Trisomy 21 and early brain development

Activation of the ISR mediates the behavioral and neurophysiological abnormalities in Down syndrome

Down syndrome (DS) is the most common genetic cause of intellectual disability. Protein homeostasis is essential for normal brain function, but little is known about its role in DS pathophysiology. In this study, we found that the integrated stress response (ISR)-a signaling network that maintains proteostasis-was activated in the brains of DS mice and individuals with DS, reprogramming translation. Genetic and pharmacological suppression of the ISR, by inhibiting the ISR-inducing double-stranded RNA-activated protein kinase or boosting the function of the eukaryotic translation initiation factor eIF2-eIF2B complex, reversed the changes in translation and inhibitory synaptic transmission and rescued the synaptic plasticity and long-term memory deficits in DS mice. Thus, the ISR plays a crucial role in DS, which suggests that tuning of the ISR may provide a promising therapeutic intervention.

Structural Magnetic Resonance Imaging-Based Brain Morphology Study in Infants and Toddlers With Down Syndrome: The Effect of Comorbidities

BACKGROUND

Down syndrome (DS) is the most prevalent chromosomal disorder characterized by intellectual disability, multiple organ anomalies, generalized muscular hypotonia, and characteristic physical features. The presence of DS-associated medical comorbidities has contributed to brain morphologic changes. The aim of this study was to evaluate brain morphologic characteristics during infant and toddler ages in patients with DS using structural brain magnetic resonance imaging.

METHODS

Structural brain T1-weighted magnetic resonance images from participants with DS with complete chromosome 21 trisomy (n = 20; 1.6 ± 0.6 [mean ± standard deviation] years old) were analyzed using FreeSurfer. The measurements were compared with those of 60 gender- and age-matched neurotypical controls by Cohen's d statistic and unpaired t test with false discovery rate correction for multiple comparisons and analyzed using a univariate general linear model with the following DS-associated medical comorbidities: congenital cardiac disease, infantile spasms, and hypothyroidism.

RESULTS

We identified 27 candidate measurements with large effect sizes (absolute d > 0.8) and statistically significant differences (P < 6.9 × 10^-3). Among them were decreased volumes in bilateral cerebellar gray matter and right cerebellar white matter and brainstem and cortical abnormalities in the right superior temporal, right rostral anterior cingulate, and left rostral middle frontal gyrus, independent of comorbid effects. Only bilateral cerebellar gray matter volumes and brainstem volume showed differences between DS and healthy groups during infancy.

CONCLUSION

These results suggest that cerebellar gray matter and brainstem may represent the primary regions affected by the presence of an additional copy of chromosome 21.

Neuronal Cell-Intrinsic Defects in Mouse Models of Down Syndrome

Down Syndrome (DS) is the most common genetic disorder associated with intellectual disability (ID). Excitatory neurons of DS patients and mouse models show decreased size of dendritic field and reduction of spine density. Whether these defects are caused by cell autonomous alterations or by abnormal multicellular circuitry is still unknown. In this work, we explored this issue by culturing cortical neurons obtained from two mouse models of DS: the widely used Ts65Dn and the less characterized Ts2Cje. We observed that, in the in vitro conditions, axon specification and elongation, as well as dendritogenesis, take place without evident abnormalities, indicating that the initial phases of neuronal differentiation do not suffer from the presence of an imbalanced genetic dosage. Conversely, our analysis highlighted differences between trisomic and euploid neurons in terms of reduction of spine density, in accordance with in vivo data obtained by other groups, proposing the presence of a cell-intrinsic malfunction. This work suggests that the characteristic morphological defects of DS neurons are likely to be caused by the possible combination of cell-intrinsic defects together with cell-extrinsic cues. Additionally, our data support the possibility of using the more sustainable line Ts2Cje as a standard model for the study of DS.

New approaches to studying early brain development in Down syndrome

Down syndrome is the most common genetic developmental disorder in humans and is caused by partial or complete triplication of human chromosome 21 (trisomy 21). It is a complex condition which results in multiple lifelong health problems, including varying degrees of intellectual disability and delays in speech, memory, and learning. As both length and quality of life are improving for individuals with Down syndrome, attention is now being directed to understanding and potentially treating the associated cognitive difficulties and their underlying biological substrates. These have included imaging and postmortem studies which have identified decreased regional brain volumes and histological anomalies that accompany early onset dementia. In addition, advances in genome-wide analysis and Down syndrome mouse models are providing valuable insight into potential targets for intervention that could improve neurogenesis and long-term cognition. As little is known about early brain development in human Down syndrome, we review recent advances in magnetic resonance imaging that allow non-invasive visualization of brain macro- and microstructure, even in utero. It is hoped that together these advances may enable Down syndrome to become one of the first genetic disorders to be targeted by antenatal treatments designed to 'normalize' brain development. WHAT THIS PAPER ADDS: Magnetic resonance imaging can provide non-invasive characterization of early brain development in Down syndrome. Down syndrome mouse models enable study of underlying pathology and potential intervention strategies. Potential therapies could modify brain structure and improve early cognitive levels. Down syndrome may be the first genetic disorder to have targeted therapies which alter antenatal brain development.

Physical therapy in Down syndrome: systematic review and meta-analysis

BACKGROUND

Down syndrome is the most common chromosomal abnormality, with a worldwide incidence of around 0.1% in live births. It is related to several conditions in which the physical therapy could take action-preventing co-morbidities. This study aims to evaluate the effectiveness of physical therapy in Down syndrome, to know and compare the effectiveness of different physical therapy interventions in this population.

METHODS

A systematic review and a meta-analysis of randomised controlled trials were conducted. The search was performed during June 2018 in the following databases: PubMed, Web of Science, Physiotherapy Evidence Database and Scopus. The studies were selected using predefined inclusion and exclusion criteria. The Physiotherapy Evidence Database scale evaluated the quality of the methods used in the studies. Subsequently, the data were extracted, and statistical analysis was performed when possible.

RESULTS

A total of 27 articles were included, of which nine contributed information to the meta-analysis. Statistical analysis showed favourable results for the strength of upper and lower limbs [standardised mean difference (SMD) = 1.46; 95% confidence interval (CI): (0.77-2.15); and SMD = 2.04; 95% CI: (1.07-3.01)] and mediolateral oscillations of balance [SMD = -3.30; 95% CI: (-5.34 to -1.26)].

CONCLUSIONS

The results show the potential benefit of certain types of physical therapy interventions, specifically in strength and balance, in people with Down syndrome. There are still many aspects to clarify and new lines of research.

Elimination of protein aggregates prevents premature senescence in human trisomy 21 fibroblasts

Chromosome abnormalities induces profound alterations in gene expression, leading to various disease phenotypes. Recent studies on yeast and mammalian cells have demonstrated that aneuploidy exerts detrimental effects on organismal growth and development, regardless of the karyotype, suggesting that aneuploidy-associated stress plays an important role in disease pathogenesis. However, whether and how this effect alters cellular homeostasis and long-term features of human disease are not fully understood. Here, we aimed to investigate cellular stress responses in human trisomy syndromes, using fibroblasts and induced pluripotent stem cells (iPSCs). Dermal fibroblasts derived from patients with trisomy 21, 18 and 13 showed a severe impairment of cell proliferation and enhanced premature senescence. These phenomena were accompanied by perturbation of protein homeostasis, leading to the accumulation of protein aggregates. We found that treatment with sodium 4-phenylbutyrate (4-PBA), a chemical chaperone, decreased the protein aggregates in trisomy fibroblasts. Notably, 4-PBA treatment successfully prevented the progression of premature senescence in secondary fibroblasts derived from trisomy 21 iPSCs. Our study reveals aneuploidy-associated stress as a potential therapeutic target for human trisomies, including Down syndrome.

