Signaling pathways and preimplantation developmentof mammalian embryos Yong Zhang1, Zhaojuan Yang1and Ji Wu1,2 1 School of Life Science and Biotechnology, Shanghai Jiao Tong University,
Trang 1Signaling pathways and preimplantation development
of mammalian embryos
Yong Zhang1, Zhaojuan Yang1and Ji Wu1,2
1 School of Life Science and Biotechnology, Shanghai Jiao Tong University, China
2 Key Laboratory of Cell Differentiation and Apoptosis of Ministry of Education of China, Shanghai Jiao Tong University, China
An embryo is a stage in the development of plants,
invertebrate and vertebrate animals Embryonic
devel-opment is a key event in the organism and is under
rigorous control Preimplantation growth is one of the
early embryonic development processes, from a
single-cell zygote, to a morula, to a blastocyst Furthermore,
preimplantation development is critical in establishing
a viable mammalian pregnancy During this period,
the zygote initiates its first cell division and the first
lineage cell begins to differentiate into the inner cell
mass and the trophectoderm These processes are
com-plex and are regulated by various cell-signaling
path-ways Each signal-transduction pathway is primarily
responsible for one or several related biological
pro-cesses, such as cell division, growth, differentiation,
migration, apoptosis, transformation, immune response and polarity By combining several functions, such as cross-linking and other interactions, these pathways form a complicated signaling network Successful embryo development requires functional signaling net-works, and any disruption to these networks may lead
to abnormal development or fatal disease
Although there is a reasonably sound understanding
of the specific events associated with mammalian pre-implantation embryo development, including activation
of the zygotic genome, development of the anterior– posterior axis, compaction, and blastocyst formation, little is known about the intracellular signaling path-ways that regulate these events [1–6] Several signal-transduction pathways have been shown to be involved
Keywords
development; preimplantation embryo;
signaling pathways; signaling transduction
network; stage-specific expression pattern
Correspondence
J Wu, School of Life Science and
Biotechnology, Shanghai Jiao Tong
University, no 800, Dongchuan Road,
Minhang District, Shanghai, 200240, China
Fax: 86 21 3420 4051
Tel: 86 21 3420 4933
E-mail: jiwu@sjtu.edu.cn
(Received 17 April 2007, revised 12 June
2007, accepted 5 July 2007)
doi:10.1111/j.1742-4658.2007.05980.x
The mammalian preimplantation embryo is a critical and unique stage in embryonic development This stage includes a series of crucial events: the transition from oocyte to embryo, the first cell divisions, and the establish-ment of cellular contacts These events are regulated by multiple signal-transduction pathways In this article we describe patterns of stage-specific expression in several signal-transduction pathways and try to give a profile
of the signaling transduction network in preimplantation development of mammalian embryo
Abbreviations
BMP, bone morphogenetic protein; BMPR, bone morphogenetic protein receptor; ERK, extracellular signal-regulated protein kinase; JAK, Janus-activated kinase; JNK, Jun N-terminal kinase; LRP, lipoprotein receptor-related protein; MAPK, mitogen-activated protein kinase; PtdIns3K, phosphatidylinositol 3-kinase; PtdIns-3,4,5-P 3 , phosphatidylinositol-3,4,5-triphosphate; PtdIns-4,5-P 2 ,
phosphatidylinositol-4,5-diphosphate; STAT, signal transducer and activator of transcription; TGF, transforming growth factor; Wnt, Wingless.
