The discovery was documented in the publication Genes and Development.
Men can continue to generate sperm throughout their adult life, in contrast to women who are born with all the eggs they will ever have. They must continually replenish the spermatogonial stem cells that give rise to sperm in order to do this.
According to research by Jeremy Wang of the University of Pennsylvania School of Veterinary Medicine and colleagues, this stem cell renewal is dependent on a newly described stem cell self-renewal factor known as DOT1L.
This novel factor was only able to be identified by finding this unusual phenotype: the fact that mice lacking DOT1L were not able to continue to produce sperm. Identifying this essential factor not only helps us understand the biology of adult germline stem cells but could also allow us to one day reprogram somatic cells, like a type of skin cell called fibroblasts, to become germline stem cells, essentially creating a gamete in a petri dish. That is the next frontier for fertility treatment,” said Jeremy Wang, corresponding author on the paper.
The scientists demonstrated that animals lacking DOT1L are unable to maintain spermatogonial stem cells, which affects their capacity to constantly make sperm.
By chance, the researchers discovered DOT1L’s function in stem cell self-renewal. The gene is widely expressed, and mice with a mutant form of DOT1L in every cell die before they reach the embryonic stage of development. However, DOT1L’s genetic expression patterns led Wang and colleagues to hypothesise that it might be involved in meiosis, the process of cell division that results in sperm and eggs. So they made the decision to investigate what would happen if they altered the gene just in these germ cells.
There could be other issues causing this decline in sperm production. However, a number of lines of research suggested a connection between DOT1L and a lack of stem cell self-renewal. The scientists discovered that the mice sequentially lost the ability to produce spermatogonia, spermatocytes, round spermatids, and elongated spermatids, as well as other sperm formation stages.
In a second experiment, the researchers looked at what would happen if DOT1L was inactivated in germ cells throughout adulthood rather than at birth. Wang and colleagues found that the identical progressive loss of sperm development they had seen in the mice born without DOT1L in their germ cells occurred as soon as the DOT1L loss was activated.
The fact that DOT1L affects gene expression by acting as a histone methyltransferase—an enzyme that modifies histones by adding a methyl group—was already known. Wang and his team treated spermatogonial stem cells with a substance that inhibits the methyltransferase activity of DOT1L to test if the same mechanism was in charge of the outcomes they had seen in sperm formation. The spermatogonia-producing capacity of the stem cells was greatly diminished as a result. Additionally, the therapy reduced stem cells’ capacity to methylate histones. Additionally, the activity of the animals’ spermatogonial stem cells was reduced in half after these treated stem cells were implanted into otherwise healthy mice.
The scientists discovered that DOT1L appeared to be controlling the Hoxc gene family, transcription factors that are important in controlling the expression of a variety of other genes.
“We think that DOT1L promotes the expression of these Hoxc genes by methylating them,” says Wang. “These transcription factors probably contribute to the stem cell self-renewal process. Finding out the details of that is a future direction for our work. That’s the future of this field: in vitro gametogenesis,” Wang says. “Reprogramming somatic cells to become spermatogonial stem cells is one of the steps. And then we’d have to figure out how to have those cells undergo meiosis. We’re in the early stages of envisioning how to accomplish this multi-step process, but identifying this self-renewal factor brings us one step closer.”
Longer term objectives include employing DOT1L and other germline stem cell self-renewal factors to assist individuals with reproductive issues. The idea is to generate germ cells from scratch.