There are only two northern white rhinos left in the world and both are females. Saving the species through natural reproduction is therefore impossible. But now a rescue from the laboratory is within reach. Scientists have managed to reprogram the skin cell of a deceased female rhino into the precursor of an egg cell. Now the researchers only have to find ways to allow this germ cell to mature into a functional egg cell. Then they could use it to create rhino embryos in the lab and preserve the species that way.
There are only two northern white rhinos left in the world: 33-year-old Najin and her 22-year-old daughter Fatu. Male specimens no longer exist. The two females live in a reserve in Kenya. Poaching and habitat destruction are driving the world's largest rhino species to the brink of extinction. And so fast that even traditional breeding programs and measures to protect their habitat could no longer intervene in time. A solution from the laboratory was needed.
How do you save an entire species with two females?
There are two approaches to saving the species from extinction from the laboratory. The first approach relies on "classic" artificial insemination. Unlike the old Najin, Fatu still produces functional egg cells. Frozen semen from four deceased bulls is also available. So you can artificially inseminate Fatu's eggs with this sperm and then have the embryo carried to term by a surrogate mother, a closely related southern white rhino. Fatu himself is not allowed to become pregnant due to problems with the Achilles tendon. Although this method has already been used several times, it cannot remain the only long-term solution for species preservation, because the gene pool would otherwise be too small.
"It was clear from the start that we would not be able to save the northern white rhino from extinction in the long term if we only used natural gametes for artificial insemination. A complementary strategy to obtain germ cells with significantly higher genetic diversity and in larger numbers is of crucial importance," explains Thomas Hildebrandt from the Leibniz Institute for Zoo and Wildlife Research. This complementary strategy consists of obtaining oocytes and sperm from other northern white rhinos that have already died. Six other individuals are eligible. However, neither egg cells nor sperm are preserved from them, only cell cultures of mature body cells, for example skin cells. These cultures must first be grown back into germ cells.
So far, the scientists had already succeeded in reprogramming mature rhinoceros horn cells into pluripotent stem cells. Such induced pluripotent stem cells are still undifferentiated and can develop into a wide variety of tissue types, including germ cells. The necessary procedure is therefore: skin cell becomes stem cell becomes germ cell.
First germ cell progenitors produced
Researchers led by Masafumi Hayashi from Osaka University have now reached a milestone on the way from the stem cell to the germ cell. They managed to create primordial germ cells from induced pluripotent stem cells of a deceased female northern white rhino, i.e. the predecessors of her egg cells. "It is the first time that primordial germ cells of a large and at the same time endangered mammalian species have been generated from stem cells," says Hayashi. So far, this has only been achieved in rodents and primates. "The exact orchestration of when cells need which signals in order to develop as desired is different for each species. Reproducing this development in cell culture is an extremely big challenge,” explains the scientist. Unlike in rodents, the researchers identified the SOX17 gene in rhinos as a key to the development of progenitor cells. The gene also plays an essential role in human germ cell development.
However, the researchers accomplished their genetic feat not only on the northern white rhino, but also on its relative, the southern white rhino, which is also threatened. There are still about 20,000 specimens of this species worldwide. Therefore, the research team did not have to grow back mature cells, but was able to access embryonic, pluripotent stem cells directly, which they then had differentiated into germ cell precursors. The researchers also identified two characteristic molecules, CD9 and ITGA6, on the surface of the progenitor cells of both rhino species. "In the future, these markers will help us to detect and isolate the primordial germ cells that have already formed in a group of pluripotent stem cells," says Hayashi.
Next stop: ova and sperm
The researchers are now faced with the next difficult task: the germ cell precursors have to mature into functional egg cells and sperm in the laboratory. "Compared to egg cells, the progenitor cells are relatively small and, above all, still have a double set of chromosomes," explains Hayashi's colleague Vera Zywitza. "So we have to find suitable conditions under which the cells can grow and halve their chromosome set." In the future, this method could be used in addition to the artificial insemination of Fatus egg cells and thus save the northern white rhino from extinction.
Source: Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Leibniz Institute for Zoo and Wildlife Research (IZW) in the Forschungsverbund Berlin eV, specialist article: Science Advances, doi: 10.1126/sciadv.abp9683)