Asynchronous excitatory neuron development in an isogenic cortical spheroid model of Down syndrome.

The intellectual disability (ID) in Down syndrome (DS) is thought to result from a variety of developmental deficits such as alterations in neural progenitor division, neurogenesis, gliogenesis, cortical architecture, and reduced cortical volume. However, the molecular processes underlying these neurodevelopmental changes are still elusive, preventing an understanding of the mechanistic basis of ID in DS. In this study, we used a pair of isogenic (trisomic and euploid) induced pluripotent stem cell (iPSC) lines to generate cortical spheroids (CS) that model the impact of trisomy 21 on brain development. Cortical spheroids contain neurons, astrocytes, and oligodendrocytes and they are widely used to approximate early neurodevelopment. Using single cell RNA sequencing (scRNA-seq), we uncovered cell type-specific transcriptomic changes in the trisomic CS. In particular, we found that excitatory neuron populations were most affected and that a specific population of cells with a transcriptomic profile resembling layer IV cortical neurons displayed the most profound divergence in developmental trajectory between trisomic and euploid genotypes. We also identified candidate genes potentially driving the developmental asynchrony between trisomic and euploid excitatory neurons. Direct comparison between the current isogenic CS scRNA-seq data and previously published datasets revealed several recurring differentially expressed genes between DS and control samples. Altogether, our study highlights the power and importance of cell type-specific analyses within a defined genetic background, coupled with broader examination of mixed samples, to comprehensively evaluate cellular phenotypes in the context of DS.

Neurodevelopment in Down syndrome: Concordance in humans and models.

Great strides have been made over the past 30 years in understanding the neurodevelopmental changes underlying the intellectual disability (ID) in Down syndrome (DS). Detailed studies of human tissue coupled with findings from rodent and induced pluripotent stem cells (iPSCs) model systems have uncovered the changes in neurogenesis, synaptic connectivity, and myelination that drive the anatomical and physiological changes resulting in the disability. However, there remain significant conflicting data between human studies and the models. To fully understand the development of ID in DS, these inconsistencies need to be reconciled. Here, we review the well documented neurodevelopmental phenotypes found in individuals with DS and examine the degree to which widely used models recapitulate these phenotypes. Resolving these areas of discord will further research on the molecular underpinnings and identify potential treatments to improve the independence and quality of life of people with DS.

Sirt2 promotes white matter oligodendrogenesis during development and in models of neonatal hypoxia.

Delayed oligodendrocyte (OL) maturation caused by hypoxia (Hx)-induced neonatal brain injury results in hypomyelination and leads to neurological disabilities. Previously, we characterized Sirt1 as a crucial regulator of OL progenitor cell (OPC) proliferation in response to Hx. We now identify Sirt2 as a critical promoter of OL differentiation during both normal white matter development and in a mouse model of Hx. Importantly, we find that Hx reduces Sirt2 expression in mature OLs and that Sirt2 overexpression in OPCs restores mature OL populations. Reduced numbers of Sirt2+ OLs were also observed in the white matter of preterm human infants. We show that Sirt2 interacts with p27Kip1/FoxO1, p21Cip1/Cdk4, and Cdk5 pathways, and that these interactions are altered by Hx. Furthermore, Hx induces nuclear translocation of Sirt2 in OPCs where it binds several genomic targets. Overall, these results indicate that a balance of Sirt1 and Sirt2 activity is required for developmental oligodendrogenesis, and that these proteins represent potential targets for promoting repair following white matter injury.

Building the Future Therapies for Down Syndrome: The Third International Conference of the T21 Research Society.

Research focused on Down syndrome has increased in the last several years to advance understanding of the consequences of trisomy 21 (T21) on molecular and cellular processes and, ultimately, on individuals with Down syndrome. The Trisomy 21 Research Society (T21RS) is the premier scientific organization for researchers and clinicians studying Down syndrome. The Third International Conference of T21RS, held June 6-9, 2019, in Barcelona, Spain, brought together 429 scientists, families, and industry representatives to share the latest discoveries on underlying cellular and molecular mechanisms of T21, define cognitive and behavioral challenges and better understand comorbidities associated with Down syndrome, including Alzheimer's disease and leukemia. Presentation of cutting-edge results in neuroscience, neurology, model systems, psychology, cancer, biomarkers and molecular and phar-ma-cological therapeutic approaches demonstrate the compelling interest and continuing advancement in all aspects of understanding and ameliorating conditions associated with T21.

