibroblast cultures from TS mice and human DS hippocampal regions show increased apoptotic cell death it has been suggested that alterations in cerebral apoptosis might be implicated in the cognitive deficits found in TS mice and in people with DS. In the present study we have evaluated brain expression levels of several proapoptotic and antiapoptotic proteins from the mitochondrial (Bcl-2, Bcl-X(L), Bax and Bad) and the extrinsic (Fas-R and Fas-L) apoptotic pathways as well as the final executioner caspase-3, in the cortex and hippocampus of TS mice. No significant alterations in the expression levels of the proapoptotic Bad and Bax or the antiapoptotic Bcl-2 proteins in the cortex or hippocampus were found in TS mice. However, TS mice showed downregulation of Bcl-X(L) in the hippocampus. In the extrinsic pathway we found unchanged levels of Fas-L in both structures and also in the expression levels of Fas-R in the hippocampus. Although Bcl-X(L) downregulation suggests that the hippocampus of TS mice is less protected against programmed cell death, we did not find any evidence for increased apoptosis in TS mice since neither TUNEL-positive cells nor active caspase-3 expression were found in cortex or hippocampus of TS or CO mice.

APP-dependent up-regulation of Ptch1 underlies proliferation impairment of neural precursors in Down syndrome

Mental retardation in Down syndrome (DS) appears to be related to severe neurogenesis impairment during critical phases of brain development. Recent lines of evidence in the cerebellum of a mouse model for DS (the Ts65Dn mouse) have shown a defective responsiveness to Sonic Hedgehog (Shh), a potent mitogen that controls cell division during brain development, suggesting involvement of the Shh pathway in the neurogenesis defects of DS. Based on these premises, we sought to identify the molecular mechanisms underlying derangement of the Shh pathway in neural precursor cells (NPCs) from Ts65Dn mice. By using an in vitro model of NPCs obtained from the subventricular zone and hippocampus, we found that trisomic NPCs had an increased expression of the Shh receptor Patched1 (Ptch1), a membrane protein that suppresses the action of a second receptor, Smoothened (Smo), thereby maintaining the pathway in a repressed state. Partial silencing of Ptch1 expression in trisomic NPCs restored cell proliferation, indicating that proliferation impairment was due to Ptch1 overexpression. The overexpression of Ptch1 in trisomic NPCs resulted from increased levels of AICD [a transcription-promoting fragment of amyloid precursor protein (APP)] and increased AICD binding to the Ptch1 promoter. Our data provide novel evidence that Ptch1 overexpression underlies derangement of the Shh pathway in trisomic NPCs with consequent proliferation impairment. The demonstration that Ptch1 overexpression in trisomic NPCs is due to an APP fragment provides a link between this trisomic gene and the defective neuronal production that characterizes the DS brain.

Effects of voluntary physical exercise on adult hippocampal neurogenesis and behavior of Ts65Dn mice, a model of Down syndrome

The Ts65Dn (TS) mouse is the most widely used model of Down syndrome (DS). This mouse shares many phenotypic characteristics with the human condition including cognitive and neuromorphological alterations. In this study the effects of physical exercise on hippocampal neurogenesis and behavior in TS mice were assessed. 10-12 month-old male TS and control (CO) mice were submitted to voluntary physical exercise for 7 weeks and the effects of this protocol on hippocampal morphology, neurogenesis and apoptosis were evaluated. Physical exercise improved performance in the acquisition sessions of the Morris water maze in TS but not in CO mice. Conversely, it did not have any effect on anxiety or depressive behavior in TS mice but it did reduce the cognitive components of anxiety in CO mice. TS mice presented a reduced dentate gyrus (DG) volume, subgranular zone area and number of granule neurons. Hippocampal neurogenesis was reduced in TS mice as shown by the reduced number of 5-bromo-2-deoxyuridine (BrdU) positive cells. Voluntary physical exercise did not rescue these alterations in TS mice but it did increase the number of doublecortin (DCX)-and phospho histone 3 (PH3)-positive neurons in CO mice. It is concluded that physical exercise produced a modest anxiolytic effect in CO mice and that this was accompanied by an increased number of immature cells in the hippocampal DG. On the other hand, voluntary physical exercise exerted a positive effect on TS mice learning of the platform position in the Morris water maze that seems to be mediated by a neurogenesis-independent mechanism.

