PRODUCTS, GENETIC-LINKAGE AND LIMB PATTERNING ACTIVITY OF A MURINE HEDGEHOG GENE

The hedgehog (hh) segmentation gene of Drosophila melanogaster encodes a secreted signaling protein that functions in the patterning of larv al and adult structures. Using low stringency hybridization and degene rate PCR primers, we have isolated complete or partial hh-like sequenc es from a range of invertebrate species including other insects, leech and sea urchin, We have also isolated three mouse and two human DNA f ragments encoding distinct hit-like sequences, Our studies have focuse d upon Hhg-1, a mouse gene encoding a protein with 46% amino acid iden tity to hh, The Hhg-1 gene, which corresponds to the previously descri bed vhh-1 or sonic class, is expressed in the notochord, ventral neura l tube, lung bud, hindgut and posterior margin of the limb bud in deve loping mouse embryos, By segregation analysis the Hhg-1 gene has been localized to a region in proximal chromosome 5, where two mutations af fecting mouse limb development previously have been mapped, In Drosoph ila embryos, ubiquitous expression of the Hhg-1 gene yields effects up on gene expression and cuticle pattern similar to those observed for t he Drosophila hh gene, We also find that cultured quail cells transfec ted with a Hhg-1 expression construct can induce digit duplications wh en grafted to anterior or mid-distal but not posterior borders within the developing chick limb; more proximal limb element duplications are induced exclusively by mid-distal grafts, Both in transgenic Drosophi la embryos and in transfected quail cells, the Hhg-1 protein product i s cleaved to yield two stable fragments from a single larger precursor . The significance of Hhg-1 genetic linkage, patterning activity and p roteolytic processing in Drosophila and chick embryos is discussed.