Expression of alternatively spliced and canonical basic fibroblast growth factor mRNAs in the early embryo and developing heart.

Previous studies in this laboratory have revealed the presence of substantial deposits of basic fibroblast growth factor (bFGF; FGF-2) in the myocardium from the earliest stages of heart development (Parlow et al. [1991] Dev. Biol. 146:139-147) and that an autocrine supply of bFGF is required for myocardial cell proliferation (Sugi et al. [1993] Dev, Biol, 157:28-37). Recently, an alternatively spliced isoform of bFGF, termed alt-bFGF, was described during later stages of embryogenesis, after heart morphogenesis is complete (Borja et al. [1993] Dev. Biol. 157:110-118). Because the antibody and nucleic acid probes used in our previous studies would have recognized canonical as well as alt-bFGF proteins and mRNAs, we have examined the expression of alt-and canonical bFGF mRNAs at early stages of embryogenesis, during which the initial differentiative and morphogenetic phases of heart development occur (Hamburger-Hamilton stages 3-24). Reverse transcription/polymerase chain reaction (RT/PCR) analysis detected the presence of both alt-bFGF and bFGF mRNAs in whole embryos as early as stage 3 and in the developing heart from the time of its initial appearance at stage 9. The presence of alt-bFGF mRNA was corroborated by RNase protection analysis which, in assessing RNA from whole embryos, revealed increasing levels of alt-bFGF mRNA between stages 5-18, suggesting that expression of alt-bFGF is developmentally regulated. Utilization of a probe that simultaneously protects segments of both alt- and canonical bFGF mRNAs indicated that alt-bFGF was the more abundant FGF isoform in the developing embryo until stage 24, when equivalent expression of each isoform was detected. Similar analysis revealed that alt-bFGF was the more abundant isoform in the embryonic heart, but that its relative expression was not decreased at stage 24.