A single-cell transcriptome atlas of the maturing zebrafish telencephalon.

The zebrafish telencephalon is composed of highly specialized subregions that regulate complex behaviors such as learning, memory, and social interactions. The transcriptional signatures of the neuronal cell types in the telencephalon and the timeline of their emergence from larva to adult remain largely undescribed. Using an integrated analysis of single-cell transcriptomes of approximately 64,000 cells obtained from 6-day-postfertilization (dpf), 15-dpf, and adult telencephalon, we delineated nine main neuronal cell types in the pallium and eight in the subpallium and nominated novel marker genes. Comparing zebrafish and mouse neuronal cell types revealed both conserved and absent types and marker genes. Mapping of cell types onto a spatial larval reference atlas created a resource for anatomical and functional studies. Using this multiage approach, we discovered that although most neuronal subtypes are established early in the 6-dpf fish, some emerge or expand in number later in development. Analyzing the samples from each age separately revealed further complexity in the data, including several cell types that expand substantially in the adult forebrain and do not form clusters at the larval stages. Together, our work provides a comprehensive transcriptional analysis of the cell types in the zebrafish telencephalon and a resource for dissecting its development and function.

Overexpression screen of chromosome 21 genes reveals modulators of Sonic hedgehog signaling relevant to Down syndrome.

Trisomy 21 and mutations in the Sonic hedgehog (SHH) signaling pathway cause overlapping and pleiotropic phenotypes including cerebellar hypoplasia, craniofacial abnormalities, congenital heart defects and Hirschsprung disease. Trisomic cells derived from individuals with Down syndrome possess deficits in SHH signaling, suggesting that overexpression of human chromosome 21 genes may contribute to SHH-associated phenotypes by disrupting normal SHH signaling during development. However, chromosome 21 does not encode any known components of the canonical SHH pathway. Here, we sought to identify chromosome 21 genes that modulate SHH signaling by overexpressing 163 chromosome 21 cDNAs in a series of SHH-responsive mouse cell lines. We confirmed overexpression of trisomic candidate genes using RNA sequencing in the cerebella of Ts65Dn and TcMAC21 mice, model systems for Down syndrome. Our findings indicate that some human chromosome 21 genes, including DYRK1A, upregulate SHH signaling, whereas others, such as HMGN1, inhibit SHH signaling. Individual overexpression of four genes (B3GALT5, ETS2, HMGN1 and MIS18A) inhibits the SHH-dependent proliferation of primary granule cell precursors. Our study prioritizes dosage-sensitive chromosome 21 genes for future mechanistic studies. Identification of the genes that modulate SHH signaling may suggest new therapeutic avenues for ameliorating Down syndrome phenotypes.

A transchromosomic rat model with human chromosome 21 shows robust Down syndrome features.

Progress in earlier detection and clinical management has increased life expectancy and quality of life in people with Down syndrome (DS). However, no drug has been approved to help individuals with DS live independently and fully. Although rat models could support more robust physiological, behavioral, and toxicology analysis than mouse models during preclinical validation, no DS rat model is available as a result of technical challenges. We developed a transchromosomic rat model of DS, TcHSA21rat, which contains a freely segregating, EGFP-inserted, human chromosome 21 (HSA21) with >93% of its protein-coding genes. RNA-seq of neonatal forebrains demonstrates that TcHSA21rat expresses HSA21 genes and has an imbalance in global gene expression. Using EGFP as a marker for trisomic cells, flow cytometry analyses of peripheral blood cells from 361 adult TcHSA21rat animals show that 81% of animals retain HSA21 in >80% of cells, the criterion for a "Down syndrome karyotype" in people. TcHSA21rat exhibits learning and memory deficits and shows increased anxiety and hyperactivity. TcHSA21rat recapitulates well-characterized DS brain morphology, including smaller brain volume and reduced cerebellar size. In addition, the rat model shows reduced cerebellar foliation, which is not observed in DS mouse models. Moreover, TcHSA21rat exhibits anomalies in craniofacial morphology, heart development, husbandry, and stature. TcHSA21rat is a robust DS animal model that can facilitate DS basic research and provide a unique tool for preclinical validation to accelerate DS drug development.

All Creatures Great and Small: New Approaches for Understanding Down Syndrome Genetics.

Human chromosome 21 (Hsa21) contains more than 500 genes, making trisomy 21 one of the most complex genetic perturbations compatible with life. The ultimate goal of Down syndrome (DS) research is to design therapies that improve quality of life for individuals with DS by understanding which subsets of Hsa21 genes contribute to DS-associated phenotypes throughout the lifetime. However, the complexity of DS pathogenesis has made developing appropriate animal models an ongoing challenge. Here, we examine lessons learned from a variety of model systems, including yeast, nematode, fruit fly, and zebrafish, and discuss emerging methods for creating murine models that better reflect the genetic basis of trisomy 21.

A non-mosaic transchromosomic mouse model of down syndrome carrying the long arm of human chromosome 21.

Animal models of Down syndrome (DS), trisomic for human chromosome 21 (HSA21) genes or orthologs, provide insights into better understanding and treatment options. The only existing transchromosomic (Tc) mouse DS model, Tc1, carries a HSA21 with over 50 protein coding genes (PCGs) disrupted. Tc1 is mosaic, compromising interpretation of results. Here, we "clone" the 34 MB long arm of HSA21 (HSA21q) as a mouse artificial chromosome (MAC). Through multiple steps of microcell-mediated chromosome transfer, we created a new Tc DS mouse model, Tc(HSA21q;MAC)1Yakaz ("TcMAC21"). TcMAC21 is not mosaic and contains 93% of HSA21q PCGs that are expressed and regulatable. TcMAC21 recapitulates many DS phenotypes including anomalies in heart, craniofacial skeleton and brain, molecular/cellular pathologies, and impairments in learning, memory and synaptic plasticity. TcMAC21 is the most complete genetic mouse model of DS extant and has potential for supporting a wide range of basic and preclinical research.