Identification of IGF-1 receptors in primitive vertebrates

This is the first report of the existence of insulin-like growth factor (IGF-1) receptors in three representatives of lower vertebrates: the osteichtyes, chondrichtyes and cyclostomi. Competitive binding studies and affinity labelling of brain membranes from Cottus scorpius (sea scorpion), Raja clavata (ray) and Myxine glutinosa (atlantic hagfish) identified a mammalian type 1 or IGF-1 receptor by its binding specificity and the molecular size of its alpha-subunit. IGF-1 and IGF-2 are almost equally potent in displacing receptor-bound 125I-IGF-1 or 125I-IGF-2, and the proteins labeled with both tracers have a molecular size of 100,000-120,000 under reducing conditions. There was no evidence for the presence of a mammalian type 2 or IGF-2/mannose 6-phosphate receptor in brains of Cottus, Raja or Myxine. In all three species the binding of 125I-IGF-1 and 125I-IGF-2 was significantly higher in brain compared with liver and gastrointestinal tract, and the IGF-1 receptor could only be identified with certainty in Raja liver. It is concluded that the brain of three lower vertebrates express mammalian IGF-1 receptors, whereas IGF-2-mannose 6-phosphate receptors could not be detected.