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Title: Investigation of the insulin-like growth factor system in the avian epiphyseal growth plate.

Author
item LEACH, R. - PENN STATE
item Richards, Mark
item PRAUL, B. - PENN STATE
item FORD, B. - PENN STATE
item McMurtry, John

Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2006
Publication Date: 5/15/2006
Citation: Leach, R.M.,Jr., Richards, M.P., Praul, B.C., Ford, B.C., McMurtry, J.P. 2007. Investigation of the insulin-like growth factor system in the avian epiphyseal growth plate. Domestic Animal Endocrinology. 33:143-153.

Interpretive Summary: Poultry producers have, over the years, intensively selected for lines of chickens that grow faster and produce more meat than previous generations. Along with such improvements has come an increase in growth-related problems including problems with bone growth. 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. To date, little is known about the function of the IGF system in the avian bone growth. The goal of this research was to further define the role of IGFs (IGF-I and IGF-II) on aspects of bone growth in broiler chickens. Each of the IGF system genes was expressed in cells (chondrocytes) derived from the growing portion of bone (the epiphyseal growth plate) responsible for longitudinal bone growth. The expression of IGF-II relative to IGF-I was very high in these cells indicating that IGF-II may play a significant role in avian bone growth. IGF-I and IGF-II were equally effective in promoting the synthesis of bone cartilage matrix, as well in stimulating maturation of chondrocytes involved in bone growth. Together, these findings indicate an important role for the IGF system in regulating bone growth in chickens. This information will be useful to researchers studying the role of the IGF system in the growth of poultry species, as well as, to producers in formulating new genetic selection strategies for commercial poultry flocks.

Technical Abstract: Components of the insulin-like growth factor (IGF) system were investigated in chondrocytes isolated from the avian growth plate. The genes for IGF-I, IGF-II, IGF receptor (IGFR), IGF binding protein-2 (IGF BP-2) and IGF BP-5 were found to be expressed in both proliferative and hypertrophic chondrocytes. The expression of IGF-II in proliferative chondrocytes was extremely high relative to IGF-I. Although IGF-I expression was significantly increased in hypertrophic chondrocytes, the level was still low relative to IGF-II. In cell culture, IGF-I and IGF-II were equally efficacious in stimulating proteoglycan synthesis and stimulating the expression of Indian hedgehog (Ihh) and type X collagen, markers of chondrocyte differentiation. The observations suggest that IGF-II may play a significant role in avian growth plate physiology, which is consistent with several reports on mammalian endochondral bone growth.