The avian IGF type 1 receptor: CDNA analysis and in situ hybridization reveal conserved sequence elements and expression patterns relevant for the development of the nervous system

Insulin-like growth factor type 1 receptor (IGF-1R) is a tyrosine kinase with a key role in development. The primary structure of IGF-1R is known for mammalian species, but not for birds. The avian embryo, however, provides an ideal system for the experimental study of neurogenesis. We therefore cloned the complete coding sequence of the chicken IGF-1R from a cDNA library and analyzed its embryonic expression by Northern blot and in situ hybridization. The deduced chicken IGF-1R precursor of 1363 amino acids was 85% identical to human IGF-1R and did not show deletions or insertions in critical positions, when compared to its mammalian homologues. Notably, all cysteine residues in the extracellular domains, and 15 of the 17 N-linked glycosylation sites found in human IGF-1R were also present in the chicken receptor. An 11 kb transcript was abundant in developing nervous tissues, kidney, pancreas and the gastrointestinal tract. The early in situ expression patterns in 20-somite embryos revealed high levels of IGF-1R mRNA in the neuroepithelia, notochord and somites. At embryonic day 4 (E4), high concentrations of IGF-1R transcripts were found again primarily in the neuroepithelia and, to a lesser degree, in the sensory ganglia and diverse mesenchymal derivatives. During the second half of embryonic development, IGF-1R expression in the CNS was particularly abundant in telencephalic regions, including the olfactory bulb, hippocampus, striatum and piriform cortex, and also in the optic tectum and cerebellum. By the use of cDNA cloning and in situ hybridization this study reveals conserved amino acid sequence elements between birds and mammals, and developmental expression patterns that an compatible with an important role of this receptor in growth, differentiation and maturation of the avian CNS.