Tài liệu Báo cáo khoa học: Bone morphogenetic protein signaling in stem cells ) one signal, many consequences pptx

MINIREVIEW
Bone morphogenetic protein signaling in stem cells ) one
signal, many consequences
Toni U. Wagner
Physiological Chemistry I, University of Wuerzburg, Germany
BMP signals in stem cells
Bone morphogenetic protein (BMP) signals have tre-
mendous effects on all kinds of cells. Most striking
and defining, however, are the reactions that stem and
progenitor cells show upon exposure to BMP ligands.
Various stem cell types utilize BMP signals in a multi-
tude of ways in order to define their fates. The integra-
tion of this pathway with a variety of other signals is
still poorly understood, but recent discoveries strongly
suggest that stem cell niches, areas with a certain sig-
nal–molecule cocktail, are responsible for the final out-
come of BMP signaling, be it by modulating the ligand
itself, or the cascade transducing the signal within
the cell. Regulation of BMP signaling is seen at all
molecular levels: ligands, receptors, transducers, tran-
scription complex composition and chromatin state.
The present review focuses on data gathered on the
role of BMP signaling in selected stem cell systems.
Due to space limitations, numerous stem cell niches
described to be influenced by BMP are not reviewed.
We try to focus on publications that represent the wide
variety of effects induced by BMP signals.
The BMP signaling cascades
The basic BMP signaling process is started by homo-
or heterodimeric BMP ligands. Upon binding to type I
receptors, formation of a heteromeric complex with

type II receptors is induced. In this simple transduction
Keywords
apoptosis; BMP; differentiation;
pluripotency; signaling; stem cells
Correspondence
T. U. Wagner, Physiological Chemistry I,
University of Wuerzburg, 97070 Wuerzburg,
Germany
Fax: +49 931 888 4150
Tel: +49 931 888 4165
E-mail: toni.wagner@biozentrum.
uni-wuerzburg.de
Website: zentrum.
uni-wuerzburg.de/pc/pc1/
(Received 1 December 2006, revised
29 March 2007, accepted 19 April 2007)
doi:10.1111/j.1742-4658.2007.05839.x
Bone morphogenetic protein (BMP) signals play key roles throughout
embryology, from the earliest patterning events, via tissue specification,
through organ development and again in germ cell differentiation. While
both input and the transducer molecules are rather well studied, the final
outcome of a BMP signal is basically unpredictable and differs enormously
between previously studied cell types. As already suggested by their name,
BMPs exhibit most of their (known) functions on stem cells and precursor
cells, usually driving them into various types of differentiation or death. In
this minireview, some prime examples of BMP effects on several very
different stem-cell types are discussed.
Abbreviations
BMP, bone morphogenetic protein; ES, embryonic stem; ID, inhibitors of DNA-binding; GDF3, growth and differentiation factor-3; GFAP, glial
fibrillary acidic protein; LIF, leukaemia inhibitory factor; MAPK, mitogen-activated protein kinase; MPC, mesodermal progenitor cell; NSC,

neural stem ⁄ progenitor cell; PGC, primordial germ cell; STAT3, signal transducer and activator of transcription-3; TGF-b, transforming growth
factor b.
2968 FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS
version, the type II receptors then phosphorylate the
type I receptors, which subsequently activate R-Smads
(Smads 1, 5 and 8 for BMP ligands) by serine-threonine
phosphorylation [1,2]. R-Smads are transcription fac-
tors, which need activation (usually by phosphoryla-
tion) and subsequent multimerization in order to
become active and accumulate in the nucleus. Once
activated, the R-Smad is able to bind to the Co-Smad
(Smad4) and translocate to, or accumulate in the nuc-
leus [3]. There, together with a wide variety of cofac-
tors, target gene transcription is usually activated when
R-Smads are in play.
Of course, this very linear pathway is far from real-
ity. BMP receptors have been shown to convey signals
not only by Smad phosphorylation, but also through
p38 activation [3]. Furthermore, transduction can as
well occur through so-called repressor-Smads and
Smads specific for transforming growth factor b
(TGF-b) signals such as Nodal (Table 1) [2].
BMP signals are strongly influenced by many addi-
tional parameters, such as the mode of oligomerization
of receptors even prior to ligand-binding and resulting
differences in downstream targets have also been clar-
ified in much more detail [1]. Regulation is further
fine-tuned by BMP receptor regulation through degra-
dation and dephosphorylation [4], different modes of
endocytosis [5], interaction with other pathways and

