A team of researchers headed by Jun Wu, PhD, assistant professor of molecular biology at UT Southwestern Medical Center, has derived a new type of embryonic pluripotent stem cell from mice, horses, and humans that they have named XPSC.
A unique feature of XPSCs is that they can not only be induced to form chimeras, but they can also be differentiated into precursors of sperm and ova. To date, no known pluripotent stem cell has demonstrated this dual competency.
Stem cells, amorphous blanks that can shape-shift into any cell-type the body needs, are endowed with distinct pluripotency programs. Establishing and characterizing the “stem-ness” of different types of stem cells based on the specificity of their pluripotency program and their progressive priming has been an active focus of research over the past decades.
In mice, “naïve” embryonic stem cells emerging at four days post-fertilization, and “primed” epiblast stem cells, emerging at around seven days post-fertilization, once the embryo is embedded into the uterine wall, have well-characterized, distinct properties.
“But there hasn’t been much progress in deriving and characterizing pluripotent stem cells that exist at intermediate stages between the naïve and primed states,” said Wu. This is because until now researchers had not zeroed in on the conditions that could maintain stem cells at this dynamic stage along the pluripotency continuum.
The authors were able to successfully tweak key molecular pathways to generate XPSCs. These include fibroblast growth factor (FGF), transforming growth factor β (TGF-β)/Smad, and WNT/β-catenin signaling pathways.
Establishing XPSCs presented a challenge because maintaining naïve pluripotent stem cells in culture requires activating the WNT/β-catenin cell-signaling pathway and suppressing the FGF and TGF-β/Smad pathways, which is an exact reversal of the conditions required to maintain primed epiblast stem cells in culture, where WNT/β-catenin signaling must be suppressed and FGF and TGF-β/Smad signaling must be activated.
In an ingenious approach to maintaining the intermediate stage between naïve and primed pluripotent stem cells, Wu and his colleagues activated all three molecular pathways. This stabilized intermediate-stage XPSCs, that divided without differentiating under these conditions for approximately two years.
Mouse XPSCs share transcriptomic similarities with five to six-day-old mouse embryos and can contribute to form both intraspecies (organisms that contain a mix of cells from different individuals of the same species) and interspecies (organisms that contain a mix of cells from different species) chimeras. In response to the transcription factor BMP4, mouse XPSCs differentiate into primordial germ cell-like cells (PGC-LCs) in vitro.
Wu and his colleagues developed intraspecies chimeras using cells derived from mice with different coat colors. Tagging mouse XPSCs with a fluorescent protein before injecting them into the embryo allowed the researchers to track the cells in the offspring, revealing that the XPSCs contributed to different fetal tissues generated from all three germ layers.
Horse XPSCs injected into early mouse embryos contributed to the development of mouse organs in the interspecies chimeras, revealing the necessity of signals from mouse cells in determining organ development.
The same culture conditions used to derive XPSCs from mice also successfully derived embryonic stem cells from large livestock species, such as horses, which lack stable embryonic stem cells, as well as transgene-free induced pluripotent stem cells (iPSCs) from both horse and human fibroblasts.
“We plan to apply these new XPSCs for the generation of functional gametes in culture for humans, horses, endangered northern white rhinos, and others,” said Wu. “Potentially these cells can be applied for the generation of functional gametes in vitro for fertility treatment. In addition, they may provide a novel platform for the generation of gametes from large livestock animals such as cows, pigs, and horses in a culture dish or in mice through interspecies chimera formation.”
These findings, published in Cell Stem Cell, could lead to advances in basic developmental biology, regenerative biology, and reproductive medicine. The array of new avenues of investigation and application that the development of XPSCs ushers in includes identification of evolutionarily conserved signatures in interspecies chimeras, development of tissues and organs from stem cells for transplantation, preservation of endangered animal species, and advancement of infertility treatments.
