3 Endocrine system controls Salmonidae growth
Our research group is investigating how Salmonidae growth is regulated by hormones. An endocrine system called the growth hormone (GH) – insulin-like growth factor (IGF)-I system plays a central role in growth regulation. Within this system, we focus on IGF-I and IGF-binding proteins that regulate its activity.
We often hear the words metabolism and growth. Their relationship is shown in the figure below. Metabolism can be divided between anabolism, which is a process through which organic matter is synthesized from simple substances through energy consumption; and catabolism, which decomposes complex substances to extract energy. The surplus from anabolism is then utilized for growth in the processes of proliferation, differentiation, and hypertrophy of the cells that support the body. However, when animals are starved, this surplus is degraded, and the process is switched to catabolism for procuring energy. Metabolism and growth are linked in these ways.
The growth of vertebrates, including fish, is primarily regulated by growth hormone (GH) and insulin-like growth factor (IGF)-I. GH is secreted from the pituitary gland either directly or indirectly through IGF-I in target organs such as the bones and muscles. IGF-I is secreted into the blood mainly through the stimulation of GH. Blood IGF-I not only mediates the action of GH but also inhibits the synthesis and secretion of GH in the pituitary gland. This is called negative feedback. The GH – IGF-I endocrine system remains autonomous in this way. IGF-I not only promotes cell proliferation but also helps improve adaptation to seawater when acting in the gills. It also contributes to the development of the gonads. Why does IGF-I, which has such a diverse range of effects, act on the appropriate organ at the appropriate time? Several IGF-binding proteins (IGFBPs) play an important role in this aspect.
Most of the IGF-I exists bound to IGFBPs. IGFBPs are capable of inhibiting IGF-I from binding to the receptor or/and promoting the effects of IGF-I by transporting it to target cells and passing it to the receptor while protecting it from degradation. Types 1-6 of IGFBPs are known; each of these exhibits different IGF-I inhibitory / promoting functions. The activity of the IGF-I is strictly regulated by multiple IGFBPs in this way, and IGFBP is an important element in understanding the growth regulation mechanism.
IGFBP regulates the growth-promoting effect of IGF-I. Three main types are found in the blood of Salmonids, namely, IGFBP-2b, -1a, and -1b in descending order of molecular weight. These are important in terms of their ability to regulate IGF-I activity. However, we believe that these are also useful as growth indicators in terms of reflecting growth status. As shown in the figure below, differences in the IGFBP patterns in the blood can be observed when juvenile Chinook salmon (Oncorhynchus tshawytscha) are reared separately in a feeding group and fasting group. For example, the amount of IGFBP-2b is high in the feeding group, whereas IGFBP-1b is high in the fasting group. Using these IGFBPs together with IGF-I as the positive and negative indicators of growth enables the possibility of more accurately evaluating the growth rate of salmon. This is expected to improve the efficiency of the aquaculture industry and hatchery release program.
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