In a study published in Nature Chemistry, Northwestern researchers have discovered that in the mammalian egg, zinc is stored in tiny packages just below the cell surface. These packages are released in waves following fertilization in events called ‘zinc sparks.’ These sparks are akin to neurotransmitter release in the brain or insulin release in the pancreas. This fluorescence image of an egg labeled for zinc (green) and DNA (blue) depicts how the egg stores zinc in thousands of tiny packages. These are released during fertilization to form the zinc spark.Northwestern UniversityUsing cutting-edge technology they developed, including new high-energy X-ray imaging techniques, the team is the first to capture images of these molecular fireworks and pinpoint the origin of the zinc sparks: tiny zinc-rich packages just below the egg’s surface.

Zinc flux plays a central role in regulating the biochemical processes that ensure a healthy egg-to-embryo transition, and this new unprecedented quantitative information should be useful in improving in vitro fertilization methods.

“The amount of zinc released by an egg could be a great marker for identifying a high-quality fertilized egg, something we can’t do now,” said Teresa K. Woodruff, an expert in ovarian biology and one of two corresponding authors of the study. “If we can identify the best eggs, fewer embryos would need to be transferred during fertility treatments. Our findings will help move us toward this goal.”

Woodruff is the Thomas J. Watkins Professor of Obstetrics and Gynecology and director of the Women’s Health Research Institute at Northwestern University Feinberg School of Medicine.

The study, to be published Dec. 15 by the journal Nature Chemistry, provides the first quantitative physical measurements of zinc localization in single cells in a mammal, using mouse eggs. The research team developed a suite of four physical methods to determine how much zinc there is in an egg and where it is located at the time of fertilization and in the two hours just after. Sensitive imaging methods allowed the researchers to see and count individual zinc atoms in egg cells and visualize zinc spark waves in three dimensions. 

After inventing a novel vital fluorescent sensor for live-cell zinc tracking, scientists discovered close to 8,000 compartments in the egg, each containing approximately one million zinc atoms. These packages release their zinc cargo simultaneously in a concerted process, akin to neurotransmitter release in the brain or insulin release in the pancreas.

These findings were further confirmed with chemical methods that trap cellular zinc stores and enable zinc mapping on the nanometer scale in a custom-designed electron microscope developed for this project with funding from the W.M. Keck Foundation. Additional experiments at the Advanced Photon Source synchrotron at Argonne enabled the scientists to precisely map the location of zinc atoms in two and three dimensions.

“On cue, at the time of fertilization, we see the egg release thousands of packages, each dumping a million zinc atoms, and then it’s quiet,” said Thomas V. O’Halloran, the other corresponding author. “Then there is another burst of zinc release. Each egg has four or five of these periodic sparks. It is beautiful to see, orchestrated much like a symphony. We knew zinc was released by the egg in huge amounts, but we had no idea how the egg did this.”

O’Halloran is the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences and director of Northwestern’s Chemistry of Life Processes Institute.

The study establishes how eggs compartmentalize and distribute zinc to control the developmental processes that allow the egg to become a healthy embryo. Zinc is part of a master switch that controls the decision to grow and change into a completely new genetic organism.

The studies reported in Nature Chemistry are the culmination of six years of work and build on prior discoveries in the Woodruff and O’Halloran labs, and work they conducted at the APS. In previous studies in mouse eggs by this research team, the egg’s tremendous zinc requirement for reaching maturity was discovered. In addition, the researchers determined that an egg loses 10 billion of its 60 billion zinc atoms upon fertilization in a series of four or five waves called “zinc sparks.” Release of zinc sparks from the egg is essential for embryo formation in the two hours following fertilization.

“The egg first has to stockpile zinc and then must release some of the zinc to successfully navigate maturation, fertilization and the start of embryogenesis,” O’Halloran said. “But exactly how much zinc is involved in this remarkable process and where is it in the cell? We needed data to better understand the molecular mechanisms at work as an egg becomes a new organism.”

One major hurdle O’Halloran and Woodruff faced was the lack of sensitive methods for measuring zinc in single cells. To address this problem, they formed a collaborative team with other researchers in Northwestern’s Chemistry of Life Processes Institute to develop the tools they needed.

Key members of the team were Vinayak P. Dravid, the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science, and Stefan Vogt, a physicist and group leader of microscopy at the Advanced Photon Source at Argonne. Dravid and Vogt are authors of the paper.

“We had to develop a slew of methods to be convinced we were seeing the right thing,” O’Halloran said. “Science is about testing and retesting ideas. All of our complementary results point to the same conclusion: the zinc originates in packages called vesicles near the cell’s surface.”

The researchers currently are working to see if they can correlate zinc sparks with egg quality, information that would be key to improving fertility treatments.

Not only are these new imaging techniques important for describing the zinc spark, they can be applied to other cells that likely use zinc in a similar way, but whose workings remain elusive due to the lack of sensitive and specific tools. This study lays the groundwork for understanding how zinc fluxes can regulate events in multiple biological systems beyond the egg, including neurotransmission from zinc-enriched neurons in the brain and insulin-release in the pancreas.

Proposed upgrades to the APS will dramatically enhance future studies in this area. At the moment, X-ray tools cannot effectively probe trace metal content in biological samples at the resolution and sensitivity required to map small clusters of metals as they interact with transporters on cell surfaces and vesicles.

“Upgrading the APS will dramatically increase the spatial resolution, sensitivity, and speed at which we can visualize trace elemental content in three dimensions in a statistically significant number of cells,” Vogt pointed out. “This will not only make it possible to visualize in 3-D across the whole egg the zinc at the membrane and the individual vesicles in the cell that emit sparks of zinc to make the egg fertilizable, but allow us to do so with dozens of samples at all relevant time points.”

The National Institute of General Medical Sciences, the National Institute of Child Health and Human Development and the W.M. Keck Foundation supported the research, which made use in part of APS, a DOE Office of Science User Facility.

The title of the paper is “Quantitative mapping of zinc fluxes in the mammalian egg reveals the origin of fertilization-induced zinc sparks.”

In addition to O’Halloran, Woodruff, Dravid and Vogt, other authors of the paper are lead author Emily L. Que, Reiner Bleher, Francesca E. Duncan, Betty Y. Kong, Seth A. Garwin and Amanda R. Bayer, of Northwestern; and Sophie C. Gleber and Si Chen, of Argonne.