Perturbation of the immune cells and prenatal neurogenesis by the triplication of the Erg gene in mouse models of Down syndrome

Some mouse models of Down syndrome (DS), including Ts1Cje mice, exhibit impaired prenatal neurogenesis with yet unknown molecular mechanism. To gain insights into the impaired neurogenesis, a transcriptomic and flow cytometry analysis of E14.5 Ts1Cje embryo brain was performed. Our analysis revealed that the neutrophil and monocyte ratios in the CD45-positive hematopoietic cells were relatively increased, in agreement with the altered expression of inflammation/immune-related genes, in Ts1Cje embryonic brain, whereas the relative number of brain macrophages was decreased in comparison to wild-type mice. Similar upregulation of inflammation-associated mRNAs was observed in other DS mouse models, with variable trisomic region lengths. We used genetic manipulation to assess the contribution of Erg, a trisomic gene in these DS models, known to regulation hemato-immune cells. The perturbed proportions of immune cells in Ts1Cje mouse brain were restored in Ts1Cje-Erg^+/+/Mld2 mice, which are disomic for functional Erg but otherwise trisomic on a Ts1Cje background. Moreover, the embryonic neurogenesis defects observed in Ts1Cje cortex were reduced in Ts1Cje-Erg^+/+/Mld2 embryos. Our findings suggest that Erg gene triplication contributes to the dysregulation of the homeostatic proportion of the populations of immune cells in the embryonic brain and decreased prenatal cortical neurogenesis in the prenatal brain with DS.

OLIG2 Drives Abnormal Neurodevelopmental Phenotypes in Human iPSC-Based Organoid and Chimeric Mouse Models of Down Syndrome

Down syndrome (DS) is a common neurodevelopmental disorder, and cognitive defects in DS patients may arise from imbalances in excitatory and inhibitory neurotransmission. Understanding the mechanisms underlying such imbalances may provide opportunities for therapeutic intervention. Here, we show that human induced pluripotent stem cells (hiPSCs) derived from DS patients overproduce OLIG2⁺ ventral forebrain neural progenitors. As a result, DS hiPSC-derived cerebral organoids excessively produce specific subclasses of GABAergic interneurons and cause impaired recognition memory in neuronal chimeric mice. Increased OLIG2 expression in DS cells directly upregulates interneuron lineage-determining transcription factors. shRNA-mediated knockdown of OLIG2 largely reverses abnormal gene expression in early-stage DS neural progenitors, reduces interneuron production in DS organoids and chimeric mouse brains, and improves behavioral deficits in DS chimeric mice. Thus, altered OLIG2 expression may underlie neurodevelopmental abnormalities and cognitive defects in DS patients.

Transcriptome and Proteome Profiling of Neural Induced Pluripotent Stem Cells from Individuals with Down Syndrome Disclose Dynamic Dysregulations of Key Pathways and Cellular Functions

Down syndrome (DS) or trisomy 21 (T21) is a leading genetic cause of intellectual disability. To gain insights into dynamics of molecular perturbations during neurogenesis in DS, we established a model using induced pluripotent stem cells (iPSC) with transcriptome profiles comparable to that of normal fetal brain development. When applied on iPSCs with T21, transcriptome and proteome signatures at two stages of differentiation revealed strong temporal dynamics of dysregulated genes, proteins and pathways belonging to 11 major functional clusters. DNA replication, synaptic maturation and neuroactive clusters were disturbed at the early differentiation time point accompanied by a skewed transition from the neural progenitor cell stage and reduced cellular growth. With differentiation, growth factor and extracellular matrix, oxidative phosphorylation and glycolysis emerged as major perturbed clusters. Furthermore, we identified a marked dysregulation of a set of genes encoded by chromosome 21 including an early upregulation of the hub gene APP, supporting its role for disturbed neurogenesis, and the transcription factors OLIG1, OLIG2 and RUNX1, consistent with deficient myelination and neuronal differentiation. Taken together, our findings highlight novel sequential and differentiation-dependent dynamics of disturbed functions, pathways and elements in T21 neurogenesis, providing further insights into developmental abnormalities of the DS brain.

Subicular hypotrophy in fetuses with Down syndrome and in the Ts65Dn model of Down syndrome

Intellectual disability in Down syndrome (DS) has been attributed to neurogenesis impairment during fetal brain development. Consistently with explicit memory alterations observed in children with DS, fetuses with DS exhibit neurogenesis impairment in the hippocampus, a key region involved in memory formation and consolidation. Recent evidence suggests that the subiculum plays a unique role in memory retrieval, a process that is also altered in DS. While much attention has been devoted to the hippocampus, there is a striking lack of information regarding the subiculum of individuals with DS and DS models. In order to fill this gap, in the current study, we examined the subiculum of fetuses with DS and of the Ts65Dn mouse model of DS. We found that in fetuses with DS (gestational week: 17-21), the subiculum had a reduced thickness, a reduced cell density, a reduced density of progenitor cells in the ventricular zone, a reduced percentage of neurons, and an increased percentage of astrocytes and of cells immunopositive for calretinin-a protein expressed by inhibitory interneurons. Similarly to fetuses with DS, the subiculum of neonate Ts65Dn mice was reduced in size, had a reduced number of neurons and a reduced number of proliferating cells. Results suggest that the developmental defects in the subiculum of fetuses with DS may underlie impairment in recall memory and possibly other functions played by the subiculum. The finding that the subiculum of the Ts65Dn mouse exhibits neuroanatomical defects resembling those seen in fetuses with DS further validates the use of this model for preclinical studies.

Anti-IL17 treatment ameliorates Down syndrome phenotypes in mice

Down syndrome (DS) is characterized by structural and functional anomalies that are present prenatally and that lead to intellectual disabilities. Later in life, the cognitive abilities of DS individuals progressively deteriorate due to the development of Alzheimer's disease (AD)-associated neuropathology (i.e., β-amyloid (Aβ) plaques, neurofibrillary tangles (NFTs), neurodegeneration, synaptic pathology, neuroinflammation and increased oxidative stress). Increasing evidence has shown that among these pathological processes, neuroinflammation plays a predominant role in AD etiopathology. In AD mouse models, increased neuroinflammation appears earlier than Aβ plaques and NFTs, and in DS and AD models, neuroinflammation exacerbates the levels of soluble and insoluble Aβ species, favoring neurodegeneration. The Ts65Dn (TS) mouse, the most commonly used murine model of DS, recapitulates many alterations present in both DS and AD individuals, including enhanced neuroinflammation. In this study, we observed an altered neuroinflammatory milieu in the hippocampus of the TS mouse model. Pro-inflammatory mediators that were elevated in the hippocampus of this model included pro-inflammatory cytokine IL17A, which has a fundamental role in mediating brain damage in neuroinflammatory processes. Here, we analyzed the ability of an anti-IL17A antibody to reduce the neuropathological alterations that are present in TS mice during early neurodevelopmental stages (i.e., hippocampal neurogenesis and hypocellularity) or that are aggravated in later-life stages (i.e., cognitive abilities, cholinergic neuronal loss and increased cellular senescence, APP expression, Aβ peptide expression and neuroinflammation). Administration of anti-IL17 for 5 months, starting at the age of 7 months, partially improved the cognitive abilities of the TS mice, reduced the expression of several pro-inflammatory cytokines and the density of activated microglia and normalized the APP and Aβ_1-42 levels in the hippocampi of the TS mice. These results suggest that IL17-mediated neuroinflammation is involved in several AD phenotypes in TS mice and provide a new therapeutic target to reduce these pathological characteristics.

