Developmental expression of rat insulin-like growth factor-II/mannose 6-phosphate receptor messenger ribonucleic acid

We have examined the developmental pattern of the insulin-like growth factor-II (IGF-II)/mannose 6-phosphate (M6P) receptor mRNA in various rat tissues from 20-day gestation fetuses and 20-day postnatal animals by Northern blotting and solution hybridization/RNase protection assays. The major mRNA species in all fetal and postnatal tissues was 9.0 kilobases. The rank order of receptor mRNA concentrations among the fetal tissues was heart greater than limb/muscle, lung, intestine, kidney, liver greater than brain, which agrees with the previously reported rank order of the tissue concentrations of receptor protein. The concentration of IGF-II/M6P receptor mRNA was significantly lower in postnatal tissues, again reflecting the relative levels of receptor protein in fetal and postnatal tissues. We measured IGF-II/M6P receptor mRNA copy number in fetal heart, the tissue with the highest concentration of receptor protein and mRNA, by including in the solution hybridization/RNase protection assay known amounts of a sense strand transcript of the receptor cDNA. This sense strand standard was quantitated by incorporating a tracer amount of [32P]UTP into the transcript and measuring the radioactivity in the product purified by gel electrophoresis. The receptor mRNA copy number in fetal heart was 74 molecules/cell. We conclude that the IGF-II/M6P receptor mRNA concentration is an important determinant of the level of receptor protein in most tissues.