Pluripotency is well defined functionally but ambiguously defined at the molecular level. In this issue of Developmental Cell, Boroviak and colleagues (2015) use a multi-species approach to differentiate between fundamental features of pluripotency in mammals and those that exhibit evolutionary plasticity

Frankenberg, Stephen R.

Elsevier - Publisher Connector 

Developmental Cell
PreviewsDifferent Species Choose
Their Own Paths to PluripotencyStephen R. Frankenberg1,*
1School of BioSciences, University of Melbourne, Parkville, 3010 VIC, Australia
*Correspondence: srfr@unimelb.edu.au
http://dx.doi.org/10.1016/j.devcel.2015.10.025
Pluripotency is well defined functionally but ambiguously defined at the molecular level. In this issue of
Developmental Cell, Boroviak and colleagues (2015) use a multi-species approach to differentiate between
fundamental features of pluripotency in mammals and those that exhibit evolutionary plasticity.Since pluripotent mouse embryonic
stems (ESCs) were first derived in the
early 1980s (Evans and Kaufman, 1981;
Martin, 1981), attempts to derive similar
cells from other species have been far
less fruitful. Although human ESCs were
also derived almost two decades later
(Thomson et al., 1998), they differ mark-
edly from mouse ESCs and for obvious
ethical reasons cannot be exploited and
characterized to anywhere near the
same extent as mouse ESCs. Applying
similar methods has yielded ESC-like
cells from many other mammalian spe-
cies, but they too have fallen short of the
manipulability and usefulness of mouse
ESCs. The mouse is thus somehow
specially predisposed to life in the labora-
tory but conversely is a rather atypical
mammal in many aspects of its biology.
In fact, mammals exhibit an amazing
degree of variation in their reproductive
strategies, with corresponding hetero-
chrony in key early developmental events,
differences in the timing of implantation,
and diversity in modes of placentation
(reviewed by Wimsatt, 1975). A major
challenge is therefore to identify those
features of early development that are
shared, and thus fundamental to all mam-
mals, versus those that are taxon-spe-
cific. Chief among these is the regulation
of pluripotency, the property of certain
early embryonic cells that enables them
ultimately to differentiate into any cell
type of the adult body. Now, to better un-
derstand how pluripotency mechanisms
in the mouse compare to those of other
mammalian species, Boroviak et al.
(2015) report a large-scale gene-expres-
sion analysis comparing different embry-
onic stages in mouse and in marmoset.
Although pluripotent stem cells can
self-renew in vitro, pluripotent cells in vivotend to be in a constant state of flux.
Capturing pluripotency in culture thus
depends on identifying developmental
stage(s) at which self-renewal happens
to be possible. Two distinct pluripotent
states have been defined based on the
properties of isolated mouse stem cells.
‘‘Naive pluripotency’’ is characteristic of
conventional mouse ESCs and of the
E4.5 preimplantation epiblast shortly after
it has segregated from the hypoblast (the
epiblast and hypoblast are both derived
from the inner cell mass, but the hypo-
blast contributes to extraembryonic tis-
sues, whereas the epiblast forms the
embryo proper). ‘‘Primed pluripotency,’’
on the other hand, is characteristic of
later, post-implantation epiblast-derived
stem cells and differs from naive pluripo-
tency in the signaling pathways on which
it depends and the lack of germline com-
petency (reviewed by Kalkan and Smith,
2014). The definition of pluripotency has
traditionally depended on functional tests
of cultured cells, but an alternative is to
describe the molecular properties of
pluripotent cells in the embryo—in vivo—
in as much detail as possible.
Now, Boroviak et al. (2015) do just that.
By applying single-cell RNA-seq methods
to pooled groups of up to 20 embryonic
cells, they explore the progression of plu-
ripotency in the early mouse conceptus
and compare it with ESCs. They use a
Pdgfra::GFP knockin reporter mouse line
to separate epiblast from hypoblast and
target four main developmental stages
of pluripotency: the 8-cell morula; whole
inner cell mass (ICM) of the early E3.5
blastocyst; isolated epiblast of the late
E4.5 blastocyst, just after segregation
from the hypoblast; and the epiblast of
the implanted E5.5 egg cylinder. The
ESC transcriptome was most similar toDevelopmental Cell 35,that of the E4.5 epiblast, supporting pre-
vious suggestions that these two cell
types share an essentially equivalent
naive pluripotency. A number of genes
were downregulated during the transition
from pre- to post-implantation epiblast,
including Esrrb, Nr0b1, Klf2, Klf4, Klf5,
Lifr, Il6st, Spp1, Tcl1, Zfp57, and Zfp42.
Other genes were upregulated, including
Foxd3, Lef1, Ccnd1, Zscan10, Phc1, and
Nr216, and thus represent candidate spe-
cific markers of primed pluripotency.
As an added bonus, Boroviak and col-
leaguesalsocompared theabovesamples
with the epiblast of the diapausing blasto-
cyst. Embryonic diapause is a dormant
phaseofdevelopmentusedasa reproduc-
tive strategy by at least 100 mammalian
species (including many rodents, carni-
vores, and marsupials) to delay breeding
until environmental or physiological condi-
tions are optimal (reviewed by Wimsatt,
1975). In the mouse, it is induced by lacta-
tion or artificially by ovariectomy. A recent
study suggested that the capacity for
diapause is an evolutionarily conserved
property of the blastocyst, at least among
eutherian mammals, and thus only the
maternal regulatory mechanism has
