Scientists grow human sperm stem cells in lab, call it an early step toward infertility treatment in men
Infertility affects one in seven men of reproductive age worldwide and it is estimated that 10% of the US population suffer from this problem. According to health experts, one idea for treating male sterility is “spermatogonial stem cell (SSC) therapy.” In this approach, sperm stem cells in the testis are transferred to a test tube, cultured, and nudged into becoming fully-fledged sperm. However, a key bottleneck has been identifying just the right conditions to get human SSCs to grow in the lab. Researchers from the University of California (UC) San Diego School of Medicine have now been able to grow human sperm stem cells grown in the lab, which is an early step toward infertility treatment.
The production of sperm — otherwise known as spermatogenesis — generates more than 1,000 sperm per second in normal males. This productivity comes, in part, from a special cell type called the spermatogonial stem cell. SSCs are what make it possible for men to father children beyond the age of 65. “These specialized cells continually self-renew, making more SSCs, and develop into sperm so prolifically that men (and some transgender, non-binary and gender-fluid people) produce more than 1,000 new sperm every few seconds,” say researchers.
Yet spermatogonial stem cells have not been well studied in humans and attempts to grow them in the lab for clinical purposes have had limited success. Progress in the field has been hindered by the fact that it is extremely difficult to distinguish SSCs from other cells in the testes. While there have been many attempts to identify the right conditions to get human SSCs to grow in the lab, in most reported cases it was not clear whether the cells being cultured were SSCs and no previously published method is routinely used. “Spermatogonial stem cells (SSCs) are essential for the generation of sperm and have potential therapeutic value for treating male infertility, which afflicts over 100 million men worldwide. While much has been learned about rodent SSCs, human SSCs remain poorly understood. Here, we molecularly characterize human SSCs and define conditions favoring their culture,” write authors in the report published in the Proceedings of the National Academy of Sciences (PNAS).
Several laboratories, including the Wilkinson team, recently used a technique called single-cell RNA sequencing to define the likely molecular characteristics specific to human SSCs. It allowed scientists to develop a clearer picture of human spermatogonial stem cells and how sperm are formed. Their study identified many unique molecules — biomarkers — that define spermatogonial stem cells. These biomarkers, which they detected with specific antibodies, allowed the researchers to efficiently capture human spermatogonial stem cells. They also developed tools to better isolate these stem cells. These advances opened the possibility that spermatogonial stem cell transplants could be developed to treat male infertility.
In the current study, authors report that they have developed a reliable method for culturing cells with the characteristics of human SSCs. “We think our approach, which is backed up by several techniques, including single-cell RNA-sequencing analysis, is a significant step toward bringing SSC therapy into the clinic,” says senior author Dr Miles Wilkinson in the analysis. Dr Wilkinson is a distinguished professor in the Department of Obstetrics, Gynecology, and Reproductive Sciences at UC San Diego School of Medicine.
Explaining further, the research team says that they used their single-cell RNA sequencing information to purify what they thought might be human SSCs. Using a method called “germ-cell transplantation,” the authors showed that the cells they purified were indeed highly enriched in human SSCs. The team then gathered the profile of genes expressed in these human SSCs to make guesses as to the conditions that might best support their growth in the lab. Using more than 30 human testis biopsies, the researchers determined just the right conditions needed to culture immature germ cells with the characteristics of SSCs. By applying an inhibitor of the molecule AKT, researchers were able to favor the culture of human spermatogonial stem cells in the lab, the first step toward lab-produced sperm as a treatment for male infertility.
“The key ingredient was an inhibitor of the AKT pathway, a cellular system that controls cell division and survival. We determined that AKT inhibition maintains human SSCs by inhibiting the development of later-stage sperm precursors. Several AKT inhibitors are currently used to treat cancer. With that approach, we were able to favor the culture of human cells with the molecular characteristics of SSCs for two-to-four weeks,” the findings state. According to Dr Wilkinson, the next main goal of the research team is to learn how to maintain and expand human spermatogonial stem cells longer so they might be clinically useful.