Supernumerary digit induction by mouse posterior limb bud cell lines
Description
It becomes apparent that the complex process of limb patterning can not be described by the activities of a single factor. Despite its initial characterization as a morphogen, Sonic Hedgehog (SHH) is tethered to membrane surfaces by cholesterol modification (Porter et al., 1996) and does not function through diffusion across the entire anterior-posterior axis as originally conceived (Marti et al., 1995, Yang et al., 1995). Indispensable signals secondary to SHH have also been identified (Laufer et al., 1994, Niswander et al., 1994). Moreover, localization of the zone of polarizing activity (ZPA), and with it shh expression, is a tightly regulated event as demonstrated by mutations that result in ectopic ZPA formation in the anterior limb (Chan et al., 1995; Masuya et al., 1995). Given this, a true understanding of limb development must include defining the mechanisms by which identity is established and maintained at all positions The data presented demonstrate the complexity of interactions necessary for the maintenance of positional identities and show that SHH is not strictly required for digit induction or patterning. Through use of the Mouse Posterior Limb Bud (MPLB) cell lines, it is shown that extracellular moieties may elicit patterning effects reminiscent of the ZPA through an indirect mechanism independent of SHH, which is not expressed by N91B cells. Rather, MPLB extracellular matrix (ECM) isolates are shown to induce supernumerary digits through disruption of epithelial-mesenchymal interactions involving FGF8. Such disregulation does not lead to an alteration of apoptosis in the limb anterior as shown by relevant gene expression, nor does disregulation result in ectopic induction of shh. Furthermore, genes normally characterized as downstream effectors of SHH are either unaffected or only weakly induced by MPLB ECM. As such, instead of an inductive cascade, the data suggest disregulation of existing identities. These results are important in that they clearly show inductive signals alone are insufficient to describe pattern formation in the developing limb