Two grass carp (Ctenopharyngodon idella) insulin-like growth factor-binding protein 5 genes exhibit different yet conserved functions in development and growth

Insulin-like growth factor binding-protein 5 (igfbp5), the most conserved member of the IGFBP family in vertebrates, plays a critical role in controlling cell survival, growth, differentiation, and apoptosis. Here, we characterized the expression patterns of igfbp5a and igfbp5b in grass carp (Ctenopharyngodon idella), which are retained in many fish species, likely from the teleost-specific whole-genome duplication. Both igfbp5a and igfbp5b encode 268- and 263-aa peptides, respectively, which share a sequence identity of 71%. Their mRNAs are not detected in zygotes. At 14 hpf, grass carp igfbp5b mRNA was detected in the somites, while igfbp5a mRNA has some possible signal around the eye and head region. At 24 hpf, both igfbp5a and igfbp5b mRNA appear to be limited to the presomitic mesoderm. At 36 hpf, igfbp5a mRNA was only detected in the midbrain, while igfbp5b mRNA was detected in both the midbrain and notochord. Overall, both mRNAs were expressed in most adult tissues. igfbp5a and igfbp5b were significantly upregulated in the muscle and liver after injection of 10 μg per kilogram body weight of zebrafish growth hormone (zGH), while their hepatic expression was downregulated by 50 μg zGH. During fasting, both igfbp5a and igfbp5b mRNAs were significantly downregulated in the muscle but upregulated in the liver. Collectively, the results suggest that the two igfbp5 genes play important but different roles in the regulation of growth and development in grass carp.