From Drosophila segmentation to human cancer therapy

The mouse Hedgehog gene family consists of three members, Sonic, Desert, and Indian hedgehog (Shh, Dhh, and Ihh, respectively), relatives of the Drosophila segment polarity gene, hedgehog (hh). All encode secreted proteins implicated in cell-cell interactions. One of these, Shh, is expressed in and mediates the signaling activities of several key organizing centers which regulate central nervous system, limb, and somite polarity. However, nothing is known of the roles of Dhh or Ihh, nor of the possible function of Shh during later embryogenesis. We have used serial-section in situ hybridization to obtain a detailed profile of mouse Hh gene expression from 11.5 to 16.5 days post coitum. Apart from the gut, which expresses both Shh and Ihh, there is no overlap in the various Hh expression domains. Shh is predominantly expressed in epithelia at numerous sites of epithelial-mesenchymal interactions, including the tooth, hair, whisker, rugae, gut, bladder, urethra, vas deferens, and lung, Dhh in Schwann and Sertoli cell precursors, and Ihh in gut and cartilage. Thus, it is likely that Hh signaling plays a central role in a diverse array of morphogenetic processes. Furthermore, we have compared Hh expression with that of a second family of signaling molecules, the Bone morphogenetic proteins (Bmps), vertebrate relatives of decapentaplegic, a target of the Drosophila Hh signaling pathway. The frequent expression of Bmp-2, -4, and -6 in similar or adjacent cell populations suggests a conserved role for Hh/Bmp interactions in vertebrate development.

Recombinant DNA carrying the 3-kilobase transposable element was injected into Drosophila embryos of a strain that lacked such elements. Under optimum conditions, half of the surviving embryos showed evidence of P element-induced mutations in a fraction of their progeny. Direct analysis of the DNA of strains derived from such flies showed them to contain from one to five intact 3-kilobase P elements located at a wide variety of chromosomal sites. DNA sequences located outside the P element on the injected DNA were not transferred. Thus P elements can efficiently and selectively transpose from extrachromosomal DNA to the DNA of germ line chromosomes in Drosophila embryos. These observations provide the basis for efficient DNA-mediated gene transfer in Drosophila.

The zone of polarizing activity (ZPA) is a region at the posterior margin of the limb bud that induces mirror-image duplications when grafted to the anterior of a second limb. We have isolated a vertebrate gene, Sonic hedgehog, related to the Drosophila segment polarity gene hedgehog, which is expressed specifically in the ZPA and in other regions of the embryo, that is capable of polarizing limbs in grafting experiments. Retinoic acid, which can convert anterior limb bud tissue into tissue with polarizing activity, concomitantly induces Sonic hedgehog expression in the anterior limb bud. Implanting cells that express Sonic hedgehog into anterior limb buds is sufficient to cause ZPA-like limb duplications. Like the ZPA, Sonic hedgehog expression leads to the activation of Hox genes. Sonic hedgehog thus appears to function as the signal for antero-posterior patterning in the limb.