Northern white rhino Nabire, from which the primordial germ cells have been produced, at Safari Park Dvur Kralove. Credit: Khalil Baalbaki
Northern white rhinoceroses face near-inevitable extinction, as the only two known surviving members of the species are the 33-year-old female Najin and her daughter Fatu. Advanced assisted reproduction technologies have the potential to save the species, as scientists have already produced northern white rhino embryos using preserved oocytes and sperm; however, Fatu is the only natural donor of oocytes left on the planet, and some of the four deceased sperm donors are closely related to Fatu, posing challenges and bottlenecks for repopulation efforts. A recent breakthrough by BioRescue, an international research consortium including scientists from Japan, Germany, Italy and the Czech Republic, opens up the possibility of producing functional gametes from preserved northern white rhino skin tissue, potentially offering scientists a new avenue to “rewind” the extinction of the species.
In a world first for large mammals, the researchers optimized a cell culture system that enabled the production of primordial germ cell-like cells (PGCLCs) from northern white rhino induced pluripotent stem cells (NWR-iPSCs). To engineer an environment that would foster PGCLC differentiation, the team first developed a culture system to produce PGCLCs from the embryonic stem cells (ESCs) of southern white rhinos, which are a near-threatened subspecies related to northern white rhinos. The consortium leveraged knowledge from previous experiments at Osaka University in which mouse PGCLCs and germ cells were produced from stem cells and ultimately resulted in the birth of healthy offspring. Constructing the right environment for rhino stem cells required the researchers to identify the hormone and protein signals to introduce into the system at specific time points to trigger their morphological and functional transformation into PGCLCs.
Once the team established a successful system for generating PGCLCs from ESCs, the system was further refined for iPSCs derived from northern white rhino skin. By fine-tuning the timing and duration of key processes such as WNT and BMP signaling, the team produced PGCLCs using stem cells from the skin of Fatu’s aunt Nabire, who died in 2015. These results represent a next step in ultimately generating functional germ cells and gametes from white rhino stem cells, which could greatly accelerate embryo production for assisted reproduction efforts. Like with the natural northern white rhino gametes currently available, the artificial gametes could be fertilized in vitro and implanted into southern white rhino surrogates. This study was published in Science Advances.
“Through repeated refinement, we produced PGCLCs from NWR-iPSCs under a defined condition. Because PGCs are the founder population for gametes, this study paves a way to produce functional gametes from NWR-iPSCs, which will contribute to the effort to rewind the extinction of the NWR,” the authors wrote.