Not content with inducing mere pluripotency, stem cells scientists have been trying to coax developing cells all the way back to totipotency, the exceptionally plastic state that is characteristic of newly fertilized cells. Now it is possible, say French and German scientific teams, to induce a totipotent-like state by restoring some of chromatin’s new-cell openness and looseness, and by doing so in a particular way—interfering with CAF-1, a chromatin assembly factor.
By the time an embryo develops into a blastocyst, a ball of cells consisting of about 30 cells, totipotency has already hardened into pluripotency, which means that cells lose the ability to differentiate into embryonic and extraembryonic tissues. Instead, they can generate only embryonic tissues. No placenta. No umbilical cord.
Even though differentiated cells are routinely reprogrammed back to pluripotency, reprogramming back to totipotency remains unusual. Nonetheless, when pluripotent stem cells are cultured, a few totipotent cells appear spontaneously. These are called “2C-like cells” because they resemble the 2-cell stage embryo.
Zeroing in on 2C-like cells, reasoned scientists based at INSERM and the Max Planck Institute for Molecular Medicine, might reveal ways to achieve totipotency more consistently.
The researchers compared 2C-like cells to 2-cell-stage embryos in order to find their common characteristics and identify telling differences. In particular, the ISERM and Max Planck teams found that the DNA was less condensed in totipotent cells, and that the amount of the protein complex CAF1 was diminished. A closer look revealed that CAF1—already known for its role in the assembly of chromatin (the organized state of DNA)—is responsible for maintaining the pluripotent state by ensuring that the DNA is wrapped around histones.
This observation animated follow-up work by INSERM team, which was led by Maria-Elena Torres-Padilla, Ph.D., a researcher from the Institut de Génétique et de Biologie Moléculaire et Cellulaire, or IGBMC. The INSERM team was able to induce a totipotent state by inactivating the expression of the CAF1 complex, which led to chromatin reprogramming into a less condensed state.
Details of this work appeared August 3 in Nature Structural & Molecular Biology, in an article entitled, “Early embryonic-like cells are induced by downregulating replication-dependent chromatin assembly.”
“We show that mouse 2C-like cells can be induced in vitro through downregulation of the chromatin-assembly activity of CAF-1,” wrote the authors. “Endogenous retroviruses and genes specific to 2-cell embryos are the highest-upregulated genes upon CAF-1 knockdown. Emerging 2C-like cells exhibit molecular characteristics of 2-cell embryos and higher reprogrammability than ES cells upon nuclear transfer.”
Moreover, the INSERM and Max Planck teams found that specific classes of repetitive elements were also up-regulated in induced totipotent-like cells, a hallmark of the 2-cell embryo. “The computational analysis of expression of repetitive elements is very challenging, since these are found many times in the genome,” said Juanma Vaquerizas, Ph.D., a researcher at the Max Planck Institute. “Now it is key to understand why these repetitive elements and gene expression programs are both up-regulated in totipotent cells.”
The authors speculated that early embryonic-like cells can be induced by modulating chromatin assembly and that atypical histone deposition may trigger the emergence of totipotent cells. If so, the authors’ work may contribute to more efficient reprogramming techniques, advancing applications in regenerative medicine.