expression of pseudo-receptors [6,7].
Further downstream, Smads are again subject to
massive functional modulation by interaction with
other transcriptional modifiers [8–11], nuclear import
and export regulation [3], as well as dephosphorylation
[12–15].
BMP signals aid to keep pluripotency
in embryonic stem cell cultures
Among the hot topics in current research is the genera-
tion and use of stem cells. The theoretical applications
of having expandable and differentiation-controllable
stem cell cultures are extremely promising. However,
basic knowledge of molecular changes happening in
cells that are transferred from the embryonic milieu to
the cell culture dish is missing.
A different side of the same problem is the lack of
information on factors that guide self-renewal and plu-
ripotency in the embryo or adult.
Since 1988, the key player in embryonic stem (ES)-
cell media for the best established culture system,
murine ES-cells, has been leukaemia inhibitory factor
(LIF) [16]. Even though cultures have to be addition-
ally supplemented by serum, feeder cells and other
factors, LIF is considered to be necessary for pluripo-
tency. The signaling cascade triggered by LIF is trans-
duced through phosphorylation and subsequent
translocation of the signal transducer and activator of
transcription-3 (STAT3) to the nucleus [17,18].
Although most of the data reviewed below has been
collected with mouse embryonic stem cells, it should

be noted that recent studies in nonhuman primate
ES-cell cultures [19] as well as in human ES-cell culture
systems [20,21], have demonstrated complete independ-
ence of LIF and STAT3.
Back in the mouse system, feeder cells and serum
can be omitted if BMP2 ⁄ 4 and LIF are present in the
medium [22], resulting in a very defined two-factor sys-
tem to study pluripotency. The same work demonstra-
ted that the downstream target genes primarily
responsible for the pluripotency maintenance effect of
BMPs under these conditions are the inhibitors of
DNA-binding (ID) genes. ID gene transcription was
previously shown to be enhanced by a Smad1–Smad4
complex directly binding GC-rich elements in combina-
tion with Smad-binding elements (SBE, sequence:
GTCT) present in the ID1 promoter region [23]. ID
gene expression is further enhanced by another well
known pluripotency associated factor called Nanog
[24] in a not yet understood way.
Adding to the picture are data obtained by micro-
array-based analysis [25] of murine stem cells, in which
Table 1. BMP and TGF-b signal transducer molecules of the Smad family and their respective function [1,2].
Name Type Ligands Receptors (type I) Function
Smad1 R-Smad AMH, BMP2 ⁄ 4 ⁄ 7 ALK1 ⁄ 2 ⁄ 3 ⁄ 6 Transcriptional regulation
Smad2 R-Smad Activin, Nodal, TGF-b ALK4 ⁄ 5 ⁄ 7 Transcriptional regulation
Smad3 R-Smad Activin, Nodal, TGF-b ALK4 ⁄ 5 ⁄ 7 Transcriptional regulation
Smad4 Co-Smad all – Co-Smad needed for All R-Smads
Smad5 R-Smad AMH, BMP2 ⁄ 4 ⁄ 7 ALK1 ⁄ 2 ⁄ 3 ⁄ 6 Transcriptional regulation
Smad6 I-Smad – All Decoy Smad, inhibition of Smad interactions
Smad7 I-Smad – All Decoy Smad, inhibition of Smad interactions

Smad8 R-Smad AMH, BMP2 ⁄ 4 ⁄ 7 ALK1 ⁄ 2 ⁄ 3 ⁄ 6 Transcriptional regulation
T. U. Wagner BMP signaling in stem cells
FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS 2969
several genes with STAT3 dependent expression were
identified. Among the list of strongly up-regulated
genes after LIF ⁄ STAT3 signaling inactivation are fac-
tors generally associated with TGF-b and BMP signa-
ling cascades including Lefty1, ID1 and ID2. These
data already indicate negative transcriptional regula-
tion between LIF ⁄ STAT3 signaling on one end, and
TGF-b ⁄ BMP signaling on the other end.
Strong activation of the BMP pathway was shown
to lead to differentiation of embryonic stem cells into
mesodermal and endodermal lineages, whereas neural
differentiation is actively suppressed [22].
Low levels of BMP, followed by transduction via
Smad1, already leads to up-regulation of typical mark-
ers for early stages of mesodermal differentiation inclu-
ding the transcription factor Brachyury. Present in
only low amounts, Brachyury forms a complex with
STAT3. This complex has been shown to bind to the
Nanog promoter and enhance its transcription. Nanog
protein in turn was proven to bind to Smad1 and sup-
press formation of Smad1 transcriptional activator
complexes [26]. Closing the regulatory circle, a prime
target of the thus inhibited Smad1-complex mediated
transcriptional activation would be Brachyury. Thus,
early stages of differentiation triggered by low level
BMP signaling are reversible by simultaneous presence
and action of Nanog and STAT3. Consequently, sup-