Down syndrome is accompanied by significantly reduced cortical grey-white matter tissue contrast

Increased cortical thickness (CT) has been reported in Down syndrome (DS) during childhood and adolescence, but it remains unclear, which components of the neural architecture underpin these increases and if CT remains altered in adults. Among other factors, differences in CT measures could be driven by reduced tissue contrast between grey and white matter (GWC), which has been reported in neurodegenerative disorders, such as Alzheimer's disease. Using structural magnetic resonance imaging, we therefore examined differences in CT and GWC in 26 adults with DS, and 23 controls, to (1) examine between-group differences in CT in adulthood, (2) establish whether DS is associated with significant reductions in GWC, and (3) determine the influence of GWC variability on between-group differences in CT. As hypothesized, we observed that DS was accompanied by wide-spread increases in CT, and significantly reduced GWC in several large clusters distributed across the cortex. Out of all vertices with a significant between-group difference in CT, 38.50% also displayed a significant reduction in GWC. This percentage of overlap was also statistically significant and extremely unlikely to be obtained by chance (p = .0002). Differences in GWC thus seem to explain some, although not all, of the differences in CT observed in DS. In addition, our study is the first to extend previous in vivo reports of altered CT in DS during childhood and adolescence to older adults, implying that the regional pattern of neuroanatomical differences associated with DS remains stable across the lifespan.

Trisomy 21 Represses Cilia Formation and Function

Trisomy 21 (T21) is the most prevalent human chromosomal disorder, causing a range of cardiovascular, musculoskeletal, and neurological abnormalities. However, the cellular processes disrupted by T21 are poorly understood. Consistent with the clinical overlap between T21 and ciliopathies, we discovered that T21 disrupts cilia formation and signaling. Cilia defects arise from increased expression of Pericentrin, a centrosome scaffold and trafficking protein encoded on chromosome 21. Elevated Pericentrin is necessary and sufficient for T21 cilia defects. Pericentrin accumulates at centrosomes and dramatically in the cytoplasm surrounding centrosomes. Centrosome Pericentrin recruits more γ-tubulin and enhances microtubules, whereas cytoplasmic Pericentrin assembles into large foci that do not efficiently traffic. Moreover, the Pericentrin-associated cilia assembly factor IFT20 and the ciliary signaling molecule Smoothened do not efficiently traffic to centrosomes and cilia. Thus, increased centrosome protein dosage produces ciliopathy-like outcomes in T21 cells by decreasing trafficking between the cytoplasm, centrosomes, and cilia.

Development of novel 2,4-bispyridyl thiophene-based compounds as highly potent and selective Dyrk1A inhibitors. Part I: Benzamide and benzylamide derivatives

The protein kinase Dyrk1A modulates several processes relevant to the development or progression of Alzheimer's disease (AD), e. g. through phosphorylation of tau protein, amyloid precursor protein (APP) as well as proteins involved in the regulation of alternative splicing of tau pre-mRNA. Therefore, Dyrk1A has been proposed as a potential target for the treatment of AD. However, the co-inhibition of other closely related kinases of the same family of protein kinases (e.g. Dyrk1B and Dyrk2) or kinases from other families such as Clk1 limits the use of Dyrk1A inhibitors, as this may cause unpredictable side effects especially over long treatment periods. Herein, we describe the design and synthesis of a series of amide functionalized 2,4-bispyridyl thiophene compounds, of which the 4-fluorobenzyl amide derivative (31b) displayed the highest potency against Dyrk1A and remarkable selectivity over closely related kinases (IC50: Dyrk1A = 14.3 nM; Dyrk1B = 383 nM, Clk1 > 2 μM). This degree of selectivity over the frequently hit off-targets has rarely been achieved to date. Additionally, 31b inhibited Dyrk1A in intact cells with high efficacy (IC50 = 79 nM). Furthermore, 31b displayed a high metabolic stability in vitro with a half-life of 2 h. Altogether, the benzamide and benzylamide extension at the 2,4-bispyridyl thiophene core improved several key properties, giving access to compound suitable for future in vivo studies.

Treatment with corn oil improves neurogenesis and cognitive performance in the Ts65Dn mouse model of Down syndrome

Individuals with Down syndrome (DS), a genetic condition due to triplication of Chromosome 21, are characterized by intellectual disability that worsens with age. Since impairment of neurogenesis and dendritic maturation are very likely key determinants of intellectual disability in DS, interventions targeted to these defects may translate into a behavioral benefit. While most of the neurogenesis enhancers tested so far in DS mouse models may pose some caveats due to possible side effects, substances naturally present in the human diet may be regarded as therapeutic tools with a high translational impact. Linoleic acid and oleic acid are major constituents of corn oil that positively affect neurogenesis and neuron maturation. Based on these premises, the goal of the current study was to establish whether treatment with corn oil improves hippocampal neurogenesis and hippocampus-dependent memory in the Ts65Dn model of DS. Four-month-old Ts65Dn and euploid mice were treated with saline or corn oil for 30 days. Evaluation of behavior at the end of treatment showed that Ts65Dn mice treated with corn oil underwent a large improvement in hippocampus-dependent learning and memory. Evaluation of neurogenesis and dendritogenesis showed that in treated Ts65Dn mice the number of new granule cells of the hippocampal dentate gyrus and their dendritic pattern became similar to those of euploid mice. In addition, treated Ts65Dn mice underwent an increase in body and brain weight. This study shows for the first time that fatty acids have a positive impact on the brain of the Ts65Dn mouse model of DS. These results suggest that a diet that is rich in fatty acids may exert beneficial effects on cognitive performance in individuals with DS without causing adverse effects.

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.

RBM4 Modulates Radial Migration via Alternative Splicing of Dab1 during Cortex Development

The RNA-binding motif 4 (RBM4) protein participates in cell differentiation via its role in regulating the expression of tissue-specific or developmentally regulated mRNA splice isoforms. RBM4 is expressed in embryonic brain during development; it is initially enriched in the ventricular zone/subventricular zone and subsequently distributed throughout the cerebral cortex. Rbm4a knockout brain exhibited delayed migration of late-born neurons. Using in utero electroporation, we confirmed that knockdown of RBM4 impaired cortical neuronal migration. RNA immunoprecipitation with high-throughput sequencing identified Disabled-1 (Dab1), which encodes a critical reelin signaling adaptor, as a potential target of RBM4. Rbm4a knockout embryonic brain showed altered Dab1 isoform ratios. Overexpression of RBM4 promoted the inclusion of Dab1 exons 7 and 8 (7/8), whereas its antagonist polypyrimidine tract-binding protein 1 (PTBP1) acted in an opposite manner. RBM4 directly counteracted the effect of PTBP1 on exon 7/8 selection. Finally, we showed that the full-length Dab1, but not exon 7/8-truncated Dab1, rescued neuronal migration defects in RBM4-depleted neurons, indicating that RBM4 plays a role in neuronal migration via modulating the expression of Dab1 splice isoforms. Our findings imply that RBM4 is necessary during brain development and that its deficiency may lead to developmental brain abnormality.

