Stem cell scientists from the laboratory of Juan Carlos Izpisua Belmonte at the Salk Institute for Biological Studies in La Jolla, California have discovered a new type of stem cell that could potentially provide a model system for early human development, and might even allow human organs to be grown in large animals for therapeutic purposes.
Izpisua Belmonte and his colleagues came across these types of cells somewhat serendipitously while transplanting human pluripotent stem cells into mouse embryos.
Other types of pluripotent stem cells have already been well-known to stem cell scientists for some time. Stem cells are “pluripotent,” if they have an intrinsic ability to differentiate into any adult cell type. Embryonic stem cells (ESCs), for example, are derived from early human embryos that have yet to implant into the inner layer of the uterus. However, epiblast stem cells (EpiSCs) have been established from post-implantation embryos and have different properties. While both are pluripotent, they bear striking differences in molecular signature, signalling dependency, colony morphology, cloning efficiency, metabolic requirements and epigenetic features (see Cell Stem Cell 4, 487–492 (2009) and et al. EMBO J. 31, 2103–2116 (2012)). Both of these cells have the ability to re-enter embryogenesis but they do so at different developmental time points (pre-implantation versus post-implantation, respectively), which distinguish ESCs and EpiSCs. Therefore, these two cell types exist in two temporally distinct pluripotent states. Even though these two types of pluripotent stem cells can be grown into large numbers in the laboratory, differentiating them into specific types of mature, adult cells has proven difficult in some cases. The cells discovered by Izpisua Belmonte and his colleagues are reportedly easier to grow in vitro and engraft into an embryo if they are injected into the right spot. Izpisua Belmonte call these cells “region-selective pluripotent stem cells” (rsPSCs).&
Because rsPSCs grow more quickly and stably than other pluripotent cells, they may be more useful for developing new therapies, according to Paul Tesar, a developmental biologist at Case Western Reserve University in Cleveland, Ohio.
Izpisua Belmonte and colleagues originally wanted to transplant various types of human pluripotent stem cells into mouse embryos in the laboratory. They prepared their cells for transplantation by growing them in various blends of culture media that contained different combinations of growth factors and other chemicals. They found that one particular blend was more effective at making the cells grow and proliferate. However, the cells that grew quite well in this concoction displayed different patterns of metabolism and gene expression in comparison to other pluripotent stem cells. These same cells not graft well into the mouse embryo.
Thus, Izpisua Belmonte and his colleagues decided to nail down those features that would cause cells to efficiently integrate into mouse embryos. They injected the human cells into three different regions of a 7.5-day-old mouse embryo. Thirty-six hours later, only those cells that had been grafted into the tail, or posterior of the embryo, integrated and differentiated into the correct cell layers to form a “chimeric” or mixed-tissue embryo. Such organisms contain cells with genomes from DNA organisms. Since these cells seemed to prefer one part of the embryo, Izpisua Belmonte and his team called them region-selective pluripotent stem cells.
From these data, Izpisua Belmonte has proposed that embryos contain multiple types of pluripotent stem cells, including rsPSCs, during their early development. It is not yet clear whether the rsPSCs play a part in designating which part of the embryo will become the head, the middle, or hind end. Identifying various types of pluripotent cells might provide researchers with the ability to study the early stages of human embryonic development by transplanting rsPSCs into animal embryos.
Izpisua Belmonte and his colleagues found that they could easily use enzymes that modify the sequences of DNA to edit the genomes of rsPSCs, which is usually difficult to do in pluripotent cell lines when grown in culture.
Gene editing could help scientists to optimize the ability of human cells to grow within animals, which might allow the creation of transgenic chimeras. Tesar says that the idea of using human pluripotent cells, such as rsPSCs, to create animals with human organs is not unrealistic, but he expects that it will be very difficult. The immune system of the animal might reject the human cells and the growth rates of the two organs might also cause problems.
Izpisua Belmonte’s lab is already starting to implant pig embryos with a different type of stem cells, and this is the only very first step for these techniques.