Isolation and characterization of a mRNA encoding a novel insulin receptor (IR) subtype, IR2, from rainbow trout (Oncorhynchus mykiss) and patterns of expression of the four IR subtypes, IR1-IR4, in tissues and during embryonic development

Insulin (INS) plays a critical role in the growth, development, and metabolism of vertebrates. In this study, a cDNA encoding a novel insulin receptor (IR) subtype was isolated, cloned, and sequenced from the liver of rainbow trout. A 1525-bp cDNA encoding a partial amino acid sequence of the β-subunit including the transmembrane domain, the tyrosine kinase domain, and the 3' untranslated region (UTR) was obtained and designated IR2 based on comparison with known IR subtypes, including the three previously reported IR subtypes of trout. Trout IR2 shares 90.0%, 82.8%, and 84.3% nucleotide identity with previously characterized trout IR1, IR3 and IR4, respectively. Quantitative real-time PCR revealed that the four IR mRNAs were differentially expressed, both in terms of distribution among tissues as well as in terms of abundance within selected tissues of juvenile trout. IR1 mRNA was most abundant in spleen, liver, kidney, and muscle (white, red and cardiac), but least abundant in adipose. IR3 mRNA was most abundant in liver, spleen, kidney, and pancreas; in other tissues, levels of IR3 mRNA were uniformly abundant. By contrast, levels of IR2 and IR4 mRNA were uniformly abundant in most tissues, except in spleen where levels of IR4 were significantly lower. All IR subtypes were detected over the course of embryonic development. In head and tail regions, levels of IR2 and IR3 mRNA declined from pre-hatch (29. days post-fertilization, dpf) to post-hatch (68-90. dpf), whereas levels of IR1 and IR4 remained relatively unchanged. These findings contribute to our understanding of the evolution, distribution, and function of insulin receptors.