Introduction

It was never clear for years if there happens to be
any irreversible changes to the genome while differentiation, which would
prohibit cell’s developmental potential, till the early works of Briggs and king and then Gurdon’s work came out which showed
that a differentiated cell’s genome carries all the important information,
those are required for an organism to develop normally. They carried out this
experiment using Somatic Cell Nuclear
Transfer (SCNT) technique to transfer nuclei from various donor from to
enucleated frog oocytes, and they were successful in developing viable
organisms. These experiments went onto Mammals and eventually to humans. By
using SCNT on genetically marked lymphoid cells and olfactory receptor neurons,
it has gone further to show that reprogramming of terminally differentiated and
post-mitotic genomes is also possible. Even, recent studies suggest that it is
quite possible that, by SCNT, Human somatic cell nuclei are capable of
reprogramming. But, unlike others, in case of humans the only way is not to
remove oocyte nucleus before nuclear transfer.

        Some silenced loci in somatic genome can
be activated using factors from different cell types, not just oocytes. Given
an example, where Cytoplasm of HeLa cells was introduced with nucleus from
chicken erythrocyte, and this resulted in chromatin decondensation and also the
erythrocyte genome which was inactive before, started coding RNAs. Further
studies on mice and humans showed pluripotent, tetraploid (4N) cells can be generated
by activating pluripotency related genes by the fusion of somatic cells and
embryonic stem cells. The creation of this type of cells has opened a way
towards the study of cellular plasticity and reprogramming, despite their
limited clinical value. Also, it was understood by performing these fusion
experiments that, reprogramming of a somatic cell is possible by using factors
from ESCs like, oocytes, zygotes and early blastomeres. Classic myoD
experiments and subsequent lineage conversions in the hematopoietic system
provided the evidence for the ability of Transcription Factors (TFs) to
regulate cellular reprogramming. By ectopic expression of only four TFs, generation
induced Pluripotent Stem Cells (iPSCs) was reported by Takahashi and Yamanaka
in 2006, which is considered to be a milestone in this field.

Converting Cell States

        Basic helix-loop-helix (bHLH) TF myoD
was discovered from some different and unlinked studies, which is capable of
inducing conversion of mouse embryonic fibroblast into myoblast. Also, myoD is
well known for its ectopic expression on different kinds of non-muscle cells,
which can induce activation of muscle-specific genes. But, it is kind of
strange to note that the response myoD is very minimal in ectodermal and
endodermal cells, while it is very well capable of altering phenotype of
mesodermal cells.

        The stages of development of
hematopoietic lineage can be understood clearly, that’s why its fate conversion
certainly helped to look into TF-mediated reprogramming. Inducing expression of
GM-CSF and myeloid commitment-inducing signals associated with it, by ectopic
expression of IL-2 receptor, induces Common Lymphoid Progenitors (CLPs) to
generate granulocytes and monocytes of myeloid lineage. It can be seen that,
CLPs and pro-T-cells can undergo this type of fate conversion, while pre-T- and
pro-B-cells does not agree to this, which addresses towards a possibility of a
connection between fate conversion and differentiation of cells. Looking into
another example, GATA-1 induces CLPs and GM progenitors to produce megakaryocytes
and erythrocytes when expressed at high levels, but forced to generate mast
cells and eosinophils when expressed at low levels in GM progenitors. This suggests
that the level at which TFs are expressed also carries a greater importance.

The fundamental regulator of specification of myeloid
and lymphoid cell lineage, TF PU.1 which is from the Ets family, acts by
interplaying with other TFs like, GATA1/2 and CCAAT enhancer-binding
protein(C/EBP) ?/?, but the selection of lineage is decided by the
PU.1 graded expression. When it acts along with GATA proteins, myeloid target
gene transactivation gets supressed, contrarily when it acts with GATA-1, it
inhibits erythroid program and formation of repressive chromatin structure at
GATA-1 target loci is induced. Accompanying the conversion into myeloid cells,
suppression of GATA-1 occurs when there will be an ectopic expression of PU.1
in multipotent progenitor cell lines.

The basic leucine zipper transcription factor C/EBP?,
required for the in vivo transition of common myeloid progenitor-to-GM
progenitor. Myelomonocyte cell-type features are educed by the ectopic
expression of C/EBP? in primary bone marrow cells, lymphocytes or in
fibroblasts, where C/EBP? function along with PU.1, as deposition of H3K4me1 at
enhancer elements of target genes demand this.