Histones may hold the key to the generation of totipotent stem cells
6 February 2014
Researchers at RIKEN in Japan have identified histone
proteins that enhance the generation of induced pluripotent stem
cells (iPS cells) and may be the key to generating induced
totipotent stem cells. These are cells capable of developing into
any cell type and can give rise to a fully formed organism.
Differentiated cells can be coaxed into returning to a stem-like
pluripotent state either by artificially inducing the expression of
four factors called the Yamanaka factors, or as recently shown by
shocking them with sublethal stress, such as low pH or pressure.
However, attempts to create 'totipotent' stem cells have so far
The study, published in the journal Cell Stem Cell and
led by Dr. Shunsuke Ishii from RIKEN, sought to identify the
molecule in the mammalian oocyte (egg cell) that induces the
complete reprograming of the genome leading to the generation of
totipotent embryonic stem cells. This is the mechanism underlying
normal fertilization, as well as the cloning technique called
Somatic-Cell Nuclear Transfer (SCNT).
SCNT has been used successfully to clone various species of
mammals, but the technique has serious limitations and its use on
human cells has been controversial for ethical reasons.
Ishii and his team chose to focus on two histone variants named
TH2A and TH2B, known to be specific to the testes where they bind
tightly to DNA and affect gene expression.
The study demonstrates that, when added to the Yamanaka cocktail
to reprogram mouse fibroblasts, the duo TH2A/TH2B increases the
efficiency of iPSC cell generation about twentyfold and the speed of
the process two- to threefold. And TH2A and TH2B function as
substitutes for two of the Yamanaka factors (Sox2 and c-Myc).
By creating knockout mice lacking both proteins, the researchers
show that TH2A and TH2B function as a pair, are highly expressed in
oocytes and fertilized eggs and are needed for the development of
the embryo after fertilization, although their levels decrease as
the embryo grows.
In the early embryo, TH2A and TH2B bind to DNA and induce an open
chromatin structure in the paternal genome, thereby contributing to
its activation after fertilization.
These results indicate that TH2A/TH2B might induce reprogramming
by regulating a different set of genes than the Yamanaka factors,
and that these genes are involved in the generation of totipotent
cells in oocyte-based reprogramming as seen in SCNT.
“We believe that TH2A and TH2B in combination enhance
reprogramming because they introduce a process that normally
operates in the zygote during fertilization and SCNT, and lead to a
form of reprogramming that bears more similarity to oocyte-based
reprogramming and SCNT” explains Dr. Ishii.
Shinagawa T, et al. Histone Variants Enriched in Oocytes Enhance
Reprogramming to Induced Pluripotent Stem Cells, Cell Stem Cell