Budding yeast in a UC San Francisco lab are yielding possible insight into a key step leading to the creation of human sperm. And the finding, say the UCSF researchers, could ultimately pave the way for an understanding of why human sperm sometimes doesn't fully develop, causing sterility.
In their study, published in the April issue of Molecular Cell, the UCSF researchers have identified the molecular steps that are involved in pushing a yeast spore, the equivalent of human sperm, through a critical stage of development. And they have determined that when these steps don't take place development is arrested--permanently.
Intriguingly, this finding may offer insight into the molecular glitch that causes a male infertility syndrome. Men with the infertility syndrome have immature sperm that are developmentally interrupted at the same stage of creation as was observed in defective yeast spores.
"This observation sets out a whole paradigm for exploring the human genetics of infertility in men," says Renee Reijo, a leading researcher on sperm-related infertility in men and a UCSF assistant professor of obstetrics-gynecology and reproductive sciences. "It is probably the most elegant model for examining the creation of sperm I've seen."
"If nothing else," says lead author Shelley Chu, a graduate student in the laboratory of Ira Herskowitz, PhD, professor of biochemistry and biophysics, and co-director of the Program in Human Genetics at UCSF, "this study provides a framework for understanding how sperm progress through this critical stage of development. Until now, we didn't have a clue about the molecular targets responsible for this transition." Chu is in the UCSF Program of Genetics and Cell Biology.
While yeast, single-celled fungi, are best known for their role in making beer brew and bread rise, their spores, which carry the genes leading to offspring, have a lot in common with sperm.
Both spore and sperm are created from other cells only after undergoing a complex process known as meiosis, in which a cell's DNA replicates and then undergoes two steps of division. Spore and sperm also have unique shapes relative to the cells that created them. In addition, two matured yeast spores can fuse to produce progeny just as sperm and egg fuse to produce offspring.
Spore and sperm are also both susceptible to being blocked in mid development, just before the cells from which they are produced undergo the first of the two divisions of their DNA. This interruption occurs at a stage of meiosis called pachytene and, in most cases, is temporary and beneficial, serving as a period during which the cell can repair any damage that may have occurred up to that point in development.
At least in human sperm, however, interruption can be permanent, preventing the further development of the cell--and thereby causing sterility, as in testicular maturation arrest.
In the UCSF study, the researchers determined that a gene, known as NDT80, plays a critical role in this pachytene stage of meiosis in yeast. They found that the NDT80 gene encodes the critical protein that acts to stimulate the synthesis of a set of proteins known as cyclins, which spark entry into DNA division. They also determined that the Ndt80 protein is responsible for the synthesis of another set of proteins, which contribute to the unique shape of the spore.
Finally, the authors showed that Ndt80 is itself controlled by a so-called "check point" protein called Rad17. Normally, check point proteins play the invaluable role of stalling further cell development when there are indications of damage, until repair has taken place. If a precursor cell's DNA has somehow accumulated damage during meiosis, for instance, Rad17 will hold off on prompting Ndt80 until the problem has been repaired. Once the damage has been fixed, Rad17 will then allow Ndt80 to act, and the cell will continue through the process of spore creation.
The question interesting the UCSF researchers is whether there is a molecular pathway in the creation of human sperm that is similar to the one they have observed in yeast. And, if so, whether a human equivalent to Ndt80 or Rad17 might be involved in progression through the pachytene stage of sperm development.
Other research groups have reported a burst in cyclin synthesis at the pachytene stage of meiosis in both fly and mouse that resembles the actions of Ndt80. This suggests, say the UCSF researchers, that an equivalent gene may have been conserved throughout evolution.
Moreover, there is some question as to whether NDT80 may parallel, in some aspects, a set of genes known as DAZ, which are found on the human Y, or male, chromosome. Both the NDT80 gene in yeast, and DAZ genes in men, are required for development beyond the pachytene stage in spore and sperm, respectively.
Previous research conducted by UCSF's human male infertility researcher Reijo and co-workers showed that the DAZ genes are deleted in 13 percent of infertile men, and in many of these men sperm development is permanently arrested--again, at the pachytene stage of meiosis.
"There are probably hundreds of yet to be discovered genes involved in the creation of human sperm," says Reijo, "and we're not saying DAZ and NDT80 are the same genes or that DAZ is regulated by the equivalent of Rad17, but since there's pachytene arrest in both, we may be on to something." "Their work makes me even more curious about what is regulating DAZ," says Reijo. "I think now we can put together the spermatogenesis pathways using the same approach as has been used in yeast."
Journal
Molecular Cell