“Zinc Sparks” – Biological fireworks in preparation for 300 million years

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Frog eggs like the ones pictured here release zinc when fertilized, much like mammalian eggs do. Credit: Image by Tero Laakso / licensed to CC BY-SA 2.0

Scientists using the advanced photon source show how “zinc sparks” – showers of zinc ions released by mammalian eggs immediately after fertilization – also occur in amphibian eggs.

Five years ago, researchers at Northwestern University made international headlines when they discovered that human eggs, when fertilized with sperm, release billions of zinc ions, dubbed “sparks.” zinc. “

Now Northwestern has teamed up with the U.S. Department of Energy’s (DOE) Argonne National Laboratory and Michigan State University (MSU) to reveal that those same sparks shoot out of highly specialized compartments laden with metals. on the surface of the eggs when the frog eggs are fertilized. This means that the earliest chemistry of conception has evolutionary roots going back at least 300 million years, to the last common ancestor between frogs and humans.

“This work may help inform our understanding of the interaction between dietary zinc status and human fertility.” – Thomas O’Halloran, Professor, Michigan State University

And the research has implications beyond this shared biology and deeply rooted history. It could also help shape future discoveries about the impact of metals on early stages of human development.

This work can help inform our understanding of the interaction between dietary zinc status and human fertility, ”said Thomas O’Halloran, lead author of the research article published in June. 21, 2021, in the review Chemistry of nature.

O’Halloran was part of the original zinc spark discovery at Northwestern, and earlier this year he joined the state of Michigan as a founding professor of microbiology, molecular genetics, and chemistry. O’Halloran was the founder of Northwestern’s Chemistry of Life Processes Institute, or CLP, and remain a member.

The team also found that fertilized frog eggs eject another metal, manganese, in addition to zinc. It seems that these ejected manganese ions collide with the sperm surrounding the fertilized egg and prevent them from entering.

These breakthroughs support an emerging image that transition metals are used by cells to regulate some of the earliest decisions in an organism’s life, ”said O’Halloran.

To make these discoveries, the team needed access to some of the world’s most powerful microscopes as well as expertise spanning the chemistry, biology and physics of x-rays. This unique combination included collaborators from the Center for Quantitative Element Mapping for the Life Sciences, or QE-Map, an interdisciplinary research center funded by the National Institutes of Health in MSU and north-west CLP. The research relied heavily on the tools and expertise available at Argonne.

The research team brought sections of frog eggs and embryos to Argonne for analysis. Using both x-rays and electron microscopy, the researchers determined the identity, concentrations, and intracellular distributions of metals before and after fertilization.

X-ray fluorescence microscopy was performed on the beamline 2-ID-D of the advanced photon source (APS), a DOVE Office of Science User Facility in Argonne. Barry Lai, group leader at Argonne and author of the article, said the x-ray analysis quantified the amount of zinc, manganese and other metals concentrated in small pockets around the outer layer of eggs. They discovered that these pockets contained more than 30 times the manganese as the rest of the eggs, and ten times the zinc.

We are able to do this analysis because of the elemental sensitivity of the beamline, ”said Lai.In fact, it is so sensitive that considerably lower concentrations can be measured. “

Additional scans were performed by transmission electron microscopy at the Center for Nanoscale Materials (CNM), a DOVE Office of Science User Facility in Argonne. Further analysis was carried out on a prototype split-scanning transmission electron microscope that includes technology developed by Argonne lead scientist Nestor Zaluzec, author of the article. These analyzes were performed at smaller scales – down to a few nanometers, approximately 100,000 times smaller than the width of a human hair – but found the same results: high concentrations of metals in the pockets around the outer layer.

X-ray and electron microscopy showed that the metals in these pockets were almost completely released after fertilization.

Argonne has the tools to examine these biological samples at these scales without destroying them with X-rays or electrons, ”Zaluzec said.It’s a combination of the right resources and the right expertise.

the APS is undergoing a massive upgrade, which will increase the brightness of its x-ray beams up to 500 time. Lai said that an upgrade APS could complete these analyzes much faster or with higher spatial resolution. What took over an hour for this search could be done in less than a minute after upgrading, Lai said.

We often think of genes as key regulatory factors, but our work has shown that atoms like zinc and manganese are essential in the early stages of development after fertilization, ”said MSU Provost Teresa K. Woodruff, Ph.D., another senior author on the paper.

Woodruff, a MSU founding professor and former member of CLP, was also a leader of the Northwestern team that discovered zinc sparks five years ago. With the discovery of manganese sparks in African clawed frogs, or Xenopus laevis, the team is excited to explore whether the element is released by human eggs when they are fertilized.

These discoveries could only be made by interdisciplinary groups, fearlessly examining the fundamental steps, ”she said.Working in multiple cutting-edge disciplines is one of the deepest ways to make new discoveries. “

Xenopus is a perfect system for such studies because their eggs are an order of magnitude larger than human or mouse eggs and are accessible in large numbers, ”said Carole LaBonne, another lead author of the study, CLP member and chair of Northwestern’s Department of Molecular Biosciences.The discovery of zinc and manganese sparks is exciting and suggests that there may be other fundamental signaling roles for these transition metals.

Reference: “Metal ion fluxes controlling the fertilization of amphibians” by John F. Seeler, Ajay Sharma, Nestor J. Zaluzec, Reiner Bleher, Barry Lai, Emma G. Schultz, Brian M. Hoffman, Carole LaBonne, Teresa K. Woodruff and Thomas V O’Halloran, June 21, 2021, Chemistry of nature.
DOI: 10.1038 / s41557-021-00705-2



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