Activator-Inhibitor Interactions in Vertebrate Limb Pattern Formation: the Role of Alternative Splicing

Natalie Bronstein^1,2 and Stuart A. Newman¹

      The skeleton of the vertebrate limb develops through an interplay between factors that induce cartilage differentiation and factors that limit the spatial propagation of the chondrogenic inducers. Earlier work has indicated that members of the TGF-beta family of growth factors are early-acting, positively-autoregulatory inducers of limb chondrogenesis. More recently, FGF receptor 2 (FGFR2), which is localized to sites of precartilage condensation, has been shown to mediate lateral inhibition of chondrogenesis during the period of skeletal pattern formation. FGFR2 exists as two distinct splice variants, FGFR2-IIIb, which is present in epithelial tissues, and FGFR2-IIIc, which is present in mesenchymal tissues. The choice between these isoforms is regulated by the splicing factor hnRNP A1. We have now found that hnRNP A1 is expressed in a spatiotemporal fashion consistent with its function in causing the appearance of FGFR2-IIIc in limb precartilage condensations. Blocking the biosynthesis, nuclear transport, or RNA recognition function of hnRNP A1 leads to missplicing of FGFR2 and consequent abrogation of lateral inhibition of chondrogenesis, i.e., fused cartilage elements, in vitro and in vivo. These results provide new information on the molecular circuitry underlying limb pattern formation. They also provide insight into the basis of severe variants of the human condition known as Apert syndrome, in which missplicing of FGFR2 (caused, in this case, by insertional mutagenesis), also leads to fusion of limb skeletal elements.

¹ Department of Cell Biology and Anatomy, New York Medical College, Valhalla, NY 10595
² Department of Natural Sciences, Mercy College, Dobbs Ferry, NY 10522