Tissue-specific expression of two isoforms of chicken fibroblast growth factor receptor, bek and Cek3.

Chicken bek and Cek3 are isoforms of the fibroblast growth factor receptor which consist of primary structures that are identical except for a variation within the last of three immunoglobulin-like repeats in the ligand-binding domain. Northern blot analysis using isoform-specific probes revealed that the bek mRNA is expressed exclusively in lung, whereas the Cek3 mRNA is expressed prominently in brain and weakly in lung. We further localized these transcripts in brain and lung by in situ hybridization histochemistry. In lung, the expression of the bek and Cek3 transcripts was distinguished in the smooth muscle of the parabronchus and in the arterial adventitia. On the other hand, in brain, the Cek3 transcript was detected in three areas: the corpus medullare of the metencephalon (cerebellum), the archiastriatum of the telencephalon, and the ependymal cells of the ventriculare of the mesencephalon. Two putative exons corresponding to isoform-specific sequences, respectively, were found to be closely located on the chicken genome. These results indicate that bek/Cek3 isoforms are derived from the same premessenger and that their expression is regulated in a tissue- or even area-specific manner. Moreover, another potential isoform produced by a new splice site within the Cek3-specific exon has been isolated.