ALTERNATIVE SPLICING OF INSULIN-LIKE-GROWTH-FACTOR-I MESSENGER-RNA ISDEVELOPMENTALLY-REGULATED IN THE RAT AND MOUSE WITH PREFERENTIAL EXON-2 USAGE IN THE MOUSE

The synthesis of an IGF-I complementary RNA (cRNA) standard containing the primer sequences for each of the four IGF-I mRNA alternative tran script forms (class 1Ea, 1Eb, 2Ea and 2Eb) and a polyA(+) tail allowed the determination of the absolute abundance of each form in growing r ats and mice by quantitative RT-PCR. In rat liver, class 1Ea mRNA was the most abundant form representing 90% of the total IGF-I mRNA. Thoug h the relative proportion was maintained, the absolute abundance was m aximal at 4 weeks of age and had declined by 6 weeks. In contrast, cla ss 2Ea mRNA was the predominant transcript in mouse liver (70% of tota l IGF-I mRNA) and the 2Eb mRNA form was also 10-fold higher than that detected in rat liver. These results suggest that rats and mice differ both in their transcription initiation at two leader exons and in the ir alternative splicing activities for exon 5. (C) 1998 Churchill Livi ngstone.