Forebrain Neural Precursor Cells Are Differentially Vulnerable to Zika Virus Infection.

Prenatal exposure to Zika virus (ZIKV) can result in microencephaly and congenital Zika syndrome, although some brain cells and structures are spared by the virus for unknown reasons. Here, a novel murine model of fetal ZIKV infection incorporating intraventricular infection and cell type-specific in utero electroporation (IUE) was used to identify the time course of ZIKV infection and to determine the identity of cells that are initially infected or spared during neocortical neurogenesis. In vivo time course studies revealed the presence of ZIKV in apical radial glial cells (aRGCs) at early time points following virus exposure, while basal intermediate progenitor cells (bIPCs) became maximally (ZIKV+) after 3 d of virus exposure. ZIKV-infected fetal brains exhibited microencephaly as early as 1 d following infection, regardless of developmental age. This change in brain size was caused in part by apoptosis and reduced proliferation that persisted until birth. While 60% of aRGC basal fibers were perturbed during infection, 40% retained normal morphology, indicating that aRGCs are not uniformly vulnerable to ZIKV infection. To investigate this heterogeneous vulnerability, we performed genetic fate mapping using cell type-specific probes derived from a mouse embryonic day (E)15.5 neocortical wall single-cell RNA sequencing (scRNAseq) dataset. The results indicate that one class of aRGCs preferentially express the putative ZIKV entry receptor AXL and that these cells are more vulnerable to ZIKV infection than other aRGC subtypes with low AXL expression. Together, these data uncover crucial temporal and cellular details of ZIKV fetal brain infection for prevention strategies and for management of congenital Zika syndrome.

Sonic Hedgehog Pathway Modulation Normalizes Expression of Olig2 in Rostrally Patterned NPCs With Trisomy 21.

The intellectual disability found in people with Down syndrome is associated with numerous changes in early brain development, including the proliferation and differentiation of neural progenitor cells (NPCs) and the formation and maintenance of myelin in the brain. To study how early neural precursors are affected by trisomy 21, we differentiated two isogenic lines of induced pluripotent stem cells derived from people with Down syndrome into brain-like and spinal cord-like NPCs and promoted a transition towards oligodendroglial fate by activating the Sonic hedgehog (SHH) pathway. In the spinal cord-like trisomic cells, we found no difference in expression of OLIG2 or NKX2.2, two transcription factors essential for commitment to the oligodendrocyte lineage. However, in the brain-like trisomic NPCs, OLIG2 is significantly upregulated and is associated with reduced expression of NKX2.2. We found that this gene dysregulation and block in NPC transition can be normalized by increasing the concentration of a SHH pathway agonist (SAG) during differentiation. These results underscore the importance of regional and cell type differences in gene expression in Down syndrome and demonstrate that modulation of SHH signaling in trisomic cells can rescue an early perturbed step in neural lineage specification.

Mitigating Cognitive Deficits in Down Syndrome by Managing Microglia Activation.

In this issue of Neuron, the comprehensive and multidisciplinary results from Pinto et al. (2020) point to novel and widespread microglial dysfunction in Down syndrome that results in impaired cognitive ability. A common pharmaceutical (acetaminophen) is shown to offset these effects in mouse models.

Transcriptional priming as a conserved mechanism of lineage diversification in the developing mouse and human neocortex.

How the rich variety of neurons in the nervous system arises from neural stem cells is not well understood. Using single-cell RNA-sequencing and in vivo confirmation, we uncover previously unrecognized neural stem and progenitor cell diversity within the fetal mouse and human neocortex, including multiple types of radial glia and intermediate progenitors. We also observed that transcriptional priming underlies the diversification of a subset of ventricular radial glial cells in both species; genetic fate mapping confirms that the primed radial glial cells generate specific types of basal progenitors and neurons. The different precursor lineages therefore diversify streams of cell production in the developing murine and human neocortex. These data show that transcriptional priming is likely a conserved mechanism of mammalian neural precursor lineage specialization.

