Stem Cells Coaxed to Make Precursors to Egg, Sperm
In long term, advance may lead to better fertility treatments, experts say
WEDNESDAY, Oct. 28, 2009 (HealthDay News) -- In a step that might someday aid infertile couples, scientists have nudged stem cells to become human germ cells -- precursors to egg and sperm.
"For the first time we have a human genome-based system for how to make a germ cell or not," said lead researcher Renee Reijo Pera, director of the Center for Human Embryonic Stem Cell Research and Education at Stanford University School of Medicine. "I think it will play out to people's benefit because 10 to 15 percent of couples cannot have children."
The study, published online Oct. 28 in the journal Nature, provides both an experimental system for germ cell development as well as a method for coaxing germ cell production from embryonic stem cells (ESCs). It also identifies three molecules called RNA-binding proteins that are critical to the process, members of the so-called DAZ gene family.
Researchers can exploit the resulting system to study germ cell formation -- and in particular, ways it can go awry -- and to develop drugs that can manipulate the process, explained Reijo Pera.
More long-term, the system should ultimately aid infertile couples by providing a mechanism for generating sperm or eggs (gametes) in the laboratory, which can then be used for in vitro fertilization.
"This research basically tells us how to make a gamete," said Dr. Raymond Anchan, a fertility specialist and stem cell researcher at Brigham and Women's Hospital in Boston. "If we understand this, then we can think about making gametes from patient-specific stem cells," said Anchan, who was not involved in the study.
According to the National Women's Health Information Center, infertility affects approximately 10 percent of U.S. women -- some 6.1 million women aged 15 to 44. For many of those individuals, and also for many men, infertility stems from low or abnormal gamete production. Though researchers have had sporadic success turning human ESCs into germ cells, the process has been inefficient and difficult to study in the laboratory. Reijo Pera decided to try to change that.
The Stanford team knew that feeding human ESCs growth factors, called bone morphogenetic proteins, would induce the cells to differentiate into germ cells. Led by postdoctoral fellow Kehkooi Kee, they developed a "reporter gene" system to tag and identify the cells they wanted -- those that were differentiating into germ cells -- by making them glow fluorescent. That fluorescence would then enable the team to count developing germ cells, and also purify them.
Using various methods, the team determined that the fluorescent cells tagged by Kee's reporter system were so-called "primordial" germ cells. Accounting for just 5 percent of the cell population in this study overall, these cells are basically early, immature germ cells.
Experimental system in hand, the team then turned to the role of the DAZ protein family in the production of these very early cells. DAZ proteins were already known to be associated with male infertility; that's how Reijo Pera first identified them in the 1990s. The question was: How exactly do they affect germ cell development?
By alternatively blocking and boosting the expression of the DAZ family members DAZ, DAZL and BOULE, and counting the cells that emerged, the team determined that DAZL is required for differentiation in primordial germ cells, while DAZ and BOULE push the cells further towards the production of true gametes. Importantly, by overexpressing all three proteins, the team produced cells with the same genetic content as gametes.
"We didn't know these genes are major players in germ cell development," said Reijo Pera. "We suspected it, but here we can directly show that we get differences in the pathway. For a biologist, that's incredibly exciting."
Reijo Pera said the system can now help researchers map germ cell development and identify factors that can enhance or inhibit that process. Longer term, her team wants to coax germ cells to differentiate from adult, non-embryonic stem cells -- using cells such as skin cells that are manipulated to regress to an embryo-like state.
DAZ proteins may also represent potential drug targets for infertility research, Reijo Pera added.
For Anchan, the study represents an "incremental, but significant, advance."
"People have shown that you can get germ cells from embryonic stem cells," said Anchan. "What is unique in this study is they take it beyond the early stages of development." In other words, the research team pushed the cells to the point of actually generating gametes.
The question remains, though, whether these cells are functional. In other words, can they actually fertilize an egg and develop into a viable organism?
"They haven't shown that these are functional gametes," Anchan said. "They look like gametes, but can they be fertilized?"
In 2004, Dr. George Daley, director of the Stem Cell Transplantation Program at Children's Hospital Boston, generated gametes from embryonic stem cells in mice. He called the new study "a very interesting extension of our earlier work."
But Daley questioned the system's ability to really aid infertile couples, at least in the near term.
"The reality is, the system is a tool for basic studies at this point," Daley said. "Anything more is years, if not decades, away."
For more information on infertility, visit the National Women's Health Information Center.