Reactive oxygen species (ROS) signal early embryonic development

Reactive oxygen species ROS can act as cellular signaling molecules. In a recently published study, Enrique Amaya and colleagues have demonstrated that fertilization signals an increase in the ROS in developing embryos of the African clawed frog, Xenopus laevis (PMID: 29298423) . Using a transgenic frog expressing the fluorescent ROS indicator, HyPer, this group has previous demonstrated that ROS signaling plays an important role in tail regeneration following amputation, in X. laevis tadpoles. Using these transgenic animals, they realized that HyPer was expressed in the fertilization-competent X. laevis eggs.

A possible role for ROS in fertilization was previously explored in sea urchin, where the dual oxidase protein (Udx1) was shown to be responsible for ROS generation following fertilization (Wong et al. 2004). Importantly, inhibition of ROS in sea urchin embryos inhibited cell division (Wong & Wessell 2005). These data suggested a prominent role for ROS in regulation of the embryonic cell cycle in invertebrates.

Here, the authors demonstrate that fertilization increases ROS levels in X. laevis eggs, and that this increase is downstream of the Ca2+ wave that initiates embryonic development. They also demonstrate that enzymes involved in the mitochondrial electron transport chain, are responsible for the ROS generation.  For example, blockade of complexes II, III, or IV, of the ETC, diminished or completely blocked ROS generation. The authors then go on to demonstrate that cleavage furrow development is halted eggs inseminated in the presence of these inhibitors. For example, embryos inseminated in 5 mM malonate (an inhibitor of complex II), fail to develop past the 4-cell stage. Similar to the sea urchin study, these data suggest that ROS generation may play a role in cell cycle progression.  Noting that these inhibitors are reversible, the authors found that embryos progress with normal development once the inhibitors are removed.

Finally, the authors sought to uncover the mechanism for ROS signaling in early embryonic development. Specifically, they turned toward the cyclin B/cyclin-dependent kinase 1 (Cdk1) complex, which plays a commanding role in cell cycle regulation. Importantly, Cdc5 activates the cyclin-B-Cdk1 complex to induce entry into mitosis, by dephosphorylating the inhibitory-phosphorylated T14 and Y15 residues in Cdk-1. Noting that some ROS species can modulate phosphatase activity, and Cdc25c is a phosphatase, they asked whether this protein may be the target of ROS.  Using immunoblots, they demonstrate that Cdc25 phosphorylation was diminished in embryos activated in the presence of ROS inhibitors, but was  not altered by treatments that didn’t alter ROS generation.

Now that a role for ROS in early embryonic development of a vertebrate has been established, it will be interesting to explore where similar signaling pathways are also evoked in mammalian fertilization.