Reprogramming Pluripotent Cells into Totipotent Cells


With the advent of the Nobel Prize-winning research of Shinya Yamanaka, scientists are presently able to reprogram mature, adult cells into pluripotent cells. These “induced pluripotent stem cells” or iPSCs are made by genetically engineering adult cells and culturing them in special conditions. These iPSCs can also be differentiated, potentially, into every cell type in the adult human body. Now Maria-Elena Torres-Padilla‘s research team is trying to push the limits of stem cell science even further.

Torres-Padilla and her coworker successfully made “totipotent” cells, which have the same characteristics as those of the earliest embryonic stages, from pluripotent stem cells. This work was the result of a fruitful collaboration between Juanma Vaquerizas from the Max Planck Institute for Molecular Biomedicine (Münster, Germany), and Maria-Elena Torres-Padilla and her colleagues at the Institute of Genetics and Molecular and Cellular Biology (IGBMC) at Illkirch, France. This work has been published in the journal Nature Structural & Molecular Biology.

Soon after fertilization, the embryo begins it first rounds of cell division, which is known as the “early cleavage” stages. At this early stage of development, when the embryo is composed of only 1 or 2 cells, the “blastomeres” or cells of the embryo are “totipotent.” Totipotent means that these blastomeres can produce an entire embryo, or the placenta and umbilical cord that accompany it.

Early Cleavage Stages

After the 12-16-cell stage, the embryo undergoes a complex process called “compaction,” in which the cells become very tightly bound together and two populations of cells become apparent.

Post-Compaction Embryo
Post-Compaction Embryo

The cells on the inside, which give rise to the cells of the “inner cell mass” (ICM), and outside cells, which give rise to the “trophectoderm” that produce the placenta. Trophectoderm cells express a specific set of genes; Yap1, Tead4, Gata3, Cdx2, Eomes and Elf5. The ICM generates the embryo proper and the ICM cells express a cadre of genes specific to these cells; Oct4, FGF4, Sall4, Sox2 and Nanog. These ICM cells are no longer totipotent, but have become “pluripotent,” can differentiate into any tissue, but they cannot alone produce the placenta or a whole embryo. As development progresses, these pluripotent cells continue to specialize and form the various tissues of the body through the process of cellular differentiation.

While it is possible to make pluripotent cells from mature, adult cells, Torres-Padilla and her coworkers have studied the characteristics of totipotent cells of the embryo and discovered factors capable of inducing a totipotent-like state.

When they cultured pluripotent stem cells in culture, a small percentage of totipotent cells appear spontaneously. Such cells are called “2C-like cells,” after their resemblance to the 2-cell stage embryo. Torres-Padilla and her team compared these 2C-like cells to totipotent cells in early embryos in order to determine their common characteristics and the features that distinguish them from pluripotent cells. In particular, Torres-Padilla and her collaborators found that the chromosomes of totipotent cells were less condensed and that the amount of the protein complex CAF1 was diminished. CAF1 (Chromatin Assembly Factor 1) is a protein complex that helps assemble histone proteins onto DNA.

CAF1

Histones act as tiny spools around which DNA is wound. Because DNA is negatively charged, and histones are positively charged, the two have a natural affinity for each other. CAF1 binds to histones and regulates the association of histones with DNA in order to ensure that the assembly of histones on DNA is and orderly process. Histones wind DNA into a tight structure called “chromatin.” CAF1, as it turns out, is responsible for maintaining the pluripotent state by ensuring that the DNA remains properly wound around histones.

chromatin+structure

As an extension of this hypothesis, Torres-Padilla and her crew were able to induce a totipotent state by inactivating the expression of the CAF1 complex. CAF1 inactivation caused the chromatin of pluripotent cells to reform into a less condensed state, and this less condensed state was conducive to totipotency.

These data provide new avenues for understanding the nature of pluripotency, and could increase the efficiency of reprogramming somatic cells to be used for applications in regenerative medicine,

Newcastle Scientists Grow Large Quantities of Cells to Aid Peripheral Nerve Repair


A research team at the University of Newcastle, UK, in the laboratory of Maya Sieber-Blum, have used a combination of small molecules to convert cells isolated from human skin into Schwann cells, which are the specialized cells that surround and insulate peripheral nerves. Schwann cells also play an integral role in nerve repair. This new protocol, pioneered by Sieber-Blum and her colleagues, generates large and pure populations of Schwann cells. Therefore, this research presents a promising step in the repair of peripheral nerve injuries. This research was published in the journal Development.

Schwann cells

Presently the repair of peripheral nerves utilizes nerve grafts from donors whose donated neural tissues are transplanted into patients in order to repair damaged peripheral nerves. Unfortunately, this approach has several disadvantages in that it can often itself cause nerve damage. In this new research study, Motoharu Sakaue, in collaboration with together Dr. Sieber-Blum, who is Professor of Stem Cell Sciences at the Institute of Genetic Medicine in Newcastle, examined the possibility of growing Schwann cells, which are known to promote nerve repair, in the laboratory. To expand these cells, Sieber-Blum and her team isolated stem cells from adult skin and differentiated them into Schwann cells by exposing them to small molecules.

“We observed that the bulge, a region within hair follicles, contains skin stem cells that are intermixed with cells derived from the neural crest – a tissue known to give rise to Schwann cells. This observation raised the question whether these neural crest-derived cells are also stem cells and whether they could be used to generate Schwann cells” said Sieber-Blum.

hair-stem-cells

“We then used pertinent small molecules to either enhance or inhibit pathways that are active or inactive, respectively, in the embryo during Schwann cell differentiation” she said.

By applying this novel approach, Sieber-Blum and others generated large and highly pure populations of human Schwann cells in culture. These cells displayed a morphology characteristic of Schwann cells and they also expressed proteins characteristic of Schwann cells. Sieber-Blum and others further investigated the functionality of these Schwann cells, and showed that they could interact with nerves in culture. “The next step is to determine, for example in animal models of peripheral nerve injury, whether grafts of these Schwann cells are conducive to nerve repair,” the authors said.

This study identifies a biologically relevant and accessible source of cells that can potentially be used for to generate sufficient quantities of Schwann cells and thus offers great potential in the repair of peripheral nerve injuries.