Trang 2in this process, including mitogen-activated
pro-tein kinase (MAPK), phosphatidylinositol 3-kinase
(PtdIns3K)⁄ Akt, Wingless (Wnt) ⁄ b-catenin, Notch,
bone morphogenetic protein (BMP)–Smad,
transform-ing growth factor (TGF)-b, Hedgehog, and
Janus-acti-vated kinase (JAK)⁄ signal transducer and activator of
transcription (STAT) signaling pathways Moreover,
these signaling pathways play a central role in the
embryonic development processes of other vertebrate
and invertebrate animals [7–13]
Detailed mechanisms of these signaling pathways are
now better understood, and most have been reviewed
previously [14,15] This article describes the patterns of
stage-specific expression of several signal-transduction
pathways and the signaling transduction network in
the preimplantation development of the mammalian
embryo
Stage-specific expression pattern of
several signal-transduction pathways
in the preimplantation embryo
We review the existing evidence for the presence of
each signaling pathway during preimplantation embryo
development, and summarize the stage-specific
expres-sion pattern of each signaling pathway (Fig 1)
MAPK pathways MAPK pathways transmit signals from ligand–receptor interactions and convert them into a variety of cellular responses, ranging from apoptosis to immune responses,
as well as proliferation, differentiation, growth and embryonic development The MAPK superfamily of proteins can be subdivided into four separate signaling cascades: extracellular signal-regulated protein kinase (ERK), Jun N-terminal kinase (JNK), p38 and ERK5
or big MAP kinase 1 pathway [16–19] All are highly conserved throughout eukaryotic systems Preimplanta-tion embryos utilize MAPK pathways to relay signals from the external environment in order to prepare appropriate responses and adaptations to a changing milieu It is therefore important to figure out the roles
of MAPK pathways during preimplantation embryo development
Using RT-PCR and immunostaining, 10 transcripts
of MAPK signaling pathway members have been detected in unfertilized eggs and⁄ or zygotes These genes include SOS1 (Son of sevenless 1), RSK1 (ribo-somal S6 kinase 1) and MAPK⁄ ERK2, the expression
of which is lowest in unfertilized eggs; RSK3 and MAPK⁄ ERK5 are expressed at extremely low levels
in blastocysts; and GAB1 (Grb2-associated binder 1)
Zygote 2-Cell 4-Cell 8-Cell
Wnt-3a
BMPR-II
Notch-3, DII-1, Dtx-1
BMRP-1B
80Kda and 110Kda subunit of PtdIns3K
BMRP-1A Notch-4, DII-4
MAPK
Raf1 MEK-1, MEK-2, MEK-5, MAPK/ERK1 SOS1, GAB1
MAPK/ERK2 MAPK/ERK5, RSK3
STAT5 JAK-STAT
Fig 1 Stage-specific expression of several signal-transduction pathways in the preimplantation development of the mammalian embryo Red, Wnt signaling pathway; blue, Notch signaling pathway; green, BMP signaling pathway; yellow, PtdIns3K signaling pathway; gray, JAK-STAT signaling pathway.
Trang 3Raf1, Rafb, MEK (MAPK or ERK kinase)-1, -2, -5,
and MAPK⁄ ERK1 are detected in unfertilized eggs
and blastocysts Transcripts and the protein
localiza-tion of p38-regulated and -activated kinase, p38
MAPK, MK2 and hsp25 have also been observed
throughout murine preimplantation embryo
develop-ment These proteins have been detected in
tropho-blasts on embryonic day (E)3.5, when they mediate
mitogenic fibroblast growth factor signals from the
embryo or colony-stimulating factor-1 signals from
the uterus [8,20] The phosphorylation state and
position of the phosphoproteins in the cells suggest
that they might function in mediating mitogenic
signals
Raf1 is expressed abundantly in unfertilized eggs
and throughout preimplantation embryo development
Expression of MEK-1, -2, -5, and MAPK⁄ ERK1 is
lowest in unfertilized eggs, and gradually increases
throughout the blastocyst stage SOS1 and GAB1 are
also expressed at a low level in unfertilized eggs, but at
the beginning of the two-cell stage expression abruptly
increases and continues throughout preimplantation
embryo development MAPK⁄ ERK2 could not be
detec-ted in unfertilized eggs but was detecdetec-ted at the two-cell
stage; it also increased throughout preimplantation