Developmental excitatory-to-inhibitory GABA polarity switch is delayed in Ts65Dn mice, a genetic model of Down syndrome

Down syndrome (DS) is the most frequent genetic cause of developmental abnormalities leading to intellectual disability. One notable phenomenon affecting the formation of nascent neural circuits during late developmental periods is developmental switch of GABA action from depolarizing to hyperpolarizing mode. We examined properties of this switch in DS using primary cultures and acute hippocampal slices from Ts65Dn mice, a genetic model of DS. Cultures of DIV3-DIV13 Ts65Dn and control normosomic (2 N) neurons were loaded with FURA-2 AM, and GABA action was assessed using local applications. In 2 N cultures, the number of GABA-activated cells dropped from ~100% to 20% between postnatal days 3-13 (P3-P13) reflecting the switch in GABA action polarity. In Ts65Dn cultures, the timing of this switch was delayed by 2-3 days. Next, microelectrode recordings of multi-unit activity (MUA) were performed in CA3 slices during bath application of the GABA_A agonist isoguvacine. MUA frequency was increased in P8-P12 and reduced in P14-P22 slices reflecting the switch of GABA action from excitatory to inhibitory mode. The timing of this switch was delayed in Ts65Dn by approximately 2 days. Finally, frequency of giant depolarizing potentials (GDPs), a form of primordial neural activity, was significantly increased in slices from Ts65Dn pups at P12 and P14. These experimental evidences show that GABA action polarity switch is delayed in Ts65Dn model of DS, and that these changes lead to a delay in maturation of nascent neural circuits. These alterations may affect properties of neural circuits in adult animals and, therefore, represent a prospective target for pharmacotherapy of cognitive impairment in DS.

GABAergic over-inhibition, a promising hypothesis for cognitive deficits in Down syndrome

Down syndrome (DS), also known as trisomy 21, is the most common genetic cause of intellectual disability. It is also a model human disease for exploring consequences of gene dosage imbalance on complex phenotypes. Learning and memory impairments linked to intellectual disabilities in DS could result from synaptic plasticity deficits and excitatory-inhibitory alterations leading to changes in neuronal circuitry in the brain of affected individuals. Increasing number of studies in mouse and cellular models converge towards the assumption that excitatory-inhibitory imbalance occurs in DS, likely early during development. Thus increased inhibition appears to be a common trend that could explain synaptic and circuit disorganization. Interestingly using several potent pharmacological tools, preclinical studies strongly demonstrated that cognitive deficits could be restored in mouse models of DS. Clinical trials have not yet provided robust data for therapeutic application and additional studies are needed. Here we review the literature and our own published work emphasizing the over-inhibition hypothesis in DS and their links with gene dosage imbalance paving the way for future basic and clinical research.

Normalizing the gene dosage of Dyrk1A in a mouse model of Down syndrome rescues several Alzheimer's disease phenotypes

The intellectual disability that characterizes Down syndrome (DS) is primarily caused by prenatal changes in central nervous system growth and differentiation. However, in later life stages, the cognitive abilities of DS individuals progressively decline due to accelerated aging and the development of Alzheimer's disease (AD) neuropathology. The AD neuropathology in DS has been related to the overexpression of several genes encoded by Hsa21 including DYRK1A (dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1A), which encodes a protein kinase that performs crucial functions in the regulation of multiple signaling pathways that contribute to normal brain development and adult brain physiology. Studies performed in vitro and in vivo in animal models overexpressing this gene have demonstrated that the DYRK1A gene also plays a crucial role in several neurodegenerative processes found in DS. The Ts65Dn (TS) mouse bears a partial triplication of several Hsa21 orthologous genes, including Dyrk1A, and replicates many DS-like abnormalities, including age-dependent cognitive decline, cholinergic neuron degeneration, increased levels of APP and Aβ, and tau hyperphosphorylation. To use a more direct approach to evaluate the role of the gene dosage of Dyrk1A on the neurodegenerative profile of this model, TS mice were crossed with Dyrk1A KO mice to obtain mice with a triplication of a segment of Mmu16 that includes this gene, mice that are trisomic for the same genes but only carry two copies of Dyrk1A, euploid mice with a normal Dyrk1A dosage, and CO animals with a single copy of Dyrk1A. Normalizing the gene dosage of Dyrk1A in the TS mouse rescued the density of senescent cells in the cingulate cortex, hippocampus and septum, prevented cholinergic neuron degeneration, and reduced App expression in the hippocampus, Aβ load in the cortex and hippocampus, the expression of phosphorylated tau at the Ser202 residue in the hippocampus and cerebellum and the levels of total tau in the cortex, hippocampus and cerebellum. Thus, the present study provides further support for the role of the Dyrk1A gene in several AD-like phenotypes found in TS mice and indicates that this gene could be a therapeutic target to treat AD in DS.

Activity-Dependent Dysfunction in Visual and Olfactory Sensory Systems in Mouse Models of Down Syndrome

Activity-dependent synaptic plasticity plays a critical role in the refinement of circuitry during postnatal development and may be disrupted in conditions that cause intellectual disability, such as Down syndrome (DS). To test this hypothesis, visual cortical plasticity was assessed in Ts65Dn mice that harbor a chromosomal duplication syntenic to human chromosome 21q. We find that Ts65Dn mice demonstrate a defect in ocular dominance plasticity (ODP) following monocular deprivation. This phenotype is similar to that of transgenic mice that express amyloid precursor protein (APP), which is duplicated in DS and in Ts65DN mice; however, normalizing APP gene copy number in Ts65Dn mice fails to rescue plasticity. Ts1Rhr mice harbor a duplication of the telomeric third of the Ts65Dn-duplicated sequence and demonstrate the same ODP defect, suggesting a gene or genes sufficient to drive the phenotype are located in that smaller duplication. In addition, we find that Ts65Dn mice demonstrate an abnormality in olfactory system connectivity, a defect in the refinement of connections to second-order neurons in the olfactory bulb. Ts1Rhr mice do not demonstrate a defect in glomerular refinement, suggesting that distinct genes or sets of genes underlie visual and olfactory system phenotypes. Importantly, these data suggest that developmental plasticity and connectivity are impaired in sensory systems in DS model mice, that such defects may contribute to functional impairment in DS, and that these phenotypes, present in male and female mice, provide novel means for examining the genetic and molecular bases for neurodevelopmental impairment in model mice in vivoSIGNIFICANCE STATEMENT Our understanding of the basis for intellectual impairment in Down syndrome is hindered by the large number of genes duplicated in Trisomy 21 and a lack of understanding of the effect of disease pathology on the function of neural circuits in vivo This work describes early postnatal developmental abnormalities in visual and olfactory sensory systems in Down syndrome model mice, which provide insight into defects in the function of neural circuits in vivo and provide an approach for exploring the genetic and molecular basis for impairment in the disease. In addition, these findings raise the possibility that basic dysfunction in primary sensory circuitry may illustrate mechanisms important for global learning and cognitive impairment in Down syndrome patients.

