The body's surfaces form the interface with the external environment, protecting the host. These epithelial barriers are also colonized by a controlled diversity of microorganisms, disturbances of which can give rise to disease. Specialized intraepithelial lymphocytes (IELs), which reside at these sites, are important as a first line of defense as well as in epithelial barrier organization and wound repair. We show here that the aryl hydrocarbon receptor (AhR) is a crucial regulator in maintaining IEL numbers in both the skin and the intestine. In the intestine, AhR deficiency or the lack of AhR ligands compromises the maintenance of IELs and the control of the microbial load and composition, resulting in heightened immune activation and increased vulnerability to epithelial damage. AhR activity can be regulated by dietary components, such as those present in cruciferous vegetables, providing a mechanistic link between dietary compounds, the intestinal immune system, and the microbiota.

In a gene trap screen we recovered a mouse mutant line in which an insertion generated a null allele of the Brd4 gene. Brd4 belongs to the Fsh/Brd family, a group of structurally related proteins characterized by the association of two bromodomains and one extraterminal domain. Members of this family include Brd2/Ring3/ Fsrg1 in mammals, fs(1)h in Drosophila, and Bdf1 in Saccharomyces cerevisiae. Brd4 heterozygotes display preand postnatal growth defects associated with a reduced proliferation rate. These mice also exhibit a variety of anatomical abnormalities: head malformations, absence of subcutaneous fat, cataracts, and abnormal liver cells. In primary cell cultures, heterozygous cells also display reduced proliferation rates and moderate sensitivity to methyl methanesulfonate. Embryos nullizygous for Brd4 die shortly after implantation and are compromised in their ability to maintain an inner cell mass in vitro, suggesting a role in fundamental cellular processes. The bromodomain is a conserved 110-amino acid motif which specifically interacts with acetyl-lysines, at least in the context of short histone H3 and H4 peptides (reference 9; see also reference 39). Although bromodomains have now been found in more than 40 different proteins (13,17,39), the function of this motif is poorly understood. The association of two N-terminal bromodomains with a C-terminal extraterminal (ET) domain defines the Fsh/Brd subgroup (23), which includes members in many species: Saccharomyces cerevisiae (Bdf1 and Bdf2) (5,23,24), Caenorhabditis elegans (cefsh) (36), Drosophila [fs(1)h, previously called fsh (Flybase nomenclature)] (14), and hagfish (hffsh; GenBank accession no. AF191032). In vertebrates, four members of the Fsh/Brd subgroup have now been identified: Brd2/RING3/fsrg1 (1, 32, 36), Brd3/ORFX/fsrg2 (37), Brd4/HUNK1/MCAP (8), and Brd5/ BRDT (19).The yeast Bdf1 protein interacts with the general transcription factor TFIID (24) as well as with histones H3 and H4 (28), and the Bdf1 mutant phenotype shares features with those of mutants affected in general transcription (23). Bdf1 mutants display a reduced rate of vegetative growth, failure to undergo one or both meiotic divisions, and sensitivity to the DNAdamaging agent methyl methanesulfonate (MMS). Some alleles of Drosophila fs(1)h cause partial or complete loss of segments and homeotic transformations in progeny of mutant females (10, 14), but little is known about the molecular properties of its product. In vertebrates, the human Brd2 protein has been identified as a mitogen-activated nuclear protein associated with a kinase activity (6, 30, 32) and a regulator of E2F-dependent cell cycle genes (7).In a gene trap screen designed to identify genes involved in mouse development, we recovered an integration producing a null allele of Brd4/MCAP, another member of the Brd/Fsh family (8). We showed that the Brd4 nullizygous condition results in early embryonic lethality. We also demonstrated that only one functional allele of Brd4 is not able to sustain normal developme...

Summary Proper functioning of the musculo-skeletal system requires the precise integration of bones, muscles and tendons. Complex morphogenetic events ensure that these elements are linked together in the appropriate 3D configuration. It has been difficult, however, to tease apart the mechanisms that regulate tissue morphogenesis. We find that deletion of Tbx5 in forelimb (or Tbx4 in hindlimbs) specifically affects muscle and tendon patterning without disrupting skeletal development thus suggesting that distinct cues regulate these processes. We identify muscle connective tissue as the site of action of these transcription factors and show that N-Cadherin and β-Catenin are key downstream effectors acting in muscle connective tissue regulating soft-tissue morphogenesis. In humans, TBX5 mutations lead to Holt-Oram syndrome, which is characterised by forelimb musculo-skeletal defects. Our results suggest that a focus on connective tissue is required to understand the aetiology of diseases affecting soft tissue formation.

The paraxial mesoderm of the somites of the vertebrate embryo contains the precursors of the axial skeleton, skeletal muscles and dermis. The Meox1 and Meox2 homeobox genes are expressed in the somites and their derivatives during embryogenesis. Mice homozygous for a null mutation in Meox1 display relatively mild defects in sclerotome derived vertebral and rib bones, whereas absence of Meox2 function leads to defective differentiation and morphogenesis of the limb muscles. By contrast, mice carrying null mutations for both Meox genes display a dramatic and wide-ranging synthetic phenotype associated with extremely disrupted somite morphogenesis, patterning and differentiation. Mutant animals lack an axial skeleton and skeletal muscles are severely deficient. Our results demonstrate that Meox1 and Meox2 genes function together and upstream of several genetic hierarchies that are required for the development of somites. In particular, our studies place Meox gene function upstream of Pax genes in the regulation of chondrogenic and myogenic differentiation of paraxial mesoderm.

Autotaxin (ATX) is a secreted glycoprotein widely present in biological fluids, originally isolated from the supernatant of melanoma cells as an autocrine motility stimulation factor. Its enzymatic product, lysophosphatidic acid (LPA), is a phospholipid mediator that evokes growth-factor-like responses in almost all cell types through G-protein coupled receptors. To assess the role of ATX and LPA signalling in pathophysiology, a conditional knockout mouse was created. Ubiquitous, obligatory deletion resulted to embryonic lethality most likely due to aberrant vascular branching morphogenesis and chorio-allantoic fusion. Moreover, the observed phenotype was shown to be entirely depended on embryonic, but not extraembryonic or maternal ATX expression. In addition, E9.5 ATX null mutants exhibited a failure of neural tube closure, most likely independent of the circulatory failure, which correlated with decreased cell proliferation and increased cell death. More importantly, neurite outgrowth in embryo explants was severely compromised in mutant embryos but could be rescued upon the addition of LPA, thus confirming a role for ATX and LPA signalling in the development of the nervous system. Finally, expression profiling of mutant embryos revealed attenuated embryonic expression of HIF-1a in the absence of ATX, suggesting a novel effector pathway of ATX/LPA.