port of pluripotency by BMP signals is not only highly
dose-dependent, but also needs to be counter-regulated
(e.g. by STAT3 and Nanog).
Recapitulating, the BMP-signaling pathway promotes
pluripotency only indirectly by driving expression of ID
genes in a Smad1-dependent manner. ID-proteins block
neural differentiation of ES-cells by sequestering tran-
scription factors needed to initiate commitment to this
lineage. Concurrently, the differentiation induction
effects of BMP are counteracted by STAT3 and Nanog,
which are able to suppress activation of Smad1-target
genes necessary for differentiation into mesodermal and
endodermal cell fates.
In other words, the essence of defined medium
murine ES-cell culture appears to be the simultaneous
action of STAT3 and Smad in a certain ratio. Down-
stream, negative regulation of differentiation pro-
grammes for mesodermal and endodermal fates
(mediated by STAT3) as well as neuro-ectodermal line-
ages (controlled by IDs) is initiated, resulting in the
blockage of any kind of differentiation (Fig. 1).
Taking these data from the culture system, it is inter-
esting to look at studies on BMP signaling proteins
in early embryonic development. Although Smad4
– ⁄ –
mouse embryos do not successfully undergo gastrula-
tion and die before embryonic day 7.5, it was possible
to derive ES-cell lines from the inner cell mass of these
mutants [27]. As the only Co-Smad, Smad4 is abso-
lutely necessary for any Smad-linked BMP signal con-

duction to the nucleus. Thus, these experiments suggest
that the pluripotent stem cell state within the embryo is
not depending on Smad signaling. Initially, this contra-
dicts a basal role of BMP in pluripotency described
before based on cell culture experiments. There is, how-
ever, dependency on BMPR-IA (ALK3) because it is
impossible to derive ES-cell lines from ALK3 null
embryos [27]. The discrepancy of BMP receptor
dependence on one hand and Smad4 independence on
the other suggests a Smad-independent mode of BMP
signal transduction. The only other known transduc-
tion pathway of BMP receptor activation is mediated
by p38, a mitogen-activated protein kinase (MAPK)
family member, via a complex of adapter proteins
including XIAP, Tab1 ⁄ 2 and Tak1 [28,29]. Here, it is
interesting to note that BMP4 treatment of mouse
ES-cell cultures results not only in up-regulation of ID
genes, but also in the up-regulation of Oct4, a definitive
marker of pluripotency, accompanied by a short-
termed drop of p38 phosphorylation levels [27]. The
mediators of these effects remain unidentified, with
many candidates from MAPK phosphatase families. In
a key experiment, simulation of the BMP induced
de-phosphorylation effect on p38 by its inhibitor
SB23580 enabled derivation of pluripotent stem cell
lines from ALK3
– ⁄ –
embryos. Even more striking is the
finding that these cell lines were subsequently able to
Fig. 1. Model of pluripotency control in cultured feeder and serum-

free mouse embryonic stem cells. Parallel activity of STAT3 and
Smad leads to inhibition of differentiation programs induced by the
other pathway, thereby upholding the pluripotent state of the cells.
The balance is easily broken upon signal increase in any direction.
BMP signaling in stem cells T. U. Wagner
2970 FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS
tolerate lack of ALK3 in absence of the inhibitor.
Although functional proof is missing, the authors
found up-regulation of ALK1 and ALK2 in these cells
and suggest these receptors to be able to compensate
ALK3 loss. Bringing the different aspects together,
alternative BMP signaling pathways all seem to be able
to support pluripotency, but a complete loss of BMP
signal transduction is not compatible with stemness. To
truly clarify this situation, additional studies using con-
ditional depletion of all combinations of the suggested
transduction ways are needed.
Yet another BMP signal influencing factor associ-
ated with pluripotency has been identified, namely
growth and differentiation factor-3 (GDF3) [30].
GDF3 is exclusively expressed in the undifferentiated
state in both mouse and human ES-cell culture. GDF3
is a secreted factor, which is able to bind to and
thereby inactivate BMP4. Reduction of GDF3 expres-
sion in murine ES-cells lead to increased independence
of LIF but, at the same time, to a lack of mesodermal
and endodermal differentiation-ability in vitro. In vivo,
GDF3 is expressed during early embryogenesis in mice,
notably in the inner cell mass. Protein localization
shows extracellular distribution throughout the blasto-