Longitudinal neuroanatomical and behavioral analyses show phenotypic drift and variability in the Ts65Dn mouse model of Down syndrome.

Mouse models of Down syndrome (DS) have been invaluable tools for advancing knowledge of the underlying mechanisms of intellectual disability in people with DS. The Ts(1716)65Dn (Ts65Dn) mouse is one of the most commonly used models as it recapitulates many of the phenotypes seen in individuals with DS, including neuroanatomical changes and impaired learning and memory. In this study, we use rigorous metrics to evaluate multiple cohorts of Ts65Dn ranging from 2014 to the present, including a stock of animals recovered from embryos frozen within ten generations after the colony was first created in 2010. Through quantification of prenatal and postnatal brain development and several behavioral tasks, our results provide a comprehensive comparison of Ts65Dn across time and show a significant amount of variability both across cohorts as well as within cohorts. The inconsistent phenotypes in Ts65Dn mice highlight specific cautions and caveats for use of this model. We outline important steps for ensuring responsible use of Ts65Dn in future research.This article has an associated First Person interview with the first author of the paper.

Lessons from single cell sequencing in CNS cell specification and function.

Modern RNA sequencing methods have greatly increased our understanding of the molecular fingerprint of neurons, astrocytes and oligodendrocytes throughout the central nervous system (CNS). Technical approaches with greater sensitivity and throughput have uncovered new connections between gene expression, cell biology, and ultimately CNS function. In recent years, single cell RNA-sequencing (scRNA-seq) has made a large impact on the neurosciences by enhancing the resolution of types of cells that make up the CNS and shedding light on their developmental trajectories and how their diversity is modified across species. Here we will review the advantages, innovations, and challenges of the single cell genomics era and highlight how it has impacted our understanding of neurodevelopment and neurological function.

Neurogenesis, Myelination, and Circuitry: The Case for a Distributed Therapeutic Regimen in Down Syndrome.

One of the overriding hopes of the Down syndrome (DS) research community is to arrive at a better understanding of how trisomy 21 affects brain development and function, and that doing so will improve quality of life and independence for people with DS. In searching for the underlying causes of intellectual disability in DS, researchers and clinicians have studied how changes to genes and cells may relate to motor and cognitive function. Thus far, alterations in many areas of the central nervous system have been found and it is now known that, beginning before birth, different changes occur in different areas over the course of life. Because of these spatial and temporal variations, multiple approaches for addressing motor and cognitive function must be considered.

Spatiotemporal development of spinal neuronal and glial populations in the Ts65Dn mouse model of Down syndrome.

BACKGROUND: Down syndrome (DS), caused by the triplication of chromosome 21, results in a constellation of clinical features including changes in intellectual and motor function. Although altered neural development and function have been well described in people with DS, few studies have investigated the etiology underlying the observed motor phenotypes. Here, we examine the development, patterning, and organization of the spinal cord throughout life in the Ts65Dn mouse, a model that recapitulates many of the motor changes observed in people with DS. METHODS: Spinal cords from embryonic to adult animals were processed for gene and protein expression (immunofluorescence) to track the spatiotemporal development of excitatory and inhibitory neurons and oligodendroglia. Postnatal analyses were focused on the lumbar region due to the reflex and gait abnormalities found in Ts65Dn mice and locomotive alterations seen in people with DS. RESULTS: Between embryonic days E10.5 and E14.5, we found a larger motor neuron progenitor domain in Ts65Dn animals containing more OLIG2-expressing progenitor cells. These disturbed progenitors are delayed in motor neuron production but eventually generate a large number of ISL1+ migrating motor neurons. We found that higher numbers of PAX6+ and NKX2.2+ interneurons (INs) are also produced during this time frame. In the adult lumbar spinal cord, we found an increased level of Hb9 and a decreased level of Irx3 gene expression in trisomic animals. This was accompanied by an increase in Calretinin+ INs, but no changes in other neuronal populations. In aged Ts65Dn animals, both Calbindin+ and ChAT+ neurons were decreased compared to euploid controls. Additionally, in the dorsal corticospinal white matter tract, there were significantly fewer CC1+ mature OLs in 30- and 60-day old trisomic animals and this normalized to euploid levels at 10-11 months. In contrast, the mature OL population was increased in the lateral funiculus, an ascending white matter tract carrying sensory information. In 30-day old animals, we also found a decrease in the number of nodes of Ranvier in both tracts. This decrease normalized both in 60-day old and aged animals. CONCLUSIONS: We show marked changes in both spinal white matter and neuronal composition that change regionally over the life span. In the embryonic Ts65Dn spinal cord, we observe alterations in motor neuron production and migration. In the adult spinal cord, we observe changes in oligodendrocyte maturation and motor neuron loss, the latter of which has also been observed in human spinal cord tissue samples. This work uncovers multiple cellular perturbations during Ts65Dn development and aging, many of which may underlie the motor deficits found in DS.

