The ZBED6–IGF2 axis has a major effect on growth of skeletal muscle and internal organs in placental mammals

Abstract

A single nucleotide substitution in the third intron of insulin-like growth factor 2 (<i>IGF2</i>) is associated with increased muscle mass and reduced subcutaneous fat in domestic pigs. This mutation disrupts the binding of the ZBED6 transcription factor and leads to a threefold up-regulation of <i>IGF2</i> expression in pig skeletal muscle. Here, we investigated the biological significance of ZBED6-<i>IGF2</i> interaction in the growth of placental mammals using two mouse models, ZBED6 knock-out (<i>Zbed6</i>^-/-) and <i>Igf2</i> knock-in mice that carry the pig <i>IGF2</i> mutation. These transgenic mice exhibit markedly higher serum IGF2 concentrations, higher growth rate, increased lean mass, and larger heart, kidney, and liver; no significant changes were observed for white adipose tissues. The changes in body and lean mass were most pronounced in female mice. The phenotypic changes were concomitant with a remarkable up-regulation of <i>Igf2</i> expression in adult tissues. Transcriptome analysis of skeletal muscle identified differential expression of genes belonging to the extracellular region category. Expression analysis using fetal muscles indicated a minor role of ZBED6 in regulating <i>Igf2</i> expression prenatally. Furthermore, transcriptome analysis of the adult skeletal muscle revealed that this elevated expression of <i>Igf2</i> was derived from the P1 and P2 promoters. The results revealed very similar phenotypic effects in the <i>Zbed6</i> knock-out mouse and in the <i>Igf2</i> knock-in mouse, showing that the effect of ZBED6 on growth of muscle and internal organs is mediated through the binding site in the <i>Igf2</i> gene. The results explain why this ZBED6 binding site is extremely well conserved among placental mammals.