IGF-2 mRNA binding protein 2 regulates primordial germ cell development in zebrafish

Insulin-like growth factor 2 mRNA binding protein-2 (IGF2BP2 or IMP2) is a member of a conserved family of RNA binding proteins. These proteins bind to and regulate target mRNA localization, stability, and translation. Their structure, expression and functions in bony fish are not well understood. Here, we characterized the zebrafish igf2bp2 gene and investigated its functional role in early development. Zebrafish igf2bp2 gives rise to 4 alternatively spliced transcripts. When expressed in cultured cells, all 4 proteins were detected in the cytoplasm. Igf2bp2-A, the longest isoform, has a domain structure similar to its mammalian counterpart. Igf2bp2-B lacks one of the C-terminal KH domains, while Igf2bp2-C lacks the two N-terminal RRM domains. Igf2bp2-D lacks both regions. In adult fish, these igf2bp2 isoforms were detected exclusively in the oocyte. After fertilization, they disappeared within 6 h post fertilization (hpf). At 20 ~ 24 hpf, igf2bp2-A mRNA, but not other mRNAs, was re-expressed in the embryos including in primordial germ cells. Targeted knockdown of Igf2bp2s reduced the numbers of primordial germ cells but did not affect global patterning or growth. The effect was rescued by overexpression of Igf2bp2-A. Likewise, dominant-negative inhibition of Igf2bp2 resulted in a similar reduction in primordial germ cell number. These results not only provide new information about the structure and expression of zebrafish Igf2bp2, but also reveal a critical role of this conserved RNA binding protein in primordial germ cell development.