The insulin-like growth factor II (IGF-II) receptor is identical to the mannose-6-phosphate receptor (M-6-P), but its role as a somatomedin transducer is uncertain. IGF-II/M-6-P receptor expression was studied by in situ hybridization (ISH) in the developing rat. Expression occurs in extra-embryonic membranes at the time of IGF-II mRNA induction and later at paracrine/autocrine sites of IGF-II action (skeletal muscle and perichondrium) in the embryo. Highest levels of receptor mRNA occur in heart and major vessels. Postnatally transcription is strongly down-regulated. This suggests a role for the IGF-II/M-6-P receptor in IGF-II action or turnover during development distinct from its role in lysosomal transport.
We describe the postnatal ontogeny and localization of insulin-like growth factors I and II (IGF-I and -II) in the rat. We have used oligodeoxyribonucleotide probes for in situ hybridization (hybridization histochemistry) and for Northern blotting. IGF-II mRNA is strongly expressed in liver, skeletal muscle, perichondrium, leptomeninges and choroid plexus of the newborn. Demonstrable levels fall dramatically in the liver at 18–20 days postnatally but persist for longer periods in muscle and remain undiminished throughout life in the pia/choroid plexus, indicating that different control mechanisms operate in these tissues. IGF-I mRNA is predominantly found in the liver. Its level in this organ rises well before levels of IGF-II fall. This suggests that distinct factors govern the expression of IGF-I and -II genes.