embryo development This is in accordance with
activation of the zygotic genome MAPK⁄ ERK5 and
RSK3 mRNA was abundantly and increasingly
detected in unfertilized eggs up to the eight-cell⁄
com-paction stage, but was not detectable at the blastocyst
stage [21,22]
According to some experimental results, the JNK or
p38 MAPK pathway is required for development from
the 8–16-cell stage to the blastocyst stage, and p38
MAPK is a regulator of filamentous actin during
preimplantation embryo development [22] Active
JNK and p38 MAPK pathways are required for cavity
formation during mouse preimplantation embryo
development, because inhibition of such signaling
pathways, excluding the ERK pathway, inhibits cavity
formation [23] Maternal RNA of fibroblast growth
factor receptor substrate 2 (FRS2alpha), GAB1,
growth factor receptor-bound protein 2(GRB2), SOS1,
Raf-B and Raf1 genes may delay the presence of the
lethal phenotype of null mutations These genes are
considered to be postimplantation lethal knockouts of
the genes for lipophilic MAPK pathway proteins
They are all expressed at the protein level in the
cyto-plasm or in the cell membrane of E3.5 embryos, at
a time when the first known mitogenic intercellular
communication takes place It is still not clear why the
lethality of these null mutants arises after implantation
[24]
Wnt signaling pathway The Wnt signaling pathway consists of 19 Wnt genes encoding secreted proteins [25], 10 Wnt receptors composed of Frizzled genes, and low-density lipopro-tein receptor-related prolipopro-tein (LRP) 5–6 as coreceptors participating in signal transmission [26] Antagonists
of Wnt signals include two categories [27] Fzb (frizzled-b) with its four homologs forms the secreted frizzle-related protein (Sfrp) family, which can block activation of the receptor through binding to Wnt proteins directly [28] Dickkopf-1 (Dkk1) and its three homologs can bind to and inactivate the LRP core-ceptors [29–31] There are several intracellular compo-nents of the Wnt signal-transduction pathway The canonical Wnt pathway (b-catenin pathway) is the best characterized, and includes a series of phospho-rylation reactions that eventually activate target genes
in the nucleus Signal pathways triggered by Wnts (Wnt1, -2, -2b, -3, -3a, -6, -7b, -8a and -8b) belong
to this phosphorylation mechanism The signal-trans-duction pathway activated by other Wnts (Wnt4, -5a, and -11) is regulated by noncanonical pathways involving the intracellular signaling cascade of Ca2+
or JNK
b-Catenin is present in the eggs and early embryos
of some vertebrate species; it is the first essential com-ponent of the signal-transduction pathway that leads
to formation of the endogenous dorsal–ventral axis Studies of immunoreactivity of total b-catenin in pre-implantation embryos, from the two-cell stage to the blastocyst stage, have shown that b-catenin accumu-lates on the cell surface rather than in the nucleus [32– 34] It has been shown that endogenous b-catenin accumulates in the prospective dorsal side of the embryo as early as the first division, and continues to accumulate in the cytoplasm of all animal and vegetal blastomeres, to a greater extent on the prospective dor-sal side than on the ventral side, during the early cleavage stages By the 16- and 32-cell stages, b-catenin accumulates in the dorsal but not the ventral nuclei when zygotic transcription begins The pattern of b-catenin accumulation after cortical rotation thus reflects the distribution of the transplantable dorsal-determining activity The nonphosphorylated isoform
of b-catenin accumulates in response to Wnt signaling [35] Recent studies have shown that b-catenin is neces-sary and sufficient for formation of the dorsal axis, and that it accumulates in cells that give rise to the dorsal side of the embryo These results indicate that the Wnt⁄ b-catenin signaling pathway is not active in embryos until the blastocyst stage They also show that activation of the Wnt signaling pathway is sufficient to
Trang 4maintain the pluripotency of embryonic stem cells, and
