CLONING AND EXPRESSION OF FIBROBLAST GROWTH-FACTOR RECEPTOR-1 ISOFORMS IN THE MOUSE HEART - EVIDENCE FOR ISOFORM SWITCHING DURING HEART DEVELOPMENT

Basic (b) fibroblast growth factor (FGF) mediates various biological r esponses including mitogenesis and angiogenesis by binding to specific cell surface receptors of the tyrosine kinase family. The bFGF recept or-1 (FGFR1) exists in short and long isoforms due to alternate RNA sp licing. Minor alterations in the amino acid sequence have also led to reports of different FGFR1 isoforms in different tissues even in the s ame species. In the absence of any sequence for heart FGFR1 and accumu lating evidence for a role of bFGF in heart growth and differentiation , we cloned FGFR1 from embryonic mouse hearts. Reverse transcriptase-p olymerase chain reaction (RT-PCR) was used to generate full-length sho rt (2259 base pairs) and long (2526 base pairs) forms of FGFR1 cDNAs w hich generated 86 and 102 kDa proteins, respectively following in vitr o translation. Embryonic mouse heart FGFR1 differed by seven amino aci ds from the reported sequence for mouse neuroepithelial FGFR1 and appe ared more similar to human placental FGFR1. A single FGFR1 transcript of similar to 4.3 kb was seen in RNA isolated from embryonic as well a s adult mouse hearts. There was a decrease (similar to 8.5-fold) in FG FR1 RNA levels in the adult. The majority of FGFR1 transcripts in the adult as well as embryonic heart contained exon IIIc (FGFR1-IIIc) whic h is associated with isoforms that display the highest affinity for bF GF. However, the relative ratio of short versus long FGFR1 RNA express ion was 0.5 in the embryonic heart compared-to 5.9 in the adult heart. These results indicate that: (i) structurally distinct short and long FGFR1 isoform RNAs are expressed in the embryonic and adult heart; (i i) FGFR1-IIIc is the major form of receptor expressed in the embryonic as well as adult heart; (iii) the transition from the embryo to the a dult stage is associated with a decrease but not absence of FGFR1 RNA expression; and (iv) long FGFR1-isoforms are more abundant in the embr yo while short FGFR1 isoforms predominate in the adult.