Pitx2c modulates Pax3+/Pax7+ cell populations and regulates Pax3 expression by repressing miR27 expression during myogenesis

Pitx2 is a paired-related homeobox gene that is expressed in muscle progenitors during myogenesis. We have previously demonstrated that overexpression of Pitx2c isoform in myoblasts maintained these cells with a high proliferative capacity and completely blocked terminal differentiation by inducing high Pax3 expression levels (Martinez et al., 2006). We now report that Pitx2c-mediated proliferation vs. differentiation effect is maintained during in vivo myogenesis. In vivo Pitx2c loss of function leads to a decrease in Pax3+/Pax7− cell population in the embryo accompanied by an increase of Pax3+/Pax7+ cells. Pitx2c transient-transfection experiments further supported the notion that Pitx2c can modulate Pax3/Pax7 expression. Pitx2c but not Pitx3 controls Pax3/Pax7 expression, although redundant roles are elicited at the terminal myoblast differentiation. Contrary to Pitx2c, Pitx3 does not regulate cell proliferation or Pax3 expression, demonstrating the specificity of Pitx2c mediating these actions in myoblasts. Furthermore we demonstrated that Pitx2c modulates Pax3 by repressing miR27 expression and that Pax3-miR-27 modulation mediated by Pitx2c is independent of Pitx2c effects on cell proliferation. Therefore, this study sheds light on previously unknown function of Pitx2c balancing the different myogenic progenitor populations during myogenesis.     

Directionality of heart looping: effects of Pitx2c misexpression on flectin asymmetry and midline structures

The insulin-like growth factors (IGFs) are well known mitogens, both in vivo and in vitro, while functions in cellular differentiation have also been indicated. Here, we demonstrate a new role for the IGF pathway in regulating head formation in Xenopus embryos. Both IGF-1 and IGF-2, along with their receptor IGF-1R, are expressed early during embryogenesis, and the IGF-1R is present particularly in anterior and dorsal structures. Overexpression of IGF-1 leads to anterior expansion of head neural tissue as well as formation of ectopic eyes and cement gland, while IGF-1 receptor depletion using antisense morpholino oligonucleotides drastically reduces head structures. Furthermore, we demonstrate that IGF signaling exerts this effect by antagonizing the activity of the Wnt signal transduction pathway in the early embryo, at the level of beta-catenin. Thus, the IGF pathway is required for head formation during embryogenesis.     

Pitx3-CreER mice showing restricted Cre expression in developing ocular lens and skeletal muscles

To generate temporally controlled inactivation or activation of interested genes in Pitx3-expressing cells, the tamoxifen-inducible form of Cre, CreER(T2), was inserted into the Pitx3 locus of a mouse BAC clone. Following a single dose of tamoxifen, Cre activity in Pitx3-CreER(T2) transgenic mice was observed in the ocular lens and skeletal muscles but not in the central nervous system at various embryonic stages. This mouse line provides a reagent for driving inducible Cre-dependent recombination in the lens and skeletal muscles during embryonic development.     

左右不对称信号分子Pitx2

同型框基因Pitx2在鸡、小鼠和爪蟾胚胎中不对称地表达在左侧板中胚层和衍生器官(如心脏、肠等)中. 转录因子Pitx2看来是Shh和Nodal等信号分子的下游效应子. Pitx2的错误表达足以产生器官逆位和身体旋转逆向,人类若有Pitx2表达缺陷就可能导致Rieger综合征. Pitx2看来是脊椎动物介导左右不对称的关键且保守的信号分子.     

Deletion of the Pitx1 genomic locus affects mandibular tooth morphogenesis and expression of the Barx1 and Tbx1 genes

Pitx1 is a bicoid-related homeodomain factor that exhibits preferential expression in the developing hindlimb, mandible, pituitary gland and teeth. Pitx1 gene-deleted mice exhibit striking abnormalities in morphogenesis and growth of both hindlimb and mandible, suggesting a proliferative defect in these two structures. Here, we studied the expression and regulation of Pitx1 in both mandible and developing teeth and analyzed tooth morphology, cell proliferation, apoptosis and expression of Pitx2, Barx1 and Tbx1 in dental tissues of Pitx1−/− mouse embryos. Pitx1 expression is restricted to the epithelium of the growing tooth anlagen. Tissue recombination and bead implantation experiments demonstrated that bone morphogenetic protein-4 down-regulates Pitx1 expression in both mandibular mesenchyme and dental epithelium. Deletion of the Pitx1 locus results in micrognathia and abnormal morphology of the mandibular molars. Although Pitx2 expression in teeth of Pitx1−/− embryos is not altered, expression of Barx1 decreased in the mesenchyme of the mandibular molars. Furthermore, Pitx1 deletion results in suppression of Tbx1 expression in dental epithelium. Taken together, these results indicate that independent genetic pathways in mandibular and maxillary processes determine tooth development and morphology.