“It’s been difficult to get a handle on early regulators of reprogramming to pluripotency,” Blau said. “The process is highly heterogeneous and asynchronous, so the earliest events have been hard to study.”
To overcome this limitation, Mai turned to a cell fusion model used successfully by Blau in the 1980s to show that specialized human cells, such as those in the liver and skin, could express muscle-specific genes when joined with mouse muscle fibers. At the time, the results provided the first evidence that adult cells could be coaxed under the right conditions to assume entirely different cell fates.
In the new study, Mai fused human skin cells called fibroblasts to mouse embryonic stem cells. After fusion, factors in the developmentally flexible stem cell quickly and efficiently reprogrammed the fibroblast nucleus along a predictable, research-amenable timeline. The fused cells are called heterokaryons, and they enabled Mai and his colleagues to closely track patterns of gene expression and DNA modification during the first 24 hours of reprogramming.
Using the heterokaryon model, the researchers discovered that NKX3-1 is expressed within about two hours of the initiation of reprogramming but quickly dissipates. If the protein’s expression is blocked, the Yamanaka factors are no longer able to reprogram the human fibroblasts, indicating NKX3-1’s crucial role in the conversion of adult cells to stem cells. The researchers also found that externally added NKX3-1 can replace Oct4 to reprogram cells without any loss of efficiency.
Finally, they also showed that NKX3-1 expression was necessary to trigger the cells’ expression of their own Oct4 protein and to promote other genetic changes that facilitate reprogramming.
Now Blau and her colleagues, in collaboration with assistant professor of genetics and of computer science Anshul Kundaje, PhD, plan to continue their studies into the earliest steps of reprogramming to pluripotency using a multipronged “omics-based” approach.
“Our goal is to study all facets of the regulatory logic, or ‘grammar,’ that underlies cellular reprogramming to pluripotency,” Blau said. “Reprogramming completely changes a cell’s fate. We want to understand the mechanistic and signaling pathways that mediate such a remarkable change.”
Other Stanford co-authors are postdoctoral scholars Glenn Markov, PhD, and Adelaida Palla, PhD; former postdoctoral scholar Jennifer Brady, PhD; senior research scientist Hong Zeng, MD, PhD; and assistant professor of obstetrics and gynecology Vittorio Sebastiano, PhD.
The research was supported by the National Institutes of Health (grants GM112425, HD007249, HL100397, AG009521 and AG020961), the California Institute of Regenerative Medicine, the National Science Foundation, Bio-X, the GSK Sir James Black Program for Drug Discovery and the Baxter Foundation.
Stanford’s Department of Microbiology and Immunology also supported the work.