that b-catenin is localized in the nuclei of the inner cell
mass, but not trophoblast cells in the blastocyst
[8,26,36] This suggests that Wnts may participate in
cell determination in preimplantation embryos
Recently, the expression patterns of several Wnts
during preimplantation stages have been reported,
and mRNAs encoding for Wnt1, -2b, -3, -3a, -4, -5a,
-5b, -6, -7a, -7b, -10b and -11 have been described
[7,8,34] Transcripts of Wnt3a, -6, -7b, -9a and -10b
have been detected in blastocysts, and Wnt1 and -4,
Sfrp1 and Dkk1 are highly expressed at this stage
[37] Receptors (Fz2, frizzled-2 and Fz4, frizzled-4),
intracellular signal transducers and modifiers
[Dishev-elled (Dsh), adenomatous polyposis coli (APC), axin],
as well as nuclear effectors (e.g homologs of
Dro-sophila arm, Tcf and groucho) are also present in
blastocysts [8] Transcripts of Wnt3a are found at the
2-cell stage, decreased at the 4-⁄ 8-cell stages, and are
strongly expressed in compact 8- and 16-cell and
early blastocysts The source of the Wnt3a transcripts
in 2-cell embryos, i.e whether of maternal or
embry-onic origin, is not clear, because the major gene
expression transition from the maternal to zygotic
stage occurs in the late two-cell embryo [38] The
onset of expression of Wnt4 is observed in the 4-⁄
8-cell stages, and is more strongly expressed at the
8- and 16-cell and blastocyst stages Both Wnt3a and
-4 transcripts have been detected in some precompact
4-⁄ 8-cell stages, with consistent expression detected in
all compact 8- and 16-cell and blastocyst stages [8]
Primers specific for Wnt11 amplified the expected size
product at the blastocyst stage, as well as in 10-week
whole fetus libraries during human preimplantation
embryo development [39] These data suggest that
Wnts play a role in cell development and in
cellu-lar interactions occurring in preimplantation embryo
development
By analyzing the expression levels of all 19 Wnt
genes and their 11 antagonists in mouse blastocysts,
pregastrula, gastrula and neurula stages, new
expres-sion domains for Wnt2b and Sfrp1 have been found
in the future primitive streak at the posterior side and
in the anterior visceral endoderm before the initiation
of gastrulation Moreover, the anterior visceral
endo-derm expresses three secreted Wnt antagonists (Sfrp1,
Sfrp5 and Dkk1) in partially overlapping domains
Notably, the predominant expression of Wnt1 and
Sfrp1 in the inner cell mass, and of Wnt9a in the
mural trophoblast and inner cell mass surrounding the
blastocele, suggests that the Wnt signal-transduction
pathway plays a novel role in preimplantation embryo
development
The PtdIns3K/Akt signal transduction pathway PtdIns3Ks consist of three types of enzymes, but they can produce lipid secondary messengers by phosphory-lation of plasma-membrane phosphoinositides at the 3¢OH group of the inositol ring [40] Class 1 PtdIns3Ks include a catalytic subunit (110 kDa, p110) and an adaptor⁄ regulatory subunit They can be sub-grouped into class 1A and 1B PtdIns3Ks according to their different catalytic subunits Class 1B PtdIns3Ks encompass a p110r catalytic subunit, associated with a
101 kDa (p101) adaptor subunit [40–43]
Class 1A PtdIns3Ks are activated through binding
of the Src homology (SH2) domain in the adaptor sub-unit to autophosphorylated tyrosine kinase receptors,
or to nonreceptor tyrosine kinases in the cytoplasm, such as the Src family kinases or JAK kinases Activa-tion of class 1B kinases occurs in the binding of the catalytic subunit to heterotrimeric GTP-binding proteins or G proteins Activated PtdIns3Ks pre-ferentially phosphorylate phosphatidylinositol-4,5-di-phosphate (PtdIns-4,5-P2) in vivo, to produce phosphatidylinositol 3,4,5 triphosphate
(PtdIns-3,4,5-P3) [42] In turn, the production of PtdIns-3,4,5-P3 is regulated by the phosphates phosphatase and tensin homolog deleted on chromosome 10 which catalyzes the dephosphorylation of PtdIns-3,4,5-P3 to PtdIns-4,5-P2 [44,45] A wide variety of signal-transduction proteins, including Akt, interact with PtdIns3K-gener-ated phosphorylPtdIns3K-gener-ated phosphoinositides via lipid-bind-ing pleckstrin homology domains [46] This facilitates recruitment of these proteins to the plasma membrane and their subsequent activation Akt, a well-known serine–threonine kinase mediator of survival signals is the best characterized downstream target of PtdIns3K
