Sea Urchin FGFR Muscle-Specific Expression: Posttranscriptional Regulation in Embryos and Adults

We have shown previously byin situhybridization that a gene encoding a fibroblast growth factor receptor (SpFGFR) is transcribed in many cell types during the initial phases of sea urchin embryogenesis (Strongylocentrotus purpuratus) (McCoonet al., J. Biol. Chem. 271,20119–20195, 1996). Here we demonstrate by immunostaining with affinity-purified antibody that SpFGFR protein is detectable only in muscle cells of the embryo and appears at a time suggesting that its function is not in commitment to a muscle fate, but instead may be required to support the proliferation, migration, and/or differentiation of myoblasts. Surprisingly, we find thatSpFGFRtranscripts are enriched in embryo nuclei, suggesting that lack of processing and/or cytoplasmic transport in nonmuscle cells is at least part of the posttranscriptional regulatory mechanism. Western blots show that SpFGFR is also specifically expressed in adult lantern muscle, but is not detectable in other smooth muscle-containing tissues, including tube foot and intestine, or in coelomocytes, despite the presence ofSpFGFRtranscripts at similar concentrations in all these tissues. We conclude that in both embryos and adults, muscle-specific SpFGF receptor synthesis is controlled primarily at a posttranscriptional level. We show by RNase protection assays that transcripts encoding the IgS variant of the ligand binding domain of the receptor, previously shown to be enriched in embryo endomesoderm fractions, are the predominant, if not exclusive,SpFGFRtranscripts in lantern muscle. Together, these results suggest that only a minority ofSpFGFRtranscripts are processed, exported, and translated in both adult and embryonic muscle cells and these contain predominantly, if not exclusively, IgS ligand binding domain sequences.