The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein better known for its participation in the physiopathology of Alzheimer disease as the source of the beta amyloid fragment. However, the physiological functions of the full length protein and its proteolytic fragments have remained elusive. APP was first described as a cell-surface receptor; nevertheless, increasing evidence highlighted APP as a cell adhesion molecule. In this review, we will focus on the current knowledge of the physiological role of APP as a cell adhesion molecule and its involvement in key events of neuronal development, such as migration, neurite outgrowth, growth cone pathfinding, and synaptogenesis. Finally, since APP is over-expressed in Down syndrome individuals because of the extra copy of chromosome 21, in the last section of the review, we discuss the potential contribution of APP to the neuronal and synaptic defects described in this genetic condition. Read the Editorial Highlight for this article on page 9. Cover Image for this issue: doi. 10.1111/jnc.13817.

Pre- and post-natal melatonin administration partially regulates brain oxidative stress but does not improve cognitive or histological alterations in the Ts65Dn mouse model of Down syndrome

Melatonin administered during adulthood induces beneficial effects on cognition and neuroprotection in the Ts65Dn (TS) mouse model of Down syndrome. Here, we investigated the effects of pre- and post-natal melatonin treatment on behavioral and cognitive abnormalities and on several neuromorphological alterations (hypocellularity, neurogenesis impairment and increased oxidative stress) that appear during the early developmental stages in TS mice. Pregnant TS females were orally treated with melatonin or vehicle from the time of conception until the weaning of the offspring, and the pups continued to receive the treatment from weaning until the age of 5 months. Melatonin administered during the pre- and post-natal periods did not improve the cognitive impairment of TS mice as measured by the Morris Water maze or fear conditioning tests. Histological alterations, such as decreased proliferation (Ki67+ cells) and hippocampal hypocellularity (DAPI+ cells), which are typical in TS mice, were not prevented by melatonin. However, melatonin partially regulated brain oxidative stress by modulating the activity of the primary antioxidant enzymes (superoxide dismutase in the cortex and catalase in the cortex and hippocampus) and slightly decreasing the levels of lipid peroxidation in the hippocampus of TS mice. These results show the inability of melatonin to prevent cognitive impairment in TS mice when it is administered at pre- and post-natal stages. Additionally, our findings suggest that to induce pro-cognitive effects in TS mice during the early stages of development, in addition to attenuating oxidative stress, therapies should aim to improve other altered processes, such as hippocampal neurogenesis and/or hypocellularity.

Decreasing the Expression of GABAA α5 Subunit-Containing Receptors Partially Improves Cognitive, Electrophysiological, and Morphological Hippocampal Defects in the Ts65Dn Model of Down Syndrome

Trisomy 21 or Down syndrome (DS) is the most common cause of intellectual disability of a genetic origin. The Ts65Dn (TS) mouse, which is the most commonly used and best-characterized mouse model of DS, displays many of the cognitive, neuromorphological, and biochemical anomalies that are found in the human condition. One of the mechanisms that have been proposed to be responsible for the cognitive deficits in this mouse model is impaired GABA-mediated inhibition. Because of the well-known modulatory role of GABA_A α5 subunit-containing receptors in cognitive processes, these receptors are considered to be potential targets for improving the intellectual disability in DS. The chronic administration of GABA_A α5-negative allosteric modulators has been shown to be procognitive without anxiogenic or proconvulsant side effects. In the present study, we use a genetic approach to evaluate the contribution of GABA_A α5 subunit-containing receptors to the cognitive, electrophysiological, and neuromorphological deficits in TS mice. We show that reducing the expression of GABA_A α5 receptors by deleting one or two copies of the Gabra5 gene in TS mice partially ameliorated the cognitive impairments, improved long-term potentiation, enhanced neural differentiation and maturation, and normalized the density of the GABAergic synapse markers. Reducing the gene dosage of Gabra5 in TS mice did not induce motor alterations and anxiety or affect the viability of the mice. Our results provide further evidence of the role of GABA_A α5 receptor-mediated inhibition in cognitive impairment in the TS mouse model of DS.

Early neurotrophic pharmacotherapy rescues developmental delay and Alzheimer's-like memory deficits in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS), caused by trisomy 21, is the most common genetic cause of intellectual disability and is associated with a greatly increased risk of early-onset Alzheimer’s disease (AD). The Ts65Dn mouse model of DS exhibits several key features of the disease including developmental delay and AD-like cognitive impairment. Accumulating evidence suggests that impairments in early brain development caused by trisomy 21 contribute significantly to memory deficits in adult life in DS. Prenatal genetic testing to diagnose DS in utero, provides the novel opportunity to initiate early pharmacological treatment to target this critical period of brain development. Here, we report that prenatal to early postnatal treatment with a ciliary neurotrophic factor (CNTF) small-molecule peptide mimetic, Peptide 021 (P021), rescued developmental delay in pups and AD-like hippocampus-dependent memory impairments in adult life in Ts65Dn mice. Furthermore, this treatment prevented pre-synaptic protein deficit, decreased glycogen synthase kinase-3beta (GSK3β) activity, and increased levels of synaptic plasticity markers including brain derived neurotrophic factor (BNDF) and phosphorylated CREB, both in young (3-week-old) and adult (~ 7-month-old) Ts65Dn mice. These findings provide novel evidence that providing neurotrophic support during early brain development can prevent developmental delay and AD-like memory impairments in a DS mouse model.

Perinatal Choline Supplementation Reduces Amyloidosis and Increases Choline Acetyltransferase Expression in the Hippocampus of the APPswePS1dE9 Alzheimer's Disease Model Mice

Prevention of Alzheimer's disease (AD) is a major goal of biomedical sciences. In previous studies we showed that high intake of the essential nutrient, choline, during gestation prevented age-related memory decline in a rat model. In this study we investigated the effects of a similar treatment on AD-related phenotypes in a mouse model of AD. We crossed wild type (WT) female mice with hemizygous APPswe/PS1dE9 (APP.PS1) AD model male mice and maintained the pregnant and lactating dams on a control AIN76A diet containing 1.1 g/kg of choline or a choline-supplemented (5 g/kg) diet. After weaning all offspring consumed the control diet. As compared to APP.PS1 mice reared on the control diet, the hippocampus of the perinatally choline-supplemented APP.PS1 mice exhibited: 1) altered levels of amyloid precursor protein (APP) metabolites-specifically elevated amounts of β-C-terminal fragment (β-CTF) and reduced levels of solubilized amyloid Aβ40 and Aβ42 peptides; 2) reduced number and total area of amyloid plaques; 3) preserved levels of choline acetyltransferase protein (CHAT) and insulin-like growth factor II (IGF2) and 4) absence of astrogliosis. The data suggest that dietary supplementation of choline during fetal development and early postnatal life may constitute a preventive strategy for AD.

