Aneuploidies are common chromosomal defects that result in growth and developmental deficits and high levels of lethality in humans. To gain insight into the biology of aneuploidies, we manipulated mouse embryonic stem cells and generated a trans-species aneuploid mouse line that stably transmits a freely segregating, almost complete human chromosome 21 (Hsa21). This "transchromosomic" mouse line, Tc1, is a model of trisomy 21, which manifests as Down syndrome (DS) in humans, and has phenotypic alterations in behavior, synaptic plasticity, cerebellar neuronal number, heart development, and mandible size that relate to human DS. Transchromosomic mouse lines such as Tc1 may represent useful genetic tools for dissecting other human aneuploidies.

Homologous sets of transcription factors direct conserved tissue-specific gene expression, yet transcription factor binding events diverge rapidly between closely related species. We used hepatocytes from an aneuploid mouse strain carrying human chromosome 21 to determine on a chromosomal scale whether interspecies differences in transcriptional regulation are primarily directed by human genetic sequence or mouse nuclear environment. Virtually all transcription factor binding locations, landmarks of transcription initiation, and the resulting gene expression observed in human hepatocytes were recapitulated across the entire human chromosome 21 in the mouse hepatocyte nucleus. Thus, in homologous tissues, genetic sequence is largely responsible for directing transcriptional programs; interspecies differences in epigenetic machinery, cellular environment, and transcription factors themselves play secondary roles.

The Rac1 guanosine triphosphatase (GTPase) has been implicated in multiple cellular functions, including actin dynamics, proliferation, apoptosis, adhesion, and migration resulting from signaling by multiple receptors, including the B cell antigen receptor (BCR). We used conditional gene targeting to generate mice with specific Rac1 deficiency in the B cell lineage. In the absence of both Rac1 and the highly related Rac2, B cell development was almost completely blocked. Both GTPases were required to transduce BCR signals leading to proliferation, survival and up-regulation of BAFF-R, a receptor for BAFF, a key survival molecule required for B cell development and maintenance.

SummaryFollicular B cell survival requires signaling from BAFFR, a receptor for BAFF and the B cell antigen receptor (BCR). This “tonic” BCR survival signal is distinct from that induced by antigen binding and may be ligand-independent. We show that inducible inactivation of the Syk tyrosine kinase, a key signal transducer from the BCR following antigen binding, resulted in the death of most follicular B cells because Syk-deficient cells were unable to survive in response to BAFF. Genetic rescue studies demonstrated that Syk transduces BAFFR survival signals via ERK and PI3 kinase. Surprisingly, BAFFR signaling directly induced phosphorylation of both Syk and the BCR-associated Igα signaling subunit, and this Syk phosphorylation required the BCR. We conclude that the BCR and Igα may be required for B cell survival because they function as adaptor proteins in a BAFFR signaling pathway leading to activation of Syk, demonstrating previously unrecognized crosstalk between the two receptors.

Cytokines are important in adult hematopoiesis, yet their function in embryonic hematopoiesis has been largely unexplored. During development, hematopoietic stem cells (HSCs) are found in the aorta-gonad-mesonephros (AGM) region, yolk sac (YS), and placenta and require the Runx1 transcription factor for their normal generation. Since IL-3 is a Runx1 target and this cytokine acts on adult hematopoietic cells, we examined whether IL-3 affects HSCs in the mouse embryo. Using Runx1 haploinsufficient mice, we show that IL-3 amplifies HSCs from E11 AGM, YS, and placenta. Moreover, we show that IL-3 mutant embryos are deficient in HSCs and that IL-3 reveals the presence of HSCs in the AGM and YS prior to the stage at which HSCs are normally detected. Thus, our studies support an unexpected role for IL-3 during development and strongly suggest that IL-3 functions as a proliferation and/or survival factor for the earliest HSCs in the embryo.