cyst embryo. During gastrulation, GDF3 mRNA was
detected in the node.
These data substantiate the notion that fine-tuning
BMP-signal strength, timing and downstream pathway
choices are strongly influencing cell fate decisions both
in early embryonic cells and stem cell cultures. How-
ever, the molecular mechanisms underlying this non-
linear behaviour are not yet identified. They likely
include cross-talk with other prominent signaling path-
ways such as Wnts on multiple levels of the cascades.
BMP action in other stem cell niches
BMP pathways are not only involved in pluripotency
control, but also in various other stem cell niches, both
in adults and embryos. The roles of BMP in those
niches are far from uniform.
BMP signaling induces differentiation of neural
stem cells
Another variant of signal integration between BMPs
and STAT3 has been identified in the case of differen-
tiation of neural stem ⁄ progenitor cells (NSCs). Here,
the neurogenesis versus astrocytogenesis decision
during early differentiation is based on a network of
negative regulation. Co-immunoprecipitation assays
showed that STAT3 and Smad1 are complexed via
the transcriptional cofactor p300 [31]. This complex
enhances the differentiation of fetal neuroepithelial
cells into astrocytes, by binding and hyper activating
the promoter of glial fibrillary acidic protein (GFAP).
In parallel, BMP signals lead to enhanced expression
of ID proteins, which in turn bind and sequester

bHLH transcription factors such as Neurogenin1 and
Mash1, both responsible for neurogenesis [32]. Addi-
tionally, BMP exposure results in down-regulation of
Olig2 expression [33]. Olig2 in turn inhibits formation
of the GFAP superactivator complex STAT3–p300–
Smad1 [34], thus clearing the path for neuronal differ-
entiation for cells exposed to low amounts of BMPs.
The p300–Smad1 complex is target for yet another reg-
ulatory input. Neurogenin has been shown to compete
with STAT3 for its recruitment. Although the GFAP
promoter is hyper activated when bound by Smad1–
p300–STAT3, the neuroD promoter is strongly driven
by binding of Smad1–p300–Neurogenin [35], giving
BMP signals a role in neurogenesis as well. Other stud-
ies [36] have shown that LIF or BMP4 alone are also
able to drive GFAP expression in neurosphere cul-
tures. Phenotypically, the resulting GFAP
+
cells gener-
ated by either LIF or BMP4 differ strongly: whereas
LIF induces GFAP expressing NSCs to become elon-
gated and stay proliferative, BMP4 application results
in cell-cycle exit and a star-like cell-morphology. Fur-
thermore, LIF treatment leads to an upkeep of progen-
itor features, such as prolonged culture ability and the
potential to undergo neural differentiation, whereas
BMP4 decreases both. These results stress that BMP
signaling is indeed an antiproliferative and differenti-
ation inductive signal for neural stem cells, again (as
shown and discussed for murine ES-cells) modulated

by LIF ⁄ STAT3 in a highly dose-dependent manner.
BMP signals as inhibitors of differentiation
of pancreatic progenitors
A prototypic example for a cell type where BMP sig-
nals play an inhibitory role for differentiation are pan-
creatic progenitor cells. Upon treatment of these
progenitors with BMP4, increased levels of ID2 result
in inhibition of NeuroD function [37]. NeuroD is a
classical target for ID proteins because it is a member
of the bHLH transcription factor family, which can be
efficiently bound and thereby inactivated by ID pro-
teins. In the rat tumour cell line AR42J, derived from
the acinar pancreas, and isolated primary interferon-c-
NOD epithelial duct cells, ID2 expression leads to
down-regulation of the NeuroD target gene PAX6, an
important factor for final differentiation of the progen-
itors into endocrine cells. In parallel, BMP4 treatment
increases proliferation activity of the progenitors.
T. U. Wagner BMP signaling in stem cells
FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS 2971
These data once more demonstrate that BMP signals
are able to promote stemness and block differentiation
in specific contexts.
BMP as a specification switch in hair-cell
progenitors
NeuroD also plays essential roles in hair-cell specifi-
cation in the inner ear. Recently, the effects of BMP
on hair-cell progenitors have been investigated. BMP4
is a well established marker for otic sensory patches
in various species. Over expression of BMP4 in

