Sperm mRNA found in eggs

Presence in newly fertilized ova provides insight into parthenogenesis and cloning

May 13, 2004
Cathy Holding(cathyholding@aol.com)

A team of researchers has observed sperm mRNA in newly fertilized eggs, according to a paper published in the May 13 Nature, in a finding that provides alternative explanations for mammalian parthenogenesis, cloning, and male infertility, the team writes. The results would also have immediate applications for treating infertile couples and for providing a screen for toxicological effects in spermatogenesis, said coauthor Steven Krawetz.

Krawetz's team identified six transcripts present in sperm, but not in unfertilized eggs, and followed the delivery of two of them—clusterin and protamine-2—into eggs using real-time reverse transcriptase polymerase chain reaction.

“It's really the first demonstration that human sperm contain a population of RNAs,” said Krawetz, a professor in the Department of Obstetrics and Gynecology at Wayne State University. “Specifically, we show messenger RNAs, and those are delivered to the oocyte upon fertilization.”

The mRNA must exist before the single fertilized zygote has divided into four and then eight cells, Krawetz told The Scientist. “After [that], you have activation of the embryonic genome, and that takes over,” he said. “So the egg has its own store of mRNA, and now we show that the sperm has its own store of mRNA.”

Krawetz believes that the type of assay developed in the paper can produce a fingerprint of the RNAs present in spermatozoa that could be used to examine differences between fertile and infertile male individuals. “We'll be able to critically examine the role of the male,” he said. In addition, the team is hoping to take this fingerprint technology “to the next level” by providing a screen for different toxicological effects to which a male may have been exposed, he said.

“We completely agree that the paternal fingerprints are obviously concerned at several steps,” said Serge Carreau, director of the Institut de Biologie Fondamentale et Appliquée at the Université de Caen Basse-Normandie in France, “not only at oocyte fertilization, as this paper said… but also we have found that the human spermatozoa is equipped in terms of specific mRNAs that are really also concerned with the quality of the sperm, for motility, and the acrosome reaction.”

“It's not only clusterin and protamine-2. We have also other markers like, for example, aromatase or the estrogen receptor—they are really relevant,” said Carreau, who was not involved in the study.

Krawetz said the team's results could explain the difficulties encountered during the creation of a mammalian parthenote recently published in Nature. “We think that actually the reason why these techniques are fraught with these problems, and the success rate is low, is that they are missing the RNA factor… So what we think is happening is that the RNAs that are supplied [by the sperm] are involved in… the developmental switch that is triggered that then makes the subsequent normal development proceed,“ Krawetz said.

“The scientists must more carefully decide the function of the messenger RNAs which are being introduced into the oocyte at fertilization,” Tomohiro Kono, whose group created the parthenogenetic mouse, told The Scientist.

While mitochondria from the sperm are introduced into the oocyte at fertilization, almost all of the mitochondria are excluded during the first cleavage, and so these mRNAs may not be useful, said Kono, professor in the Department of Biosciences at Tokyo University of Agriculture, who was not involved in the study.

“I think it boils down to knowing whether these messages are necessary for development,” Hugh Clarke, research director in the Department of Obstetrics and Gynecology, Royal Victoria Hospital, Montreal, said. “If they are, then it has major implications for cloning because likely when one made cloned animals, one wouldn't be delivering those messages, like the ones they mention here.”

Clarke said he thinks it all depends on whether the messages delivered by the sperm are needed by the embryo. “And we don't know that yet,” he said. “We know that the embryo gets them, but whether the embryo cares? Those are tough experiments to do, for sure.”

Correction (posted May 14): When originally posted, this article gave the incorrect affiliation for Tomohiro Kono. He is at the Tokyo University of Agriculture, not the Tokyo University of Agriculture and Technology. The Scientist regrets the error.