The homeodomain transcription factors Cdx1, Cdx2 and Cdx4 play essential roles in anteroposterior vertebral patterning through regulation of Hox gene expression. Cdx2 is also expressed in the trophectoderm commencing at E3.5 and plays an essential role in implantation, thus precluding assessment of the cognate-null phenotype at later stages. Cdx2 homozygous null embryos generated by tetraploid aggregation exhibit an axial truncation indicative of a role for Cdx2 in elaborating the posterior embryo through unknown mechanisms. To better understand such roles, we developed a conditional Cdx2 floxed allele in mice and effected temporal inactivation at post-implantation stages using a tamoxifen-inducible Cre. This approach yielded embryos that were devoid of detectable Cdx2 protein and exhibited the axial truncation phenotype predicted from previous studies. This phenotype was associated with attenuated expression of genes encoding several key players in axial elongation, including Fgf8, T, Wnt3a and Cyp26a1, and we present data suggesting that T, Wnt3a and Cyp26a1 are direct Cdx2 targets. We propose a model wherein Cdx2 functions as an integrator of caudalizing information by coordinating axial elongation and somite patterning through Hoxindependent and -dependent pathways, respectively. DEVELOPMENT 4100 system to derive a conditional null allele. In agreement with previous studies (Chawengsaksophak et al., 2004), loss of Cdx2 in the postimplantation embryo resulted in axial truncation posterior to the forelimb. This axial truncation, together with the nature of many of the genes impacted by Cdx2 loss, suggested that precocious cessation of the generation of presomitic mesoderm (PSM) is the primary basis for this phenotype. To further define the role of Cdx2 and to elucidate the mechanistic basis for this phenotype, we sought to determine whether any of the affected genes were direct Cdx2 targets. Chromatin immunoprecipitation (ChIP) demonstrated occupancy of the Wnt3a, Cyp26a1 and T promoters by Cdx2 in vivo. Moreover, all of these promoters harbor functional Cdx response elements (CDREs) as determined by transfection analysis or transgenic assays. Thus, Cdx2 directly regulates the expression of multiple players essential for the development of the posterior embryo. Taken together with previous work, these findings suggest that Cdx2 is required to couple the generation of paraxial mesoderm through multiple Hox-independent mechanisms with Hoxdependent AP vertebral patterning. MATERIALS AND METHODS Gene targeting and the generation of Cdx2 -/-mutantsA 5 kb fragment of genomic Cdx2 sequence encompassing the first intron through to the 3Ј UTR was subcloned into pBluescript II KS + . A floxed thymidine kinase/neomycin resistance cassette (loxPGK-TK-Neolox) (Iulianella and Lohnes, 2002) was cloned into the BglII site in intron 1, and a single loxP site was inserted into the NruI site in intron 2, generating the targeting vector (see Fig. S1A in the supplementary material). R1 embryonic stem cells were electroporated with...

Chromatin compaction mediates progenitor to post-mitotic cell transitions and modulates gene expression programs, yet the mechanisms are poorly defined. Snf2h and Snf2l are ATP-dependent chromatin remodelling proteins that assemble, reposition and space nucleosomes, and are robustly expressed in the brain. Here we show that mice conditionally inactivated for Snf2h in neural progenitors have reduced levels of histone H1 and H2A variants that compromise chromatin fluidity and transcriptional programs within the developing cerebellum. Disorganized chromatin limits Purkinje and granule neuron progenitor expansion, resulting in abnormal post-natal foliation, while deregulated transcriptional programs contribute to altered neural maturation, motor dysfunction and death. However, mice survive to young adulthood, in part from Snf2l compensation that restores Engrailed-1 expression. Similarly, Purkinje-specific Snf2h ablation affects chromatin ultrastructure and dendritic arborization, but alters cognitive skills rather than motor control. Our studies reveal that Snf2h controls chromatin organization and histone H1 dynamics for the establishment of gene expression programs underlying cerebellar morphogenesis and neural maturation.

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Mutations in the survival motor neuron (SMN1) gene lead to the neuromuscular disease spinal muscular atrophy (SMA). Although SMA is primarily considered as a motor neuron disease, the importance of muscle defects in its pathogenesis has not been fully examined. We use both primary cell culture and two different SMA model mice to demonstrate that reduced levels of Smn lead to a profound disruption in the expression of myogenic genes. This disruption was associated with a decrease in myofiber size and an increase in immature myofibers, suggesting that Smn is crucial for myogenic gene regulation and early muscle development. Histone deacetylase inhibitor trichostatin A treatment of SMA model mice increased myofiber size, myofiber maturity and attenuated the disruption of the myogenic program in these mice. Taken together, our work highlights the important contribution of myogenic program dysregulation to the muscle weakness observed in SMA.

Natural resistance to infection with mouse cytomegalovirus (MCMV) is controlled by a dominant locus, Cmv1. Cmv1 is linked to the Ly49 family of natural killer receptors on distal chromosome 6. While some studies localized Cmv1 as distal to the Ly49 gene cluster, genetic and functional analysis identified Ly49h as a pivotal factor in resistance to MCMV. The role of these two independent genomic domains in MCMV resistance was evaluated by functional complementation using transgenesis of bacterial artificial chromosomes (BAC) in genetically susceptible mice. Phenotypic and genetic characterization of the transgenic animals traced the resistance gene to a single region spanning the Ly49h gene. The appearance of the Ly49H protein in NK cells of transgenic mice coincided with the emergence of MCMV resistance, and there was a threshold Ly49H protein level associated with full recovery. Finally, transgenic expression of Ly49H in the context of either of the two independent susceptibility alleles, Cmv1 sBALB or Cmv1 sFVB, conferred resistance to MCMV infection. These results demonstrate that Ly49h is necessary and sufficient to confer MCMV resistance, and formally demonstrate allelism between Cmv1 and Ly49h. This panel of transgenic animals provides a unique resource to study possible pleiotropic effect of Cmv1.