Distinct Neocortical Progenitor Lineages Fine-tune Neuronal Diversity in a Layer-specific Manner.

How the variety of neurons that organize into neocortical layers and functional areas arises is a central question in the study of cortical development. While both intrinsic and extrinsic cues are known to influence this process, whether distinct neuronal progenitor groups contribute to neuron diversity and allocation is poorly understood. Using in vivo genetic fate-mapping combined with whole-cell patch clamp recording, we show that the firing pattern and apical dendritic morphology of excitatory neurons in layer 4 of the barrel cortex are specified in part by their neural precursor lineage. Further, we show that separate precursors contribute to unique features of barrel cortex topography including the intralaminar position and thalamic innervation of the neurons they generate. Importantly, many of these lineage-specified characteristics are different from those previously measured for pyramidal neurons in layers 2-3 of the frontal cortex. Collectively, our data elucidate a dynamic temporal program in neuronal precursors that fine-tunes the properties of their progeny according to the lamina of destination.

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.

CLASP2 Links Reelin to the Cytoskeleton during Neocortical Development.

The Reelin signaling pathway plays a crucial role in regulating neocortical development. However, little is known about how Reelin controls the cytoskeleton during neuronal migration. Here, we identify CLASP2 as a key cytoskeletal effector in the Reelin signaling pathway. We demonstrate that CLASP2 has distinct roles during neocortical development regulating neuron production and controlling neuron migration, polarity, and morphogenesis. We found downregulation of CLASP2 in migrating neurons leads to mislocalized cells in deeper cortical layers, abnormal positioning of the centrosome-Golgi complex, and aberrant length/orientation of the leading process. We discovered that Reelin regulates several phosphorylation sites within the positively charged serine/arginine-rich region that constitute consensus GSK3ß phosphorylation motifs of CLASP2. Furthermore, phosphorylation of CLASP2 regulates its interaction with the Reelin adaptor Dab1 and this association is required for CLASP2 effects on neurite extension and motility. Together, our data reveal that CLASP2 is an essential Reelin effector orchestrating cytoskeleton dynamics during brain development.

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.

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.

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

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

New Perspectives for the Rescue of Cognitive Disability in Down Syndrome.

Down syndrome (DS) is a relatively common genetic condition caused by the triplication of human chromosome 21. No therapies currently exist for the rescue of neurocognitive impairment in DS. This review presents exciting findings showing that it is possible to restore brain development and cognitive performance in mouse models of DS with therapies that can also apply to humans. This knowledge provides a potential breakthrough for the prevention of intellectual disability in DS.

Neural precursor lineages specify distinct neocortical pyramidal neuron types.

Several neural precursor populations contemporaneously generate neurons in the developing neocortex. Specifically, radial glial stem cells of the dorsal telencephalon divide asymmetrically to produce excitatory neurons, but also indirectly to produce neurons via three types of intermediate progenitor cells. Why so many precursor types are needed to produce neurons has not been established; whether different intermediate progenitor cells merely expand the output of radial glia or instead generate distinct types of neurons is unknown. Here we use a novel genetic fate mapping technique to simultaneously track multiple precursor streams in the developing mouse brain and show that layer 2 and 3 pyramidal neurons exhibit distinctive electrophysiological and structural properties depending upon their precursor cell type of origin. These data indicate that individual precursor subclasses synchronously produce functionally different neurons, even within the same lamina, and identify a primary mechanism leading to cortical neuronal diversity.