Sox2 is important for two crucial processes in lung development: branching morphogenesis and epithelial cell differentiation

The primary lung bud originates from the foregut and develops into the bronchial tree by repetitive branching and outgrowing of the airway. The Sry related HMG box protein Sox2 is expressed in a cyclic manner during initiation and branching morphogenesis of the lung. It is highly expressed in non-branching regions and absent from branching regions, suggesting that downregulation of Sox2 is mandatory for airway epithelium to respond to branch inducing signals. Therefore, we developed transgenic mice that express a doxycycline inducible Sox2 in the airway epithelium. Continuous expression of Sox2 hampers the branching process resulting in a severe reduction of the number of airways. In addition, the bronchioli transiently go over into enlarged, alveolar-like airspaces, a pathology described as bronchiolization of alveoli. Furthermore, a substantial increase was observed of cGRP positive neuroendocrine cells and ΔNp63 isoform expressing (pre-) basal cells, which are both committed precursor-like cells. Thus, Sox2 prevents airways from branching and prematurely drives cells into committed progenitors, apparently rendering these committed progenitors unresponsive to branch inducing signals. However, Sox2 overexpression does not lead to a complete abrogation of the epithelial differentiation program.     

The 3D structure of the immunoglobulin heavy-chain locus: implications for long-range genomic interactions

The immunoglobulin heavy-chain (Igh) locus is organized into distinct regions that contain multiple variable (V(H)), diversity (D(H)), joining (J(H)) and constant (C(H)) coding elements. How the Igh locus is structured in 3D space is unknown. To probe the topography of the Igh locus, spatial distance distributions were determined between 12 genomic markers that span the entire Igh locus. Comparison of the distance distributions to computer simulations of alternative chromatin arrangements predicted that the Igh locus is organized into compartments containing clusters of loops separated by linkers. Trilateration and triple-point angle measurements indicated the mean relative 3D positions of the V(H), D(H), J(H), and C(H) elements, showed compartmentalization and striking conformational changes involving V(H) and D(H)-J(H) elements during early B cell development. In pro-B cells, the entire repertoire of V(H) regions (2 Mbp) appeared to have merged and juxtaposed to the D(H) elements, mechanistically permitting long-range genomic interactions to occur with relatively high frequency.     

X inactivation counting and choice is a stochastic process: evidence for involvement of an X-linked activator

Female mammalian cells achieve dosage compensation of X-encoded genes by X chromosome inactivation (XCI). This process is thought to involve X chromosome counting and choice. To explore how this process is initiated, we analyzed XCI in tetraploid XXXX, XXXY, and XXYY embryonic stem cells and found that every X chromosome within a single nucleus has an independent probability to initiate XCI. This finding suggests a stochastic mechanism directing XCI counting and choice. The probability is directly proportional to the X chromosome:ploidy ratio, indicating the presence of an X-encoded activator of XCI, that itself is inactivated by the XCI process. Deletion of a region including Xist, Tsix, and Xite still results in XCI on the remaining wild-type X chromosome in female cells. This result supports a stochastic model in which each X chromosome in a nucleus initiates XCI independently and positions an X-encoded trans-acting XCI-activator outside the deleted region.     

Hypersensitive site 4 of the human beta globin locus control region.

The Locus Control Region (LCR) of the human beta globin gene domain is defined by four erythroid-specific DNasel hypersensitive sites (HSS) located upstream of this multigene cluster. The LCR confers copy number dependent high levels of erythroid specific expression to a linked transgene, independent of the site of integration. To assess the role of the individual hypersensitive sites of the LCR, we have localized HSS4 to a 280bp fragment that is functional both in murine erythroleukaemia (MEL) cells and in transgenic mice. This fragment coincides with the major area of hypersensitivity 'in vivo' and contains a number of DNasel footprints. Bandshift analysis shows that these footprints correspond to binding sites for the erythroid specific proteins GATA1 and NF-E2 and a number of ubiquitous proteins, including jun/fos, Sp1 and TEF2.     

Expression of adult and tadpole specific globin genes from Xenopus laevis in transgenic mice.

Transgenic mice were generated which carried the adult alpha and beta-globin genes and the major tadpole specific beta-globin gene of Xenopus laevis. The adult specific alpha and beta genes were found to express in erythroid tissues in adult mice, while the major tadpole specific beta gene (beta T1) was expressed in blood from 12.5 day embryos. The pattern of expression of the beta T1 gene during mouse development was consistent with its being regulated as an embryonic globin gene in the mouse. This observation suggests that some of the factors mediating globin switching have been conserved during the evolution of modern amphibia and mammals and raises interesting questions concerning the evolution of vertebrate globin gene switching.