Gene network disruptions and neurogenesis defects in the adult Ts1Cje mouse model of Down syndrome

BACKGROUND

Down syndrome (DS) individuals suffer mental retardation with further cognitive decline and early onset Alzheimer's disease.

METHODOLOGY/PRINCIPAL FINDINGS

To understand how trisomy 21 causes these neurological abnormalities we investigated changes in gene expression networks combined with a systematic cell lineage analysis of adult neurogenesis using the Ts1Cje mouse model of DS. We demonstrated down regulation of a number of key genes involved in proliferation and cell cycle progression including Mcm7, Brca2, Prim1, Cenpo and Aurka in trisomic neurospheres. We found that trisomy did not affect the number of adult neural stem cells but resulted in reduced numbers of neural progenitors and neuroblasts. Analysis of differentiating adult Ts1Cje neural progenitors showed a severe reduction in numbers of neurons produced with a tendency for less elaborate neurites, whilst the numbers of astrocytes was increased.

CONCLUSIONS/SIGNIFICANCE

We have shown that trisomy affects a number of elements of adult neurogenesis likely to result in a progressive pathogenesis and consequently providing the potential for the development of therapies to slow progression of, or even ameliorate the neuronal deficits suffered by DS individuals.

Down syndrome and the molecular pathogenesis resulting from trisomy of human chromosome 21

Chromosome copy number aberrations, anueploidies, are common in the human population but generally lethal. However, trisomy of human chromosome 21 is compatible with life and people born with this form of aneuploidy manifest the features of Down syndrome, named after Langdon Down who was a 19(th) century British physician who first described a group of people with this disorder. Down syndrome includes learning and memory deficits in all cases, as well as many other features which vary in penetrance and expressivity in different people. While Down syndrome clearly has a genetic cause - the extra dose of genes on chromosome 21 - we do not know which genes are important for which aspects of the syndrome, which biochemical pathways are disrupted, or, generally how design therapies to ameliorate the effects of these disruptions. Recently, with new insights gained from studying mouse models of Down syndrome, specific genes and pathways are being shown to be involved in the pathogenesis of the disorder. This is opening the way for exciting new studies of potential therapeutics for aspects of Down syndrome, particularly the learning and memory deficits.

Aneuploidy causes tissue-specific qualitative changes in global gene expression patterns in maize

Segmental aneuploidy refers to the relative excess or deficiency of specific chromosome regions. This condition results in gene dosage imbalance and often causes severe phenotypic alterations in plants and animals. The mechanisms by which gene dosage imbalance affects gene expression and phenotype are not completely clear. The effects of aneuploidy on the transcriptome may depend on the types of cells analyzed and on the developmental stage. We performed global gene expression profiling to determine the effects of segmental aneuploidy on gene expression levels in two different maize (Zea mays) tissues and a detailed analysis of expression of 30 genes affected by aneuploidy in multiple maize tissues. Different maize tissues varied in the frequency at which genes located outside of the aneuploid regions are positively or negatively regulated as well as in the degree of gene dosage compensation. Multiple genes demonstrated qualitative changes in gene expression due to aneuploidy, when the gene became ectopically expressed or completely silenced in aneuploids relative to wild-type plants. Our data strongly suggested that quantitative changes in gene expression at developmental transition points caused by variation in gene copy number progressed through tissue development and resulted in stable qualitative changes in gene expression patterns. Thus, aneuploidy in maize results in alterations of gene expression patterns that differ between tissues and developmental stages of maize seedlings.

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.