It is a central player in multiple signaling pathways, and acts as a transducer of many functions initiated by growth factor receptors that activate PtdIns3K [47] The PtdIns3K⁄ Akt signaling pathway is a major pathway that has been found to regulate cell survival downstream of activated growth-factor receptors The expression and function of this pathway have been documented during early and late stages of the repro-ductive process, including in murine preimplantation embryos PtdIns3K signaling is required to suppress apoptosis in preimplantation embryos, because pro-grammed cell death is rapidly induced by inhibition of PtdIns3K with LY294002 [48] Riley et al [13] found, using confocal immunofluorescence microscopy and western blot analysis, that the p85 and p110 subunits
of PtdIns3K and Akt are expressed from the one-cell stage through to the blastocyst stage of murine pre-implantation embryo development These proteins are
Trang 5localized predominantly at the cell surface at the
one-cell stage through to the morula stage Both PtdIns3K
and Akt exhibit an apical staining pattern in
trophec-toderm cells at the blastocyst stage Phosphorylated
Akt was determined throughout murine
preimplanta-tion embryo development, and its presence at the
plasma membrane is a reflection of its activation
sta-tus Inhibition of Akt activity has significant effects on
the normal physiology of the blastocyst Specifically,
inhibition of this pathway results in a reduction in
insulin-stimulated glucose uptake Moreover, inhibiting
Akt activity can cause a significant delay in blastocyst
hatching, a developmental step facilitating
implanta-tion Taken together, these data demonstrate the
pres-ence and function of the PtdIns3K⁄ Akt pathway in
mammalian preimplantation embryos These results
further our knowledge of the PtdIns3K⁄ Akt signaling
pathway [13]
The Notch signaling pathway
The Notch signaling pathway is evolutionarily
con-served, and it is essential for cell fate decisions in many
different tissues in multicellular organisms The data
show that the Notch signaling pathway blocks
differ-entiation towards a primary differdiffer-entiation fate in a
cell, rather than directing the cell to a second,
alterna-tive differentiation program, or forcing the cell to
remain in an undifferentiated state [14,49–53]
Relatively few signal proteins are involved in the
function of the Notch signaling pathway, in which
sig-nals from the cell surface are conveyed to the nuclear
transcription machinery The Notch receptor is
synthe-sized in the endoplasmic reticulum, undergoes
matura-tion in the trans-Golgi network, and is transferred to
the cell surface, where it interacts with ligands from
neighboring cells This interaction occurs only when
cells are in physical contact with each other The Notch
receptor is activated by this interaction and is
prototyp-ically cleaved, releasing the Notch intracellular domain
which translocates from the membrane to the nucleus,
where it interacts with the CSL DNA-binding protein
(CBF1 or Rbpsuh in vertebrates, suppressor of hairless
in Drosophila, Lag-1 in Caenorhabditis elegans) to
regu-late selected target gene expression [53,54] The Notch
signaling pathway is modulated by numerous accessory
proteins, such as members of the Deltex family [50]
Cormier et al [9] systematically examined the
expression profiles of genes that directly or indirectly
participate in the Notch signaling pathway in
pre-implantation embryo development These include
Notch1–4, Jagged1–2 (Jag1–2), Delta-like1 (Dll-1),
Rbpsuhand Deltex1 (Dtx1) Notch1, -2, Jag1–2, Dll-3,
