Regulatory modulation of the T-box gene Tbx5 links development, evolution, and adaptation of the sternum.

The sternum bone lies at the ventral midline of the thorax where it provides a critical attachment for the pectoral muscles that allow the forelimbs to raise the body from the ground. Among tetrapods, sternum morphology is correlated with the mode of locomotion: Avians that fly have a ventral extension, or keel, on their sterna, which provides an increased area for flight muscle attachment. The sternum is fused with the ribs attaching on either side; however, unlike the ribs, the sternal precursors do not originate from the somites. Despite the crucial role of the sternum in tetrapod locomotion, little attention has been given to its acquisition, evolution, and embryological development. We demonstrate an essential role for the T-box transcription factor gene Tbx5 in sternum and forelimb formation and show that both structures share an embryological origin within the lateral plate mesoderm. Consistent with this shared origin and role of Tbx5, sternum defects are a characteristic feature of Holt-Oram Syndrome (OMIM 142900) caused by mutations in TBX5. We demonstrate a link between sternum size and forelimb use across avians and provide evidence that modulation of Tbx5 expression underlies the reduction in sternum and wing size in a flightless bird, the emu. We demonstrate that Tbx5 is a common node in the genetic pathways regulating forelimb and sternum development, enabling specific adaptations of these features without affecting other skeletal elements and can also explain the linked adaptation of sternum and forelimb morphology correlated with mode of locomotion.

The mouse Runx1 +23 hematopoietic stem cell enhancer confers hematopoietic specificity to both Runx1 promoters.

The transcription factor Runx1 plays a pivotal role in hematopoietic stem cell (HSC) emergence, and studies into its transcriptional regulation should give insight into the critical steps of HSC specification. Recently, we identified the Runx1 +23 enhancer that targets reporter gene expression to the first emerging HSCs of the mouse embryo when linked to the heterologous hsp68 promoter. Endogenous Runx1 is transcribed from 2 alternative promoters, P1 and P2. Here, we examined the in vivo cis-regulatory potential of these alternative promoters and asked whether they act with and contribute to the spatiotemporal specific expression of the Runx1 +23 enhancer. Our results firmly establish that, in contrast to zebrafish runx1, mouse Runx1 promoter sequences do not confer any hematopoietic specificity in transgenic embryos. Yet, both mouse promoters act with the +23 enhancer to drive reporter gene expression to sites of HSC emergence and colonization, in a +23-specific pattern.

Alternative Runx1 promoter usage in mouse developmental hematopoiesis.

The interest in stem cell based therapies has emphasized the importance of understanding the cellular and molecular mechanisms by which stem cells are generated in ontogeny and maintained throughout adult life. Hematopoietic stem cells (HSCs) are first found in clusters of hematopoietic cells budding from the luminal wall of the major arteries in the developing mammalian embryo. The transcription factor Runx1 is critical for their generation and is specifically expressed at sites of HSC generation, prior to their formation. To understand better the transcriptional hierarchies that converge on Runx1 during HSC emergence, we have initiated studies into its transcriptional regulation. Here we systematically analyzed Runx1 P1 and P2 alternative promoter usage in hematopoietic sites and in sorted cell populations during mouse hematopoietic development. Our results indicate that Runx1 expression in primitive erythrocytes is largely P2-derived, whilst in definitive hematopoietic stem and/or progenitor cells from the yolk sac or AGM and vitelline and umbilical arteries both the distal P1 and proximal P2 promoters are active. After cells have migrated to the fetal liver, the P1 gradually becomes the main hematopoietic promoter and remains this into adulthood. In addition, we identified a novel P2-derived Runx1 isoform.