Jayanti Tokas 1 , Puneet Tokas 2 , Shailini Jain 3 , Hariom Yadav 3
1 Department of Biotechnology, JMIT, Radaur, Haryana, India
2 KITM, Kurukshetra, Haryana, India
3 NIDDK, National Institute of Health, Bethesda, MD 20892, USA
Email: yadavhariom@gmail.com

• Stem cells are master cells with two important characteristics
– Unspecialized cells capable of their own renewal
– Ability to differentiate into different cell types
• The stem cells may have various differentiation potentials
• Totipotent
• Pluripotent
• Multipotent
• Unipotent

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Pluri derived from latin word plures – means several or many
Most commonly the term is used to describe stem cells that give rise to cells derived from all three embryonic germ

 Embryonic stem cells
 Embryonic carcinoma cells
 Embryonic germ cells

Origins of pluripotent cells
Teratomas
Embryonic Carcinoma Cells
Abnormal karyotypes,
Germline transmission unlikely
Adult
Zygote
Fetus Blastocyst
Primordial germ Cells Inner cell mass (ICM)
Embryonic Germ Cells Embryonic Stem Cells
Undergo spontaneous differentiation
Unable to support normal development due to epigenetic modifications during PGCs
Pluripotent cells
Partially understood
Great potential

 Immortality
 Undifferentiation
 Clonality
 Broad developmental potential

a) Differentiation triggered when grown in suspension,
Embryoid Body formation b) Different cells obtained spontaneously c) Specific growth factors can be used to direct the differentiation of ES cells into specific cells

a) Teratoma formation when injected into nude mouse b) When injected into host blastocysts, the ES cells integrate, proliferate and differentiate into all germ lineages including germ cells

Regulation of pluripotency in ES cells
“Pluripotency is maintained by promotion of proliferation and
Inhibiting differentiation”

Epigenetic modifications
ES Cells pluripotency
Transcription Factors
Stage specific functioning
Require other processes
Target genes/ receptors required

 Nanog
 Oct 3/4
 Sox 2
 LIF
 c-Myc
 Klf4
 Zic3

Core regulatory circuitry in ES cells
Activation of target genes

 Transcription factor containing homeobox domain
 Downstream effectors of signals of LIF and BMP
 Elevated levels excludes inclusion of LIF and feeder layer
 Works with other key factors including Oct4 and Sox2

 POU-domain transcription factor
 Maintains pluripotency (ESCs, EGCs, ECCs, GSCs)
 Tightly regulated transcription factor, associated with a number of target genes implicated in pluripotency maintenance
 Regulatory elements in target genes are in close vicinity of Sox2binding sites
 Key factor in the transcriptional framework of self-renewing stem cells

Sox2
 Member of HMG-domain DNA-BP family
 Necessary for embryonal development and to prevent ES cell differentiation
 Many ES cell pluripotency-associated genes are co-regulated by Sox2 and Oct3/4
 A ternary complex formed with Oct4 or Oct1 on enhancer sequence of Fgf4 is must for functioning
 Cooperate with other TFs, e.g. Nanog to activate transcription of pluripotency markers

Leukaemia inhibitory factor,
LIF
 Interleukin-6 cytokine family
 Essential for maintaining pluripotency in vitro in the presence of serum
 Binds to a heterodimeric receptor comprising of LIF-receptor
(LIFR) and gp130 on cell membrane
 Binding results in the activation of Jak/ Stat signal transduction pathway
 Activated Stat3 maintains pluripotency

Combinatorial signaling pathways
(involving LIF and master regulator genes) in maintaining mouse ESC pluripotency
Boiani and Scholer, 2005

 Helix-loop-helix/leucine zipper transcription factor
 Takes part in a variety of cellular functions
 D ownstream effecter of STAT3 in LIF receptor signaling pathway
 c-Myc is a substrate for GSK3b i n Wnt signalling pathway
 Compensates anti-proliferative effects of Klf4, e.g. in iPS cells

 Member of the quartet, a Krueppel-type zinc finger transciption factor
 Can act as an oncogene and as a tumor suppressor protein
 Over expression inhibits differentiation of ES cells
 Klf4 upregulates, in concert with Oct3/4, but the role as cofactor for Oct3/4 may be limited to only a few targets
 Can repress p53, a negative regulator of Nanog

Pluripotent lineages in the mouse embryo ll

Pluripotent cells form the ICM of the blastocyst
After giving rise to the primitive endoderm on the surface of the ICM, pluripotent stem cells form the epiblast and start to proliferate rapidly after implantation
They then form the primitive ectoderm, a monolayer epithelium that has restricted pluripotency which goes on to give rise to the germ cell lineage and to the somatic lineages of the embryo
Certain key transcription factors (blue) are required for the differentiation of the various embryonic lineages

Differentiation of mouse ES cells

Differentiation of mouse ES cells
ES cells differentiate into three cell types –
 primitive endoderm
 trophectoderm (TE)
 primitive ectoderm mimicking the differentiation potential of pluripotent stem cells in preimplantation embryos
In the absence of LIF and in the presence of an excess of Oct3/4,
ES cells differentiate into primitive endoderm-like cells
In the absence of Nanog and in the presence of Gata6, they differentiate into parietal endoderm-like cells
Removing Oct3/4 and adding Cdx2 to, ES cell culture induces TElike differentiation.
MEFc, mouse embryonic fibroblast conditioned medium

Regulation of proliferation of mouse ES cells

Regulation of proliferation of mouse ES cells
 Pluripotent transcription factors activate the expression of certain effectors that drive
ES cell proliferation
 Eras and Tcl1 stimulate the phosphoinositide-3kinase (PI3K)/Akt signaling pathway to promote the cell cycle, whereas b-Myb and c-Myc activate the progression of the cell cycle directly

A
Transcriptional regulation of the mouse
Oct3/4 gene
B

Oct4
Sox2 Nanog
Zic3
Sox17, PDGFRA
Gata4, Gata6
Foxa2, Sox7
Endoderm
 Zic3 contributes to the maintenance of pluripotency by operating downstream of Oct4, Nanog, and Sox2 to inhibit endoderm lineage specification as characterized by endodermal markers
Sox17, PDGFRA, Gata4, Gata6, Foxa2, and Sox7
 The presence of Zic3 also maintains the expression of the homeodomain protein Nanog, a key regulator of pluripotency in embryonic stem cells

Characteristics of the pluripotent epigenome
The nucleus shrinks and the distribution of electrondense areas (mainly heterochromatin) changes dramatically when ES cells are induced to differentiate into primitive endoderm by the ectopic expression of Gata6

Epigenetic features of the pluripotent cell nucleus
 Small regions of perinuclear heterochromatin exist, but most of the chromatin exists as euchromatin, bearing histone marks associated with transcriptional activity
 The hyperdynamics of chromatin proteins (green) might contribute to the maintenance of euchromatin
 Bivalent domains are also a feature of the pluripotent epigenome, in which active histone marks (such as H3K4me) are flanked by transcriptionally repressive histone marks (such as H3K9me)