Rbpsuh and Dtx2 transcripts are synthesized in unfer-tilized oocytes and at later blastocyst stages; Notch4 and Dll-4 mRNAs can be detected from the two-cell stage to the hatched blastocyst stage; and Notch3, Dll-1 and Dtx1 mRNAs are found in two-cell embryos and in hatched blastocysts, but are absent or present at a low levels at the morula stage These results suggest that the Notch signaling pathway may be active during these stages [9] Using cDNA microarray technology, researchers have also found that other genes of the Notch pathway are expressed in the mouse embryo, such as homologs of Drosophila N, Delta, deltex, fringe, serrateand presenilin [8]
The JAK–STAT signaling pathway The JAK–STAT5 signaling pathway plays a crucial role in the growth and differentiation of mammalian cells RT-PCR analysis shows the expression of STAT5 throughout preimplantation embryo development; inhibiting the activation of JAK might interfere with the localization of STAT5 to the nucleus, and reduce the embryo development rate, suggesting that the JAK–STAT5 signaling pathway has a key function in preimplantation embryo development [55]
The BMP signaling pathway BMPs are members of the TGF-b superfamily of growth factors, which plays a critical role in develop-mental and regenerative processes BMPs were origi-nally identified as regulators of bone formation in rodents [56] More than 30 BMPs have been identified
to date BMPs are broadly conserved across the animal kingdom, including vertebrates, arthropods and nema-todes
The BMPs fulfill their signaling function by binding
to a heterodimeric complex of two transmembrane receptors, type 1 and type 2, which have serine–threo-nine kinase activity [57–59] When ligand binding is required for type 1 receptor activation, the kinase activity of the type 2 receptors is constitutive Although BMPs can bind to each of these weakly, and subsequently recruit the second subunit, optimal ligand binding is achieved when both type 1 and type 2 recep-tors are present The type 2 receptor transphosphory-lates the type 1 receptor by ligand binding The type 1 receptor then phosphorylates members of the Smad family of transcription factors which are subsequently translocated to the nucleus, activating the expression
of target genes [60–62]
At the very beginning of the preimplantation stage, embryonic polarity and spatial patterns start to
Trang 6develop [10,11] BMP receptors (BMPRs) are essential
for this process, and BMPs exert their function by
binding to BMPRs In the preimplantation mouse
embryo, large-scale cDNA analysis has been
per-formed and has provided some insight into the phased
gene expression patterns [12] Activation of the
Xeno-pus BMP signaling pathway is coincident with the
onset of zygotic transcription, but requires maternal
signaling proteins Analysis of the expression profiles
of several BMPRs has shown that BMPR-II mRNAs
are present in the zygote, two-cell and blastocyst
stages However, no BMPR-II mRNA can be detected
at the four-cell and morula stages Expression of
ActR-I one of the BMPR-Is, similar to BMPR-II, can
be observed at the zygote, two- and four-cell, and late
blastocyst stages, but not at the uncompacted or
com-pacted morula stages BMPR-IA mRNA is detected
only in blastocysts; BMPR-IB transcripts are found at
all stages from the zygote to the uncompacted morula,
but are absent from the compacted morula and
blast-ocysts Because maternal gene products are degraded
rapidly after the start of zygotic transcription [63],
transcripts of BMPR-IB at the one- and two-cell stages
are probably maternal derivations However, at the
four-cell and uncompacted morula stages, the
tran-scripts may be from the embryonic genome Therefore,
one- and two-cell stage embryos may have the ability
to respond to BMPs, either via an ActR-I⁄ BMPR-II
receptor complex, or by forming a BMPR-IB⁄
BMPR-II receptor complex Transcripts of BMPs have been
found in preimplantation embryos However, some
researchers have detected several BMP proteins,
mito-tic arrest-deficient proteins, and other components of
the BMP signaling pathway, as well as homologs of
the receptors, in blastocysts, using cDNA microarray
analysis BMP6, a member of the 60A subgroup of
BMPs, is expressed in diverse sites in the developing