explanted chicken otic vesicles leads to fewer hair
cells [38]. Inhibition of BMP signals by Noggin
results in increasing numbers of hair-cells. Cell-cycle
and apoptosis analyses of these experiments reveal
that BMP4 not only suppresses expression of prosen-
sory markers (itself including), but also drives the
proliferative sensory precursor cells into apoptosis. In
the same context, Noggin application is able to
expand the sensory patches without increasing pro-
genitor proliferation. This suggests that BMP4 has a
double function in restriction of hair-cell number:
block of differentiation and stop of progenitor prolif-
eration, with both effects leading to apoptosis. The
molecular mechanism at work in these progenitor
cells has not yet been addressed, but might include
ID protein mediated block of bHLH factors such as
NeuroD, which are responsible for correct progres-
sion of final differentiation steps.
BMP signals lead to proliferation, apoptosis
and cell-cycle arrest within the eye
Studies in the chick embryo have revealed a role for
BMP4 in eye development. Implantation experiments
of beads soaked in BMP4 have shown that BMP is
able to induce programmed cell death (apoptosis).
Blocking the BMP pathway by using Noggin-leaded
beads does not lead to over proliferation but, on the
contrary, restricts growth and, when applied for longer
periods, will result in reduced size of the optic cup
[39]. Surprisingly, apoptosis is inhibited at the same
time. In line with this, BMP4, even though responsible

for programmed cell death in the optic cup, increases
cell proliferation.
The effects on lens tissue are completely different:
Noggin cannot stop apoptosis there, and BMP4 ⁄ 7
application leads to over proliferation.
This set of experiments clearly demonstrates how
versatile the BMP pathway really is. Within a very
small region, clear subdivision of pro-apoptotic, pro-
proliferative and ignorant cell responses to BMP signal
effects alternate. To date, the intracellular mechanisms
are not well understood. In almost all cases studied,
BMP signalling will directly lead to target gene up-
regulation. The best studied ones with functions in
stem and progenitor cells are ID1-4 and Msx1 ⁄ 2.
Probably, there are many more direct as well as
cell-type specific targets with yet unknown functions to
be found once more in-depth molecular analyses are
performed.
Cell fate determination as a consequence
of Nodal versus BMP signals
Patterning events during early development are often
guided by BMP activity gradients. In a recently des-
cribed case, mesodermal progenitor cell commitment
was shown to be controlled by a graded exposure to
BMP and Nodal ligands. With zebrafish as a model
system, Szeto and Kimmelman [40] used elegant trans-
plantation experiments to demonstrate that the somites
along the anterior–posterior body axis are divided into
three regions: the anterior trunk, the posterior trunk
and the tail. According to their experiments, the fate

of mesodermal progenitor cells (MPCs) is set at gastru-
lation. A BMP signal gradient originating from the
posterior end of the embryo establishes a boundary
between trunk and tail domains. Determination of the
two trunk domains is probably due to higher levels of
Nodal signaling and weakening of BMP signaling by
antagonists such as Chordin or Follistatin, which are
most likely genetically downstream of Nodal. The
MPCs are then able to integrate and interpret the
signal strengths of Nodal and BMP by entering
the somite regions at different somites. Strong Nodal
drives them in early, at somite 1, whereas strong BMP
delays their entry until somite 16. MPCs receiving both
weak Nodal and weak BMP input enter in-between, at
somite 9.
So far, it has not been possible to truly visualize gra-
ded signal activities in living embryos. Furthermore,
the cellular machinery for signal integration also
remains elusive. Whether this process is strictly nuc-
lear, transcriptional control of sets of target genes or
happens in the cytoplasm where translocation and acti-
vation of transducer molecules is modulated in is also
unclear. There is a significant gap between the avail-
able in vivo knowledge coming from analyses of pheno-
types as a result of BMP signal strength and in vitro
knowledge about intracellular processes downstream of
signal triggering.
There is another example of stemness decisions by
Nodal versus BMP signaling emerging from human
embryonic stem cell research. Testing human ES-cells

