Regulation of expression of zebrafish (Danio rerio) insulin-like growth factor 2 receptor: implications for evolution...

The insulin-like growth factor 2 receptor (IGF2R) is an unusual multifunctional receptor that interacts with a diverse variety of ligands. While the receptor has been well-characterized in mammals, little is known of its biology in other vertebrates. In this report, we characterize the expression of the zebrafish (Danio rerio) ortholog of the IGF2R gene. We show that two distinct, cell-type-specific promoters drive transcription of zebrafish igf2r and that these encode receptor isoforms that differ in their amino termini. Both promoters are active in adult fish and during embryonic development, but the proximal promoter generates more abundant transcripts. The 5'-UTR of the more abundantly expressed transcript contains several AUGs upstream of the main start codon, and these negatively regulate translation of a downstream reporter gene. Comparative sequence analysis shows that upstream AUGs (uAUGs) are a feature of IGF2R genes in several other vertebrates, including Xiphophorus, Xenopus, chicken, platypus, and opossum, but not in eutherian mammals. The IGF2R is imprinted in marsupial and placental mammals, and this transcriptional control of receptor abundance was proposed to have evolved following acquisition of an insulin-like growth factor 2 (IGF2) binding site by the ancestral receptor. Our observations suggest that receptor abundance was regulated at translation in ancestral vertebrates, before acquisition of an IGF2 binding site. We propose that evolution of imprinting at the mammalian IGF2R may have facilitated the loss of negative regulation of translation conferred by uAUGs.