ZAP-70, a member of the Syk family of tyrosine kinases, has been reported to be expressed exclusively in T and NK cells. We show here that it is expressed throughout B cell development and that it plays a role in the transition of pro-B to pre-B cells in the bone marrow, a checkpoint controlled by signals from the pre-B cell receptor (pre-BCR), which monitors for successful rearrangement of immunoglobulin heavy chain genes. Whereas mice deficient in Syk show a partial block at this step, mice mutant in both Syk and ZAP-70 show a complete block at the pro-B cell stage and a failure of heavy chain allelic exclusion, hallmarks of defective pre-BCR signaling.

Adhesion and migration of T cells are controlled by chemokines and by adhesion molecules, especially integrins, and play critical roles in the normal physiological function of T lymphocytes. Using an RNA interference screen we have identified the WNK1 kinase as a regulator of both integrin-mediated adhesion and T cell migration. We demonstrate that WNK1 is a negative regulator of integrin-mediated adhesion, whereas it acts as a positive regulator of migration via OXSR1 and STK39 kinases and the SLC12A2 ion co-transporter. WNK1-deficient T cells home less efficiently to lymphoid organs, and migrate more slowly through them. Our results reveal that a pathway hitherto known only to regulate salt homeostasis in the kidney functions to balance T cell adhesion and migration.

Down syndrome (DS) is a genetic disorder arising from the presence of a third copy of the human chromosome 21 (Hsa21). Recently, O'Doherty and colleagues in an earlier study generated a new genetic mouse model of DS (Tc1) that carries an almost complete Hsa21. Since DS is the most common genetic cause of mental retardation, we have undertaken a detailed analysis of cognitive function and synaptic plasticity in Tc1 mice. Here we show that Tc1 mice have impaired spatial working memory (WM) but spared long-term spatial reference memory (RM) in the Morris watermaze. Similarly, Tc1 mice are selectively impaired in short-term memory (STM) but have intact long-term memory (LTM) in the novel object recognition task. The pattern of impaired STM and normal LTM is paralleled by a corresponding phenotype in long-term potentiation (LTP). Freely-moving Tc1 mice exhibit reduced LTP 1 h after induction but normal maintenance over days in the dentate gyrus of the hippocampal formation. Biochemical analysis revealed a reduction in membrane surface expression of the AMPAR (␣-amino-3-hydroxy-5-methyl-4-propionic acid receptor) subunit GluR1 in the hippocampus of Tc1 mice, suggesting a potential mechanism for the impairment in early LTP. Our observations also provide further evidence that STM and LTM for hippocampus-dependent tasks are subserved by parallel processing streams.Down syndrome (DS) results from trisomy of human chromosome 21 (Hsa21) (Lejeune et al. 1959) and is the most common genetically defined cause of mental retardation. Cognitive impairment in individuals with DS is characterized by developmental delay and deficits in language and memory (for reviews, see Chapman and Hesketh 2000;Silverman 2007). Although Hsa21 has been sequenced (Hattori et al. 2000), the functions of many of the Hsa21 genes are unknown, and their contributions to mental retardation remain to be clarified.The Tc1 mouse is the most complete animal model for DS currently available. Tc1 mice are trisomic for ∼92% of Hsa21 (O'Doherty et al. 2005). They not only exhibit many aspects of human DS but also recapitulate several of the DS features present in other mouse models (Reeves 2006). Tc1 mice show alterations in cerebellar neuronal number, heart development, and mandible size. In addition, they have impaired short-term recognition memory and display reduced long-term potentiation (LTP) in the dentate gyrus of the hippocampus of the anesthetized animal (O'Doherty et al. 2005). The hippocampus is critically important for episodic memory processing in humans (for review, see Squire 1992) and various forms of spatial and nonspatial learning in rodents (Morris et al. 1982;Phillips and LeDoux 1992;Kim et al. 1995). The hippocampus is among the most severely and consistently affected cortical structures in DS (Aylward et al. 1999;Pinter et al. 2001;Pennington et al. 2003).In order to provide insights into the neurobiology of DS, we have undertaken a more detailed analysis of synaptic plasticity and cognition in Tc1 mice. In 1966, McGaugh (1966 proposed thr...