Correlation between receptor-interacting protein 140 expression and directed differentiation of human embryonic stem cells into neural stem cells

Overexpression of receptor-interacting protein 140 (RIP140) promotes neuronal differentiation of N2a cells via extracellular regulated kinase 1/2 (ERK1/2) signaling. However, involvement of RIP140 in human neural differentiation remains unclear. We found both RIP140 and ERK1/2 expression increased during neural differentiation of H1 human embryonic stem cells. Moreover, RIP140 negatively correlated with stem cell markers Oct4 and Sox2 during early stages of neural differentiation, and positively correlated with the neural stem cell marker Nestin during later stages. Thus, ERK1/2 signaling may provide the molecular mechanism by which RIP140 takes part in neural differentiation to eventually affect the number of neurons produced.

Changing Paradigms in Down Syndrome: The First International Conference of the Trisomy 21 Research Society

Down syndrome (DS) is the most common genetic cause of intellectual disability (ID) in humans with an incidence of ∼1:1,000 live births worldwide. It is caused by the presence of an extra copy of all or a segment of the long arm of human chromosome 21 (trisomy 21). People with DS present with a constellation of phenotypic alterations involving most organs and organ systems. ID is present in all people with DS, albeit with variable severity. DS is also the most frequent genetic cause of Alzheimer's disease (AD), and ∼50% of those with DS will develop AD-related dementia. In the last few years, significant progress has been made in understanding the crucial genotype-phenotype relationships in DS, in identifying the alterations in molecular pathways leading to the various clinical conditions present in DS, and in preclinical evaluations of potential therapies to improve the overall health and well-being of individuals with DS. In June 2015, 230 scientists, advocates, patients, and family members met in Paris for the 1st International Conference of the Trisomy 21 Research Society. Here, we report some of the most relevant presentations that took place during the meeting.

An Integrated Human/Murine Transcriptome and Pathway Approach To Identify Prenatal Treatments For Down Syndrome

Anatomical and functional brain abnormalities begin during fetal life in Down syndrome (DS). We hypothesize that novel prenatal treatments can be identified by targeting signaling pathways that are consistently perturbed in cell types/tissues obtained from human fetuses with DS and mouse embryos. We analyzed transcriptome data from fetuses with trisomy 21, age and sex-matched euploid controls, and embryonic day 15.5 forebrains from Ts1Cje, Ts65Dn, and Dp16 mice. The new datasets were compared to other publicly available datasets from humans with DS. We used the human Connectivity Map (CMap) database and created a murine adaptation to identify FDA-approved drugs that can rescue affected pathways. USP16 and TTC3 were dysregulated in all affected human cells and two mouse models. DS-associated pathway abnormalities were either the result of gene dosage specific effects or the consequence of a global cell stress response with activation of compensatory mechanisms. CMap analyses identified 56 molecules with high predictive scores to rescue abnormal gene expression in both species. Our novel integrated human/murine systems biology approach identified commonly dysregulated genes and pathways. This can help to prioritize therapeutic molecules on which to further test safety and efficacy. Additional studies in human cells are ongoing prior to pre-clinical prenatal treatment in mice.

Specific age-related molecular alterations in the cerebellum of Down syndrome mouse models

Down syndrome, or trisomy 21, has been modeled with various trisomic and transgenic mice to help understand the consequences of an altered gene dosage in brain development and function. Though Down syndrome has been associated with premature aging, little is known about the molecular and cellular alterations that target brain function. To help identify alterations at specific ages, we analyzed the cerebellum of Ts1Cje mice, trisomic for 77 HSA21 orthologs, at three ages-young (4 months), middle-age (12 months), and old (17 months)-compared to age-matched controls. Quantification of neuronal and glial markers (n=11) revealed increases in GFAP, with an age effect, and S100B, with age and genotype effects. The genotype effect on S100B with age was unexpected as Ts1Cje has only two copies of the S100b gene. Interestingly, the different increase in GFAP observed between Ts1Cje (trisomic segment includes Pcp4 gene) and controls was magnified in TgPCP4 mice (1 extra copy of the human PCP4 gene) at the same age. S100B increase was not found in the TgPCP4 confirming a difference of regulation with aging for GFAP and S100B and excluding the calcium signaling regulator, Pcp4, as a potential candidate for increase of S100B in the Ts1Cje. To understand these differences, comparison of GFAP and S100B immunostainings at young and middle-age were performed. Immunohistochemical detection of differences in GFAP and S100B localization with aging implicate S100B+ oligodendrocytes as a new phenotypic target in this specific aging process.

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.

The HSA21 gene EURL/C21ORF91 controls neurogenesis within the cerebral cortex and is implicated in the pathogenesis of Down Syndrome

Copy number variations to chromosome 21 (HSA21) cause intellectual disability and Down Syndrome, but our understanding of the HSA21 genetic factors which contribute to fetal brain development remains incomplete. Here, we focussed on the neurodevelopmental functions for EURL (also known as C21ORF91, Refseq Gene ID:54149), a protein-coding gene at the centromeric boundary of the Down Syndrome Critical Region (DSCR) of HSA21. We report that EURL is expressed during human and mouse cerebral cortex development, and we report that alterations to EURL mRNA levels within the human brain underlie Down Syndrome. Our gene perturbation studies in mice demonstrate that disruptions to Eurl impair progenitor proliferation and neuronal differentiation. Also, we find that disruptions to Eurl impair the long-term positioning and dendritic spine densities of cortical projection neurons. We provide evidence that EURL interacts with the coiled-coil domain-containing protein CCDC85B so as to modulate β-catenin levels in cells. Further, we utilised a fluorescent reporter (8xTOPFLASHd2EGFP) to demonstrate that disruptions to Eurl alter β-catenin signalling in vitro as well as in vivo. Together, these studies highlight EURL as an important new player in neuronal development that is likely to impact on the neuropathogenesis of HSA21-related disorders including Down Syndrome.

Down syndrome: An overview

Optometrists as primary eye care providers examine patients from diverse populations, including those with special needs such as Down syndrome. Down syndrome is a chromosomal abnormality associated with several health conditions including vision anomalies such as refractive, accommodative and vergence anomalies, as well as ocular pathology. In this article, a narrative review of Down syndrome including the background, historical perspective, aetiology and genetic mechanisms, types, epidemiology, as well as the physical and medical profile of Down syndrome is presented.Keywords: Down syndrome review; Trisomy 21; historical perspective; etiology; types and epidemiology; features; Optometrist

Down Syndrome Developmental Brain Transcriptome Reveals Defective Oligodendrocyte Differentiation and Myelination

Trisomy 21, or Down syndrome (DS), is the most common genetic cause of developmental delay and intellectual disability. To gain insight into the underlying molecular and cellular pathogenesis, we conducted a multi-region transcriptome analysis of DS and euploid control brains spanning from mid-fetal development to adulthood. We found genome-wide alterations in the expression of a large number of genes, many of which exhibited temporal and spatial specificity and were associated with distinct biological processes. In particular, we uncovered co-dysregulation of genes associated with oligodendrocyte differentiation and myelination that were validated via cross-species comparison to Ts65Dn trisomy mice. Furthermore, we show that hypomyelination present in Ts65Dn mice is in part due to cell-autonomous effects of trisomy on oligodendrocyte differentiation and results in slower neocortical action potential transmission. Together, these results identify defects in white matter development and function in DS, and they provide a transcriptional framework for further investigating DS neuropathogenesis.