mouse embryo from preimplantation onwards [8,64]
A profile of the signal transduction
network in the preimplantation
development of the mammalian embryo
All of the above-mentioned signaling pathways in
pre-implantation development are essential for various cell
events, such as cell proliferation, differentiation and
growth, as well as apoptosis, and interactions between
them have been established in many other types of
cells and biological processes, suggesting that there
exists a complex signal network controlling
mamma-lian preimplantation development
SOX7 protein, one of the SRY box-containing
tran-scription factors (SOX proteins), can repress Wnt
signaling by inhibiting the ability of TGF⁄ lymphoid enhancer factor–b-catenin to transactivate a T-cell factor⁄ lymphoid enhancer factor-dependent reporter construct [65] Dickkopf-1 (Dkk1) is another potent antagonist of Wnt signaling [29] It specifically blocks Wnt⁄ b-catenin signaling by interacting with low-den-sity lipoprotein receptor-related protein 6 [66] Its expression is regulated by the Ap-1 family member c-Jun and it is activated by BMP-4 to induce apoptosis [67] Both Dkk1 and SOX7 have been identified during mouse preimplantation embryo development as direct targets of the p38 and JNK pathways [23] Inhibition
of the p38 or JNK pathway leads to decreased expres-sion of Dkk1 and SOX7 [23]
Dishevelled (Dsh⁄ Dvl) proteins are important trans-ducers for divergent Wnt pathways that lead to different cell events: cell proliferation, apoptosis, or differentiation [68,69] Recently, this type of protein has been identified in mouse oocytes and during preim-plantation embryo development, and has an important function in the regulation of cell adhesion in mouse blastocysts [70] The changes in expression of Dvl pro-teins are coincident with those of b-catenin and p120 catenin transcription during preimplantation embryo development, implying upregulation of Wnt signaling activity before implantation [70] Furthermore, Dvls can induce JNK MAPK signaling [71–73] The reason might be that Dvl can form Wnt-induced complexes with Rac and Rho, and Rac stimulates JNK activity independent of Rho [72,73] Rac-1 protein has been demonstrated throughout murine preimplantation embryo development and is a potential regulator of E-cadherin⁄ catenin interactions during this develop-ment progress [74]
The expression profiles of some genes that directly
or indirectly participate in the Notch signaling path-way, including Notch1–4, Jag1–2, Dll-1, Rbpsuh and Dtx1, have been observed in mammalian preimplanta-tion embryos [9] In other biological progresses, Notch signaling has been suggested to repress the activity of p38 MAPK by specifically inducing the expression of MKP-1, a member of the dual-specificity MAPK phos-phatase [75] Notch negatively regulates the JNK path-way by interfering with the interaction between JNK and JNK-interacting protein 1 (JIP1) [76] During
C elegans vulval development, LIN-12⁄ NOTCH inac-tivates MAPK to specify the secondary fate through up-regulation of isoenzymes from Candida rugosa lipase (LIP1) transcription [77] Ras activation inter-feres with endocytic routing of LIN-12, resulting in downregulation of LIN-12⁄ Notch [78] Whether the interaction between the Notch and MAPK signaling pathways similarly exists during preimplantation
Trang 7embryo development needs to be clarified by further
research
Some proto-oncogenes, including c-fos, c-jun,
c-ha-ras and c-myc, have been studied in bovine
preim-plantation blastocysts [79,80] The c-fos, c-jun and
c-ha-ras transcripts, as well as c-Fos, c-Jun and
c-Myc proteins have been detected in 13–14-day-old
preimplantation blastocysts [80] Wang et al
investi-gated whether Ras mRNA is expressed in
tropho-blasts in E3.5 embryos [61] The PtdIns3K⁄ Akt
signaling pathway, which is a major pathway for
reg-ulating cell survival downstream of activated growth
factor receptors [81], has also been demonstrated
throughout murine preimplantation embryo
develop-ment [13] The proto-oncogene Ras may suppress
