Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 2005
Publication Date: June 7, 2005
Citation: Richards, M.P., Poch, S.M., McMurtry, J.P. 2005. Expression of insulin-like growth factor system genes in liver and brain tissue during embryonic and post-hatch development of the turkey. Comparative Biochemistry and Physiology. 141A:76-86. Interpretive Summary: Poultry producers have, over the years, intensively selected for lines of chickens and turkeys that grow faster and produce more meat than previous generations. The insulin-like growth factor (IGF) system plays a very important role in the regulation of growth. In birds, it is comprised of two peptide hormones (IGF-I and IGF-II), a common receptor that binds both hormone peptides (IGF-1R) on the surface of many types of cells, and two binding proteins (IGFBP-2 and IGFBP-5) that bind the IGF hormones as they circulate in the bloodstream. Working together the components of the IGF system are responsible for regulating growth and development of important tissues such as muscle and bone. This system is also very important to regulating the progress of embryonic development. To date little is known about the existence and function of the IGF system in birds, especially in turkeys. Our goal in this research was to identify and characterize IGF system genes in turkeys during embryonic and early post-hatch development. The turkey IGF-II gene was identified and characterized for the first time as part of this work. We also found that expression of the IGF-1 gene in liver was low throughout embryonic development and only increased after hatching. In contrast, expression of the IGF-II gene was higher in liver during embryonic development and peaked just prior to hatching. Thus, it appears that IGF-II is more dominant during embryonic development and IGF-I may be more important to post-hatch growth and development. Moreover, there appeared to be links between the expression of both IGF hormones and the IGF-1R, as well as, the IGFBP-2. Such coordination in gene expression offers new insight into how the IGF system functions in different tissues and cells. Differences in the expression of IGF system genes between liver and brain tissue further suggested the possibility of tissue-specific regulation of growth and development in the turkey. This information will be useful to researchers studying the role of the IGF system in growth and body composition in avian species, as well as, producers in formulating new genetic selection strategies for commercial poultry flocks.
Technical Abstract: A molecular cloning strategy employing primer-directed reverse transcription polymerase chain reaction (RT-PCR) was devised to sequence 1300 bp of a turkey liver-derived cDNA corresponding to the complete coding region and the 5'- and 3'-untranslated regions of the insulin-like growth factor (IGF)-II mRNA transcript (GenBank Accession No. AF074980). The turkey IGF-II gene codes for a 187 amino acid precursor protein that includes a signal peptide, the mature IGF-II hormone, and a C-terminal extension peptide comprised of 24, 67 and 96 amino acids, respectively. Turkey IGF-II showed greater than 95% sequence identity at both the nucleotide and amino acid level with chicken IGF-II. Expression of IGF-I, IGF-II, IGF type-I receptor (IGF-IR), and IGF binding protein (IGFBP)-2 and -5 genes was quantified relative to an internal 18S rRNA standard by RT-PCR in liver and whole brain tissue on days 14, 16, 18, 20, 22, 24 and 26 of embryonic development, as well as at hatch (H, day 28) and at 3 wk post-hatching (PH). Expression of liver IGF-I was low throughout embryonic development, but increased more than 8-fold by 3 wk PH. In contrast, IGF-I was expressed in brain tissue at much higher levels than liver throughout development and this level of expression in brain increased gradually, reaching its highest point at 3 wk PH. IGF-II was expressed at comparable levels in brain and liver tissue during embryonic development, except for transient increases in liver just prior to hatching (days 24 and 26) and at 3 wk PH. Expression of IGF-IR declined in brain throughout development reaching its lowest level at 3 wk PH. In liver, IGF-IR expression was lower than that of brain throughout development. An inverse relationship was observed for the expression of IGF-I and IGF-IR genes in brain, but not in liver, through 3 wk PH. Expression of the IGFBP-2 gene increased in liver around the time of hatch (days 26-28) and declined by 3 wk PH, whereas the level of expression of IGFBP-5, which was higher than IGFBP-2, remained fairly constant in both brain and liver throughout the developmental period studied. Our data indicates differential expression of selected genes that comprise the IGF system in the turkey during embryonic and PH growth and development.