In mammals, when a sperm cell fertilizes an egg, it triggers the release of zinc cations in waves called zinc sparks. Researchers led by chemist Thomas V. O’Halloran and biologist Teresa K. Woodruff, a husband-and-wife team at Northwestern University, recently examined mouse eggs with a combination of live-cell imaging and elemental mapping and determined that these sparks originate from vesicles located near an egg’s outer membrane (Nat. Chem. 2014, DOI: 10.1038/nchem.2133).
To do the live-cell imaging, the researchers developed a probe molecule that fluoresces when it binds zinc. The compound signals zinc’s presence at probe concentrations as low as 50 nM—significantly lower than previous zinc probes, O’Halloran says.
Lead author Emily L. Que also used two synchrotron X-ray fluorescence methods and electron microscopy to map and quantify the zinc distribution within the mouse eggs. The combined data reveal a system of some 8,000 zinc-containing vesicles near the egg’s outer membrane on one side of the cell. Each vesicle holds about a million zinc atoms. All together, these vesicles contain approximately 15% of the zinc in the egg.
“But where is the remaining 85%, and what does it do?” asks Anthony C. F. Perry, a developmental biologist at the University of Bath, in England. “Is it also compartmentalized? These are questions for the future.”
The images the researchers obtained reveal that the zinc vesicles are located alongside another set of vesicles called cortical granules. The granules are known to release enzymes and other proteins—including ones that depend on zinc to function—during fertilization.
Scientists have also known that cortical granules are involved in the modification of the zona pellucida, a glycoprotein matrix surrounding an egg. After fertilization, chemical changes in the zona pellucida form a barrier that prevents other sperm cells from entering the egg. The zinc vesicles may be involved in the chemical changes, the team says. Measurements by O’Halloran, Woodruff, and coworkers reveal that the zinc concentration in the vesicles is as high as 0.2 M. Each zinc spark releases the contents of about 2,000 vesicles into the zona pellucida.
“That’s an extraordinary change in zinc concentration,” O’Halloran says. “What happens to proteins with zinc-binding side chains when they are exposed to that high a concentration of zinc? What happens to the sugars in the glycoproteins at the cell surface and the zona pellucida when they see that much zinc? These are questions that we’re just beginning to explore.”
According to Amy E. Palmer, an associate chemistry professor at the University of Colorado, Boulder, the new research is “particularly striking” because the team was able to account for all the zinc atoms during the release. “Such an approach will likely also be important for tracking the distribution of zinc during cell-signaling processes.”