Altered intrinsic and network properties of neocortical neurons in the Ts65Dn mouse model of Down syndrome

All individuals with Down syndrome (DS) have a varying but significant degree of cognitive disability. Although hippocampal deficits clearly play an important role, behavioral studies also suggest that deficits within the neocortex contribute to somatosensory deficits and impaired cognition in DS. Using thalamocortical slices from the Ts65Dn mouse model of DS, we investigated the intrinsic and network properties of regular spiking neurons within layer 4 of the somatosensory cortex. In these neurons, the membrane capacitance was increased and specific membrane resistance decreased in slices from Ts65Dn mice. Examination of combined active and passive membrane properties suggests that trisomic layer 4 neurons are less excitable than those from euploid mice. The frequencies of excitatory and inhibitory spontaneous synaptic activities were also reduced in Ts65Dn neurons. With respect to network activity, spontaneous network oscillations (Up states) were shorter and less numerous in the neocortex from Ts65Dn mice when compared to euploid. Up states evoked by electrical stimulation of the ventrobasal nucleus (VBN) of the thalamus were similarly affected in Ts65Dn mice. Additionally, monosynaptic EPSCs and polysynaptic IPSCs evoked by VBN stimulation were significantly delayed in layer 4 regular spiking neurons from Ts65Dn mice. These results indicate that, in the Ts65Dn model of DS, the overall electrophysiological properties of neocortical neurons are altered leading to aberrant network activity within the neocortex. Similar changes in DS individuals may contribute to sensory and cognitive dysfunction and therefore may implicate new targets for cognitive therapies in this developmental disorder.

RCAN1 links impaired neurotrophin trafficking to aberrant development of the sympathetic nervous system in Down syndrome

Down syndrome is the most common chromosomal disorder affecting the nervous system in humans. To date, investigations of neural anomalies in Down syndrome have focused on the central nervous system, although dysfunction of the peripheral nervous system is a common manifestation. The molecular and cellular bases underlying peripheral abnormalities have remained undefined. Here, we report the developmental loss of sympathetic innervation in human Down syndrome organs and in a mouse model. We show that excess regulator of calcineurin 1 (RCAN1), an endogenous inhibitor of the calcineurin phosphatase that is triplicated in Down syndrome, impairs neurotrophic support of sympathetic neurons by inhibiting endocytosis of the nerve growth factor (NGF) receptor, TrkA. Genetically correcting RCAN1 levels in Down syndrome mice markedly improves NGF-dependent receptor trafficking, neuronal survival and innervation. These results uncover a critical link between calcineurin signalling, impaired neurotrophin trafficking and neurodevelopmental deficits in the peripheral nervous system in Down syndrome.

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.

Dissecting Alzheimer disease in Down syndrome using mouse models

Down syndrome (DS) is a common genetic condition caused by the presence of three copies of chromosome 21 (trisomy 21). This greatly increases the risk of Alzheimer disease (AD), but although virtually all people with DS have AD neuropathology by 40 years of age, not all develop dementia. To dissect the genetic contribution of trisomy 21 to DS phenotypes including those relevant to AD, a range of DS mouse models has been generated which are trisomic for chromosome segments syntenic to human chromosome 21. Here, we consider key characteristics of human AD in DS (AD-DS), and our current state of knowledge on related phenotypes in AD and DS mouse models. We go on to review important features needed in future models of AD-DS, to understand this type of dementia and so highlight pathogenic mechanisms relevant to all populations at risk of AD.

Identification of a novel actin-dependent signal transducing module allows for the targeted degradation of GLI1

The Down syndrome-associated DYRK1A kinase has been reported as a stimulator of the developmentally important Hedgehog (Hh) pathway, but cells from Down syndrome patients paradoxically display reduced Hh signalling activity. Here we find that DYRK1A stimulates GLI transcription factor activity through phosphorylation of general nuclear localization clusters. In contrast, in vivo and in vitro experiments reveal that DYRK1A kinase can also function as an inhibitor of endogenous Hh signalling by negatively regulating ABLIM proteins, the actin cytoskeleton and the transcriptional co-activator MKL1 (MAL). As a final effector of the DYRK1A-ABLIM-actin-MKL1 sequence, we identify the MKL1 interactor Jumonji domain demethylase 1A (JMJD1A) as a novel Hh pathway component stabilizing the GLI1 protein in a demethylase-independent manner. Furthermore, a Jumonji-specific small-molecule antagonist represents a novel and powerful inhibitor of Hh signal transduction by inducing GLI1 protein degradation in vitro and in vivo.

Receptor-interacting protein 140 overexpression promotes neuro-2a neuronal differentiation by ERK1/2 signaling

Background:

Abnormal neuronal differentiation plays an important role in central nervous system (CNS) development abnormalities such as Down syndrome (DS), a disorder that results directly from overexpression of genes in trisomic cells. Receptor-interacting protein 140 (RIP140) is significantly upregulated in DS brains, suggesting its involvement in DS CNS development abnormalities. However, the role of RIP140 in neuronal differentiation is still not clear. The current study aimed to investigate the effect of RIP140 overexpression on the differentiation of neuro-2a (N2a) neuroblastoma cells, in vitro.

Methods:

Stably RIP140-overexpressing N2a (N2a-RIP140) cells were used as a neurodevelopmental model, and were constructed by lipofection and overexpression validated by real-time polymerase chain reaction and Western blot. Retinoic acid (RA) was used to stimulate N2a differentiation. Combining the expression of Tuj1 at the mRNA and protein levels, the percentage of cells baring neurites, and the number of neurites per cell body was semi-quantified to determine the effect of RIP140 on differentiation of N2a cells. Furthermore, western blot and the ERK1/2 inhibitor U0126 were used to identify the specific signaling pathway by which RIP140 induces differentiation of N2a cells. Statistical significance of the differences between groups was determined by one-way analysis of variance followed by the Dunnett test.

Results:

Compared to untransfected N2a cells RIPl40 expression in N2a-RIP140 cells was remarkably upregulated at both the mRNA and protein levels. N2a-RIP140 cells had a significantly increased percentage of cells baring neurites, and numbers of neurites per cell, as compared to N2a cells, in the absence and presence of RA (P < 0.05). In addition, Tuj1, a neuronal biomarker, was strongly upregulated in N2a-RIP140 cells (P < 0.05) and phosphorylated ERK1/2 (p-ERK1/2) levels in N2a-RIP140 cells were dramatically increased, while differentiation was inhibited by the ERK1/2-specific inhibitor U0126.

Conclusions:

RIP140 overexpression promotes N2a cell neuronal differentiation by activating the ERK1/2 pathway.