evolved independently in different taxa
(Ptak et al., 2012). Counter to this notion,
diapause in the mouse was proposed to
explain the amenability of blastocysts
from this species to ESC derivation (Boro-
viak and Nichols, 2014), suggesting that
the mouse blastocyst has properties not
conserved in those of most other species.
Thus, a better understanding of the mech-
anisms of diapause is likely to provide
valuable insights into how pluripotent cells
choose between self-renewal, differentia-
tion, proliferation, and quiescence.
Previous studies compared transcrip-
tomes or proteomes of diapausing versusNovember 9, 2015 ª2015 Elsevier Inc. 267
Developmental Cell
Previewsnon-diapausing blastocysts (reviewed
by Hondo and Stewart, 2005) but used
whole blastocysts rather than dissected
epiblasts. The data produced by those
studies were thus confounded by other
tissues and developmental events,
such as differentiation of the extraembry-
onic endoderm and preparation of the
trophoblast for implantation. As would
be expected, Boroviak and colleagues
identified a large number of genes with
roles in metabolism that were differen-
tially expressed between diapausing
and non-diapausing blastocysts. Howev-
er, when they focused only on genes
that were dynamically expressed during
normal (non-diapause) development, two
signaling pathways stood out. Profiles
suggested activation of PPAR signaling
and repression of mTOR signaling specif-
ically in dormant epiblasts. In addition,
upregulation of Wnt4 suggested a spe-
cific role for WNT signaling in maintaining
pluripotency in the diapausing epiblast,
consistent with known roles for the
pathway in ESC self-renewal (reviewed
by Merrill, 2012).
Human ESCs aremore similar tomouse
late epiblast-derived pluripotent stem
cells than to conventional mouse ESCs,
in both culture requirements and expres-
sion profiles. To determine the extent
to which mouse studies are applicable to
human, it is necessary to characterize a
species that can be used as a proxy for
human. To this end, Boroviak and col-
leagues also performed transcriptome
profiling of marmoset early, mid, and late
blastocysts and compared these data to
the mouse transcriptome data. Although268 Developmental Cell 35, November 9, 201only whole marmoset blastocysts could
be used in the marmoset studies, rather
than dissected epiblasts, several key
features were still apparent. Marmoset
blastocysts differed from mouse blas-
tocysts by an absence of expression
of some naive pluripotency-associated
genes, such as Klf2, Nrob1, Fbxo15,
Gbx2, and Bmp4, but importantly were
similar to previously published expression
patterns from human blastocysts (e.g.,
epiblast-specific expression of KLF17,
LEFTY1, and NODAL), showing that the
marmoset is likely to be an appropriate
model for early human development. Bor-
oviak et al.’s mouse-marmoset compara-
tive analysis also revealed differences
in FGF, WNT, and TGFb/NODAL signaling
components. By performing treatments of
cultured blastocysts with small-molecule
pathway inhibitors, the authors demon-
strated that whereas FGF/ERK signaling
is the principle driver of epiblast-hypo-
blast segregation in the mouse, WNT
signaling also contributes to this pro-
cess in the marmoset, with a somewhat
reduced role for FGF/ERK signaling. This
may help to explain data from previous
studies on an apparently minimal role for
FGF/ERK signaling in human hypoblast
differentiation, as well as a greater role
for WNT signaling in differentiation of hu-
man ESCs compared with mouse ESCs.
It is possible that species differences in
the signaling pathways involved in hypo-
blast specification are closely linked with
differences in the mechanisms regulating
pluripotency.
The study by Boroviak and colleagues
highlights the importance of a multi-spe-5 ª2015 Elsevier Inc.cies approach for understanding early
development ofmammals. Signaling path-
ways identified by their study, including
PPAR, mTOR, and WNT4, should be
functionally tested for potential roles in
diapause and ESC maintenance. With the
aid of recently developed tools such as
CRISPR, functional studies of naive and
primed pluripotency-specific markers in
marmoset or other non-murine models
could provide much-needed insights into
themost conserved and thus fundamental
mechanisms of early development and its
evolution in mammals.
REFERENCES
Boroviak, T., and Nichols, J. (2014). Philos. Trans.
R. Soc. Lond. B Biol. Sci. 369, 20130541.
Boroviak, T., Loos, R., Lombard, P., Okahara, J.,
Behr, R., Sasaki, E., Nichols, J., Smith, A., and
Bertone, P. (2015). Dev. Cell 35, this issue,
366–382.
Evans, M.J., and Kaufman, M.H. (1981). Nature
292, 154–156.
Hondo, E., and Stewart, C.L. (2005). Genome Biol.
6, 202.
Kalkan, T., and Smith, A. (2014). Philos. Trans. R.
Soc. Lond. B Biol. Sci. 369, 20130540.
Martin, G.R. (1981). Proc. Natl. Acad. Sci. USA 78,
7634–7638.
Merrill, B.J. (2012). Cold Spring Harb. Perspect.
Biol. 4, a007971.
Ptak, G.E., Tacconi, E., Czernik, M., Toschi, P.,
Modlinski, J.A., and Loi, P. (2012). PLoS ONE 7,
e33027.
Thomson, J.A., Itskovitz-Eldor, J., Shapiro, S.S.,
Waknitz, M.A., Swiergiel, J.J., Marshall, V.S., and
Jones, J.M. (1998). Science 282, 1145–1147.
Wimsatt, W.A. (1975). Biol. Reprod. 12, 1–40.


Different Species Choose Their Own Paths to Pluripotency 

file:///data/core-remote/dit/data/elsevier/pdf/02c/aHR0cDovL2FwaS5lbHNldmllci5jb20vY29udGVudC9hcnRpY2xlL3BpaS9zMTUzNDU4MDcxNTAwNjkwNQ==.pdf

Different Species Choose Their Own Paths to Pluripotency

Abstract

Similar works

Full text

Available Versions

Elsevier - Publisher Connector