Full-term mouse development by abolishing Zn2+-dependent metaphase II arrest without Ca2+ release

Abstract

In vertebrates, a rise in intracellular free Ca2+ (Ca2+i) levels during fertilization initiates second metaphase (mII) exit and the developmental programme. The Ca2+ rise has long been considered to be crucial for development, but verifying this contribution would benefit from defining its role during fertilization. Here, we delineate the role of Ca2+ release during mII exit in wild-type mouse eggs and show that it is dispensable for full-term development. Exit from mII can be induced by Zn2+-specific sequestration without Ca2+ release, eliciting Cyclin B degradation in a manner dependent upon the proteasome pathway and intact microtubules, but not accompanied by degradation of the meiotic regulator Emi2. Parthenogenotes generated by Zn2+ sequestration developed in vitro with normal expression of Ca2+-sensitive genes. Meiotic exit induced by either Ca2+ oscillations or a single Ca2+ rise in oocytes containing a signaling-deficient sperm resulted in comparable developmental rates. In the absence of Ca2+ release, full-term development occurred ∼50% less efficiently, but at readily detectable rates, with the birth of 27 offspring. These results show in intact mouse oocytes that Zn2+ is essential for mII arrest and suggest that triggering meiotic exit is the sole indispensable developmental role of Ca2+ signaling in mammalian fertilization.

Oocyte–cumulus cell interactions regulate free intracellular zinc in mouse oocytes

“…Cumulus cells regulate the timing of the increase in free intracellular zinc in the oocyte during maturation. This effect of cumulus cells is important because an increase in oocyte zinc is required for completion of meiosis (Kim et al 2010 and successful establishment of MII arrest (Suzuki et al 2010b. Thus, acute regulation of free intracellular zinc is a new process regulated by cumulus cell-oocyte interactions in the follicle.…”

“…Problems during maturation can disrupt programming of the maternal and/or embryonic genome resulting in epigenetic defects that affect development and even postnatal health (Doherty et al 2000, Mann et al 2004, Rivera et al 2008. Dramatic effects of zinc deficiency have been uncovered in oocytes (Kim et al 2010, Suzuki et al 2010b) and cumulus cells (Tian & Diaz 2012) during the periovulatory transition. Currently, the success of human IVM is highly variable (4-35% clinical pregnancy/ transfer) (Cha et al 1991, Trounson et al 1994, Child et al 2001, Gremeau et al 2012, Guzman et al 2012.…”

“…Total zinc increases during maturation and is required for the completion of the first meiotic division (Kim et al 2010), although this is independent of the MOS-MAPK pathway . Zinc is also required for establishment of metaphase II (MII) arrest through the regulation of the zinc-binding protein early meiosis inhibitor 2 (EMI2) (Suzuki et al 2010b. EMI2 inhibits the anaphasepromoting complex (APC) to block cyclin B1 ubiquitination and degradation during MII and is thus an essential component of the cytostatic factor activity in the mature oocyte (Masui & Markert 1971, Shoji et al 2006.…”

“…EMI2 inhibits the anaphasepromoting complex (APC) to block cyclin B1 ubiquitination and degradation during MII and is thus an essential component of the cytostatic factor activity in the mature oocyte (Masui & Markert 1971, Shoji et al 2006. EMI2 contains a zinc-binding region that is essential for blocking APC activity during MII arrest (Shoji et al 2006, Suzuki et al 2010a, 2010b. Exit from MII during oocyte activation requires a lowering of cellular zinc through the rapid export of zinc from the oocyte.…”

Single Ca2+ transients vs oscillatory Ca2+ signaling for assisted oocyte activation: limitations and benefits

“…CAMKII cRNA allowed oocyte activation in mouse (Knott et al 2006 (Suzuki et al 2010). Artificial Zn 2+ sequestration, bypassing the Ca 2+ mobilization and mediated by the specific Zn 2+ chelator N,N,N′,Ntetrakis (2-pyridylmethyl)ethane-1,2-diamine (TPEN), allows oocyte activation in several mammalian species, including human (Suzuki et al 2010, Lee et al 2015, Duncan et al 2016. In addition, full-term embryonic development was achieved in mice and pigs by using TPEN as the only agent for oocyte activation.…”