A quantitative transcriptome reference map of the normal human hippocampus

We performed an innovative systematic meta-analysis of 41 gene expression profiles of normal human hippocampus to provide a quantitative transcriptome reference map of it, i.e. a reference typical value of expression for each of the 30,739 known mapped and the 16,258 uncharacterized (unmapped) transcripts. For this aim, we used the software called TRAM (Transcriptome Mapper), which is able to generate transcriptome maps based on gene expression data from multiple sources. We also analyzed differential expression by comparing the hippocampus with the whole brain transcriptome map to identify a typical expression pattern of this subregion compared with the whole organ. Finally, due to the fact that the hippocampus is one of the main brain region to be severely affected in trisomy 21 (the best known genetic cause of intellectual disability), a particular attention was paid to the expression of chromosome 21 (chr21) genes. Data were downloaded from microarray databases, processed, and analyzed using TRAM software. Among the main findings, the most over-expressed loci in the hippocampus are the expressed sequence tag cluster Hs.732685 and the member of the calmodulin gene family CALM2. The tubulin folding cofactor B (TBCB) gene is the best gene at behaving like a housekeeping gene. The hippocampus vs. the whole brain differential transcriptome map shows the over-expression of LINC00114, a long non-coding RNA mapped on chr21. The hippocampus transcriptome map was validated in vitro by assaying gene expression through several magnitude orders by "Real-Time" reverse transcription polymerase chain reaction (RT-PCR). The highly significant agreement between in silico and experimental data suggested that our transcriptome map may be a useful quantitative reference benchmark for gene expression studies related to human hippocampus. Furthermore, our analysis yielded biological insights about those genes that have an intrinsic over-/under-expression in the hippocampus.

DSCAM promotes refinement in the mouse retina through cell death and restriction of exploring dendrites

In this study we develop and use a gain-of-function mouse allele of the Down syndrome cell adhesion molecule (Dscam) to complement loss-of-function models. We assay the role of Dscam in promoting cell death, spacing, and laminar targeting of neurons in the developing mouse retina. We find that ectopic or overexpression of Dscam is sufficient to drive cell death. Gain-of-function studies indicate that Dscam is not sufficient to increase spatial organization, prevent cell-to-cell pairing, or promote active avoidance in the mouse retina, despite the similarity of the Dscam loss-of-function phenotype in the mouse retina to phenotypes observed in Drosophila Dscam1 mutants. Both gain- and loss-of-function studies support a role for Dscam in targeting neurites; DSCAM is necessary for precise dendrite lamination, and is sufficient to retarget neurites of outer retinal cells after ectopic expression. We further demonstrate that DSCAM guides dendrite targeting in type 2 dopaminergic amacrine cells, by restricting the stratum in which exploring retinal dendrites stabilize, in a Dscam dosage-dependent manner. Based on these results we propose a single model to account for the numerous Dscam gain- and loss-of-function phenotypes reported in the mouse retina whereby DSCAM eliminates inappropriately placed cells and connections.

Abnormal mineralization of the Ts65Dn Down syndrome mouse appendicular skeleton begins during embryonic development in a Dyrk1a-independent manner

The relationship between gene dosage imbalance and phenotypes associated with Trisomy 21, including the etiology of abnormal bone phenotypes linked to Down syndrome (DS), is not well understood. The Ts65Dn mouse model for DS exhibits appendicular skeletal defects during adolescence and adulthood but the developmental and genetic origin of these phenotypes remains unclear. It is hypothesized that the postnatal Ts65Dn skeletal phenotype originates during embryonic development and results from an increased Dyrk1a gene copy number, a gene hypothesized to play a critical role in many DS phenotypes. Ts65Dn embryos exhibit a lower percent bone volume in the E17.5 femur when compared to euploid embryos. Concomitant with gene copy number, qPCR analysis revealed a  ~1.5 fold increase in Dyrk1a transcript levels in the Ts65Dn E17.5 embryonic femur as compared to euploid. Returning Dyrk1a copy number to euploid levels in Ts65Dn, Dyrk1a(+/-) embryos did not correct the trisomic skeletal phenotype but did return Dyrk1a gene transcript levels to normal. The size and protein expression patterns of the cartilage template during embryonic bone development appear to be unaffected at E14.5 and E17.5 in trisomic embryos. Taken together, these data suggest that the dosage imbalance of genes other than Dyrk1a is involved in the development of the prenatal bone phenotype in Ts65Dn embryos.

Modeling a model: Mouse genetics, 22q11.2 Deletion Syndrome, and disorders of cortical circuit development

Understanding the developmental etiology of autistic spectrum disorders, attention deficit/hyperactivity disorder and schizophrenia remains a major challenge for establishing new diagnostic and therapeutic approaches to these common, difficult-to-treat diseases that compromise neural circuits in the cerebral cortex. One aspect of this challenge is the breadth and overlap of ASD, ADHD, and SCZ deficits; another is the complexity of mutations associated with each, and a third is the difficulty of analyzing disrupted development in at-risk or affected human fetuses. The identification of distinct genetic syndromes that include behavioral deficits similar to those in ASD, ADHC and SCZ provides a critical starting point for meeting this challenge. We summarize clinical and behavioral impairments in children and adults with one such genetic syndrome, the 22q11.2 Deletion Syndrome, routinely called 22q11DS, caused by micro-deletions of between 1.5 and 3.0 MB on human chromosome 22. Among many syndromic features, including cardiovascular and craniofacial anomalies, 22q11DS patients have a high incidence of brain structural, functional, and behavioral deficits that reflect cerebral cortical dysfunction and fall within the spectrum that defines ASD, ADHD, and SCZ. We show that developmental pathogenesis underlying this apparent genetic "model" syndrome in patients can be defined and analyzed mechanistically using genomically accurate mouse models of the deletion that causes 22q11DS. We conclude that "modeling a model", in this case 22q11DS as a model for idiopathic ASD, ADHD and SCZ, as well as other behavioral disorders like anxiety frequently seen in 22q11DS patients, in genetically engineered mice provides a foundation for understanding the causes and improving diagnosis and therapy for these disorders of cortical circuit development.

Disruption of neurogenesis and cortical development in transgenic mice misexpressing Olig2, a gene in the Down syndrome critical region

The basic helix-loop-helix (bHLH) transcription factor Olig2 is crucial for mammalian central nervous system development. Human ortholog OLIG2 is located in the Down syndrome critical region in trisomy 21. To investigate the effect of Olig2 misexpression on brain development, we generated a developmentally regulated Olig2-overexpressing transgenic line with a Cre/loxP system. The transgenic mice with Olig2 misexpression in cortical neural stem/progenitor cells exhibited microcephaly, cortical dyslamination, hippocampus malformation, and profound motor deficits. Ectopic misexpression of Olig2 impaired cortical progenitor proliferation and caused precocious cell cycle exit. Massive neuronal cell death was detected in the developing cortex of Olig2-misexpressing mice. In addition, Olig2 misexpression led to a significant downregulation of neuronal specification factors including Ngn1, Ngn2 and Pax6, and a defect in cortical neurogenesis. Chromatin-immunoprecipitation and sequencing (ChIP-Seq) analysis indicates that Olig2 directly targets the promoter and/or enhancer regions of Nfatc4, Dscr1/Rcan1 and Dyrk1a, the critical neurogenic genes that contribute to Down syndrome phenotypes, and inhibits their expression. Together, our study suggests that Olig2 misexpression in neural stem cells elicits neurogenesis defects and neuronal cell death, which may contribute to developmental disorders including Down syndrome, where OLIG2 is triplicated on chromosomal 21.