BMP signaling in stem cells T. U. Wagner
2972 FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS
for signaling activity, James et al. [41] found constantly
active TGF-b signaling as demonstrated by nuclear
Smad2 staining and detectable phosphorylation of this
signal transducer. The main transducer of BMPs how-
ever, Smad1, resided mostly in the cytoplasm and was
not phosphorylated. As expected, addition of BMP to
the medium of these cultures resulted in a down-regu-
lation of Oct4 and an accordant change in cell mor-
phology reminiscent of differentiation. Interestingly,
supplementation with activin A did not alter Oct4 lev-
els and the cells retained their typical embryonic stem
cell morphology. Only recently, proof for the involve-
ment of TGF-b signaling in pluripotent cell cultures
has been extended to the murine system. Ogawa et al.
[42] used either ectopic expression of Smad7 or the
chemical inhibitor SB-431542 to block activin ⁄ TGF-b
signaling in murine stem cell cultures. Both experi-
ments resulted in a strong growth inhibition of the
stem cells but, fascinatingly, did not interfere with their
pluripotency, as judged by mRNA levels of Oct4,
Nanog and Sox2. Inhibition of BMP signaling by
ectopic expression of Smad6 neither interfered with
proliferation, nor did it lead to changes in pluripotency
[42]. These results suggest that there is not only a
mechanistic difference between proliferation and pluri-
potency of stem cells, but also that both BMP and
TGF-b signaling are dispensable for stemness, even
though they are capable of supporting it under specific

conditions as described before.
BMP as an initiator of the germ line
Cells of the germ line are unique in many aspects.
Their DNA and differentiation state have to be con-
trolled over generations. They are extremely mobile
and on their long way from being defined to arriving
in the gonad they are exposed to, and ignore,
practically all extracellular cues used for building
and patterning the embryo. Generally, with the
appearance of primordial germ cells (PGCs), the
germ line is usually the earliest cell-lineage that
is determined in the embryo. In many lower ani-
mals, such as flies and fish, they are defined by
maternally deposited factors. Strikingly, they have
even been shown to be pluripotent after in vitro
expansion [43].
In the mouse embryo, PGCs are formed during
embryonic day 6 at the posterior proximal epiblast
through a location dependent mechanism. Using gene
knockouts, the molecules responsible were identified as
BMPs. The primary induction of PGCs is driven by
BMP4 [44], whereas the number of PGCs is guided by
BMP2, BMP4 and BMP8b in synergistic action [45].
As demonstrated by in vitro culture assays [30], inde-
pendent BMP signals originating from the visceral
endoderm and the extra embryonic ectoderm are
necessary for proper PGC induction. The receiving
receptors were found by knockout experiments, where
no PGCs were present in ALK2
– ⁄ –

embryos, and lower
than wild-type numbers are found in ALK2 heterozy-
gous animals. The same effect is true for BMP4 knock-
outs and heterozygotes, which can then be rescued by
ectopic expression of a constantly active variant of
ALK2. How these different BMP signals are finally
integrated to first guide induction and later number is
yet unclear.
Summary
BMP signals influence various kinds of stem cells, inter-
estingly, with very diverse outcomes (Table 2). This
pathway is a prime example of how cells are able to
integrate the very same signal into their current
molecular state. How exactly this is achieved is far
from being understood. There are examples of this
integration on many levels of signal transduction
(Table 3). In some cases, the interaction of downstream
signal transducers produces different transcriptional
Table 2. BMP signal outcome in described stem cell types.
Stem cell type BMP signal outcome
Embryonic stem cells (i) Stemness upkeep when counteracting STAT3 is balancing BMP effects [10]
Embryonic stem cells (ii) Differentiation when dominating other signals (especially LIF, STAT3) [14]
Hair-cell progenitors (inner ear) Apoptosis of proliferative progenitors [24]
Mesodermal progenitor cells Exposure to BMP during gastrulation defines mesodermal progenitors in their
identity along the anterior–posterior axis of the embryo [26]
Neural progenitor cells Differentiation into astrocytes [22]
Pancreatic progenitor cells Inhibition of differentiation and increased proliferation of progenitors [23]
Primordial germ cells Cell lineage definition and cell number control [29]
Progenitors of the eyefield Depending on the exact cell type increases proliferation, induces cell-cycle
arrest or drives cells into apoptosis [25]

