Histone deacetylase inhibitor, trichostatin A, affects gene expression patterns during morphogenesis of chicken limb buds in vivo.

Acetylation is one of the key chromatin modifications that control gene transcription during embryonic development and tumorigenesis. The types of genes sensitive to such modifications in vivo are not known to date. We investigated the expression of a number of genes involved in embryonic development after treatment with trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, in the limbs of chicken embryos. Our results show that TSA affects the expression profiles of some genes that play important roles during limb development. The expression of BMP4, SF/HGF and Twist1 increased, whereas the expression of BMP2, FGF8, Shh, Scleraxis, Myf5 and MyoD was decreased or even inhibited. In contrast, the expression of Pax3, Paraxis, Msx1, CREB, and PCNA was not affected. Our results indicate that the chicken embryo can serve as an effective in vivo model for studying the effect of HDAC inhibitors on gene expression and can be helpful for understanding the role of chromatin remodeling and epigenetic control of gene expression.

Avian somitogenesis: translating time and space into pattern.

Vertebrates have a metameric bodyplan that is based on the presence of paired somites. Somites develop from the segmental plate in a cranio-caudal sequence. At the same time, new material is added from Hensen's node, the primitive streak and the tailbud. In this way, the material residing in the segmental plate remains constant and comprises 12 prospective somites on each side. Prospective segment borders are not yet determined in the caudal segmental plate. Prior to segmentation, the cranial segmental plate undergoes epithelialization, which is controlled by signals from the neural tube and ectoderm. The bHLH transcription factor Paraxis is critically involved in this process. Formation of a new somite from the cranial end of the segmental plate is a highly controlled process involving complex cell movements in relation to each other. Hox genes specify regional identity of the somites and their derivatives. In the chicken a transposition of thoracic into cervical vertebrae has occurred as compared to the mouse. Transcription factors of the bHLH and homeodomain type also specify the cranio-caudal polarity and that of particular cell groups within the somites. According to segmentation models, somitogenesis is under the control of a "segmentation clock" in combination with a morphogen gradient. This hypothesis has recently found support from molecular data, especially the cycling expression of genes such as cHairy1 and Lunatic Fringe, which depend on the Notch/Delta pathway of signal transduction. FGF8 has been described to be distributed along a cranio-caudal gradient. The first oscillating gene described shown to be independent of Notch is Axin2, encoding a negative regulator of the canonical Wnt pathway and a target of Wnt3a. Wnt3a and Axin2 show a similar distribution as FGF8 with high levels in the tailbud. The chick embryo has recently become accessible to molecular approaches such as overexpression by electroporation and RNA interference which can be expected to help elucidating some of the still open questions concerning somitogenesis.

Revisiting vimentin expression in early chick development.

To identify somite-specific antigens exhibiting novel expression patterns in the developing chick embryo, epithelial somites were isolated and used for intrasplenic immunization. Immunohistochemical screening of chick embryos of various stages focussed our attention on a monoclonal antibody, Som5H5, which stained somitic derivatives, spinal nerves, and neural tubes as well as the costameres of adult skeletal muscle. Western blot and mass spectrometric analysis revealed the Som5H5 antigen to be vimentin, the distribution of which has been well characterized before. In addition to the described vimentin pattern, Som5H5 stained a region in the proximal portion of the developing limb bud. This novel expression domain was confirmed by in situ hybridization using vimentin riboprobes. Signaling molecules (Shh and BMP-2), known to play a role in limb development, did not influence vimentin expression. Thus, no functional or cellular correlate to this vimentin-positive region could be determined.