IGFs Mediate the Action of LH on Oocyte Maturation in Zebrafish

Abstract

LH signaling is required for oocyte maturation in fish and other vertebrates. However, the downstream factors mediating LH signaling are largely unexplored in fish. In this study, we investigated whether IGFs could mediate LH action on oocyte maturation in zebrafish. Our results show that all igfs, including igf1, igf2a, igf2b, and igf3, are dynamically expressed during folliculogenesis, with the expression of igf3 reaching its maximal level in full grown stage follicles. The expression of igfs is regulated by LH through a cAMP pathway in intact follicles as well as in primary cultured follicular cells, with igf3 expression being the most sensitive to human chorionic gonadotropin (hCG) treatment. Moreover, recombinant zebrafish IGF-2a, IGF-2b, and IGF-3 proteins significantly enhanced oocyte maturation via IGF-1 receptors (IGF-1rs), with IGF-3 exhibiting the most potent stimulatory action on oocyte maturation. Furthermore, we have demonstrated that IGF-3 or hCG treatment could stimulate IGF-1rs phosphorylation, and hCG-induced oocyte maturation could be attenuated by IGF-1r inhibitors as well as by an anti-IGF-3 antiserum in vitro and in vivo, indicating that the IGF system especially IGF-3 plays a crucial role in mediating LH action on oocyte maturation. In addition, igf3 expression is significantly attenuated in LH β-subunit (lhb) mutant zebrafish and treatment with recombinant IGF-3 could partially rescue the oocyte maturation defects of the lhb mutants in vitro and in vivo. Collectively, our results clearly demonstrated that IGFs, particularly the gonad-specific IGF-3, act as important mediators of LH action on oocyte maturation in zebrafish.

Releasing prophase arrest in zebrafish oocyte: synergism between maturational steroid and Igf1

“…activin and Igf family members) in mediating Lh action in teleosts ovary (Pang & Ge 2002, Li et al 2015. While Egf has recently been shown to act as an essential mediator of Lh action during mammalian OM (Park et al 2004, Jamnongjit et al 2005, presence of an active Igf system consisting of Igf ligands (Igf1, Igf2a, Igf2b and Igf3), receptors (Igf1ra and Igf1rb) and Igf binding proteins have been reported earlier in zebrafish ovary (Nelson & Van Der Kraak 2010b, Reinecke 2010, Li et al 2015.…”

“…While mPRa as well as Igf1r have been identified in zebrafish oocyte (Nelson & Van Der Kraak 2010b, Hanna & Zhu 2011, a close association between the putative mPRa and OM in seatrout and zebrafish has been indicated earlier as rapid increase in receptor levels occurred in fully grown oocytes in vitro during hCGinduced GVBD (Zhu et al 2003, Hanna & Zhu 2011. Besides, involvement of Igf1r in hCG-induced meiotic maturation has been demonstrated in zebrafish oocytes (Li et al 2015). Further, involvement of DHP and/or Igf1 in activation of down-stream signalling molecules like Akt phosphorylation and PKA down-regulation, during the natural course of GVBD response, provides evidence in favour of synergistic action between MIS and Igf1.…”

“…Besides, earlier evidences have established that in addition to its role in positive regulation of mPR expression (Peng & Thomas 2010), DHP at higher concentration can activate nuclear progestin receptor (nPR) to promote genomic mechanism for OM (Hanna & Zhu 2011) or reduce Gper levels in the oocytes (Pang & Thomas 2010). Alternatively, while relative abundance of igf2a, igf2b and igf3 outweighs igf1 expression in the zebrafish ovary (Nelson & Van Der Kraak 2010b, Li et al 2015, recently it has been demonstrated that hCG stimulation promotes a robust increase in igf3 expression and igf1 to a lesser extent prior to resumption of meiosis (Li et al 2015). Besides, binding of Igf1 to insulin and/or insulin/Igf1 hybrid receptor may activate signalling molecules at higher magnitude (Belfiore et al 2009), suggesting that activation of maturational steroid and Igfmediated signalling pathways together may engage in complex interaction for alleviating prophase arrest in this species.…”

“…activin and Igf family members) in mediating Lh action in teleosts ovary (Pang & Ge 2002, Li et al 2015. While Egf has recently been shown to act as an essential mediator of Lh action during mammalian OM (Park et al 2004, Jamnongjit et al 2005, presence of an active Igf system consisting of Igf ligands (Igf1, Igf2a, Igf2b and Igf3), receptors (Igf1ra and Igf1rb) and Igf binding proteins have been reported earlier in zebrafish ovary (Nelson & Van Der Kraak 2010b, Reinecke 2010, Li et al 2015. Even though Igf1 could induce GVBD in mummichog and striped bass denuded oocytes or follicles when incubated with steroidogenesis inhibitors (Negatu et al 1998, Weber & Sullivan 2000, Igf2 has been shown to up-regulate synthesis of mPRa in southern flounder full-grown follicles (Picha et al 2012).…”

“…Even though Igf1 could induce GVBD in mummichog and striped bass denuded oocytes or follicles when incubated with steroidogenesis inhibitors (Negatu et al 1998, Weber & Sullivan 2000, Igf2 has been shown to up-regulate synthesis of mPRa in southern flounder full-grown follicles (Picha et al 2012). Moreover, available data indicate that while it requires almost 16 h for relatively high dose (2 mm) of Igf3, Igf1 at 100 nM could trigger a significant increase in meiotic maturation within the first 6 h in zebrafish full-grown and mid-vitellogenic follicles in vitro (Nelson & Van Der Kraak 2010a, Li et al 2015. Further, in vitro incubation of zebrafish ovarian follicles with Igf1 alone or in combination with hCG stimulates synthesis of DHP, but not E 2 (Nelson & Van Der Kraak 2010a).…”