T. U. Wagner BMP signaling in stem cells
FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS 2973
outcomes. This has been seen when Smad1 and STAT3
interact to guide astrocyte fates. For embryonic
stem cell cultures, it is more likely an effect of negative
regulation of target genes: here, STAT3 and its targets
seem to block transcription of BMP signal targets and
vice versa. Certainly, the signal integration process
has to be extended beyond the transducer molecules.
One such level of signal modulation occurs outside
of the signal receiving cell, by secretion of blocking
ligands such as Noggin, Chordin and Follistatin or
GDF3. Another way to modulate ligand ⁄ receptor inter-
action is by receptor localization within the membrane
[46]. Additionally, the makeup of BMP-receptor com-
plexes is crucial for the choice of the transduction route
and resulting cellular response. BMP2, for example,
can bind a type I receptor (e.g. BRIa) with high affinity
[47] and induce subsequent BRII recruitment to the
complex. This mode of complex formation leads to
signal conveyance via the p38 MAPK pathway. Alter-
natively, a preformed receptor complex including
type I and type II receptors is able to bind BMP2, in
which case transduction will occur through Smad
activation [48].
There is further evidence for higher-order cross-talk
between BMP signaling proteins and those of other
pathways close to the membrane (e.g. with PI3-Kin-
ase [49] or Dullard [4]), in cytoplasmic complexes
(e.g. Smad1 and Enofin [50]) and nuclear complexes

(e.g. b-catenin, Tcf4 and Smad1 [9] or Notch-IC and
Smad3 [11]).
Taken together, the currently available evidence
strongly suggests that cells are defined in their identity
by the sequence of signals they are exposed to, whereas
their respective responses to a molecule cocktail (often
referred to as niche) appear to be highly variable.
BMP signaling cannot be attributed to define or differ-
entiate a stem cell of any kind on its own. Rather, it is
an integral part of stemness and also differentiation
signaling depending on the the current transduction
programme of receiving cell, which involves multilevel
integration of a variety of signals.
Acknowledgements
I would like to thank Dr M. Schartl for critical read-
ing and general help. Furthermore, I need to thank Dr
A. Herpin for his patience and inspiring discussions. I
also have to thank the reviewers for helpful comments,
enhancing this article and making it more comprehen-
sive. I want to thank sources of funding: Deutsche
Forschungsgemeinschaft through GK1048 ‘Vertebrate
Organogenesis’ and the European Community through
Plurigenes. I have to apologize to a high number of
scientists whom I was not able to cite due to space
limitations, especially those cited indirectly via more
general BMP signaling reviews.
Table 3. BMP signal modulation. PIAS, protein inhibitor of activated STAT proteins.
Site Mechanism Example
Extracellular space Modification of ligand-receptor affinity ) usually by
binding to ligand dimers

Dimeric secreted proteins such as noggin,
chordin, follistatin and GDF3 bind and therby
inactivate BMP dimers [16]
Cell membrane Predimerization of type I receptors,
type I–type II receptors
Receptor heterooligomers induced by ligand
binding signal via Smads, preformed complexes
signal via p38 [31]
Receptor complex inhibiton BAMBI pseudoreceptors block receptor activation
[1]
Receptor–adapter junction R-Smads are kept from interacting
with receptor complexes
Smad7 binds to type I receptors and thus blocks
R-Smads from being activated by the receptors
[1]
Adapters inhibit receptor activation FKBP12 inhibits type I receptor phosphorylation
[1]
Cytoplasm Smad expression and covalent modification Sumoylation by PIAS, ubiquitylation by Smurf1 ⁄ 2
[1]
Competition for Smad4 binding Smad6 sequesters Smad4, thereby blocks Smad1
binding and nuclear accumulation [1]
Nucleus Co-factor-availability, corepressors and coactivators
can form a complex
Ngn competes with Stat3 for Smad1–p300
complexes in glial differentiation [21]
Nuclear import, export and retention MAPK phosphorylation of Smads leads to export
from the nucleus [2]
Dephosphorylation of Smads to end a signal Several phosphatases targeting the SXS
C-terminal regions of Smad1 ⁄ 2 ⁄ 3 [12–15]
BMP signaling in stem cells T. U. Wagner

2974 FEBS Journal 274 (2007) 2968–2976 ª 2007 The Author Journal compilation ª 2007 FEBS
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