c-Myc-induced apoptosis, via activation of the
PtdIns3-K⁄ Akt pathway [82] Sears et al [83] found that Ras
can control Myc protein (a regulator of the cell cycle
and essential for cell growth) accumulation via the
PtdIns3K⁄ Akt pathway by downregulating the kinase
GSK-3 that promotes Myc degradation However,
Ras regulates the accumulation of Myc activity by
stabilizing the Myc protein [84], depending on the
action of the Raf⁄ ERK pathway [85] Furthermore,
STAT5 has been identified throughout preimplantation
embryo development using RT-PCR [55] It has been
shown that STAT5 may induce cell proliferation by
activating PtdIns3K, by interacting with p85 and
Grb2-associated binder-2 (Gab2) [86,87] Nyga et al
have found that Gab2 seems to activate ERK1⁄ 2 via
PtdIns3K in Ba⁄ F3 cells that express constitutively
activated STAT5 [87] TEL-JAK2 can constitutively
activate the PtdIns3K signaling pathway independent
of the STAT5 pathway [88]
This illustrates the complex interactions among these
signaling pathways during mammalian preimplantation
embryo development (Fig 2), even if it is just a profile
of the signaling network In different preimplantation
events and different species, there should be a different
regulative pattern of this transduction network For
example, Wnt⁄ b-catenin and BMP signal pathways
play a key role in the polarization of frog eggs [89,90]
In Xenopus, stabilization of b-catenin leads to the
acti-vation of dorsal-specific genes, while a member of the
Nodal family of TGF-b signals, Squint, serves as a
dorsal determinant in zebrafish embryos [91,92]
How-ever, which part of this network is essential for
trigger-ing the first cell division and which is essential for the
first cell differentiation in preimplantation
develop-ment? These questions need further study Moreover,
much more research is required on the whole
signal-transduction network in preimplantation embryo
development
Perspectives Preimplantation development is a unique and critical stage during embryo development It is not only the very beginning of a new life, but also the beginning of many biological reactions Understanding the role of signaling pathways in embryos is important for knowl-edge about life processes However, due to the diffi-culties in manipulating preimplantation embryos, progression in this field has been delayed, especially in discovering how these signaling cascades function dur-ing each particular event To date, little of our knowl-edge about these cascades has come from direct experimental evidence, some has come from indirect experimental observation, and much is speculation based on the cascades functions in somatic cells The fact is that we still know little about this process, and dozens of questions remain unanswered, such as which signaling pathway triggers the activation of the zygote genome? Is it exogenous or endogenous? What is the exact means of maternal signaling proteins passing on new life? Which signaling cascade participates in cavity formation during development of the blastocyst? Which cascades participate in preimplantation embryo
STAT5
PI3K
AKT
Myc
Cell Growth division, apoptosis differentiation
SOX7 c-Jun
Dkk1 Bmp-4
DvI
Rac
ERK p38 JNK
active repress
directly indirectly
Fig 2 Signal network predicted during mammalian preimplantation embryo development.STAT5 may regulate preimplantation develop-ment by activating PtdIns3K Ras may suppress c-Myc-induced apoptosis by activation of the PtdIns3K ⁄ Akt pathway Ras can therefore stabilize the Myc protein, depending on the action of the Raf ⁄ ERK pathway Notch signaling represses the activity of p38 MAPK by specifically inducing the expression of MKP-1 Dvl pro-teins are important transducers for the Wnt ⁄ b-catenin pathway Dvls can forms Wnt-induced complexes with Rac, and Rac can induce JNK ⁄ MAPK signaling Dkk1 and SOX7 are antagonists of Wnt signaling Both are identified as direct targets of the p38 and JNK pathways.
Trang 8apoptosis, and explain why 15–50% of embryos die
during preimplantation development? Interactions
among these signaling pathways in somatic cells have
been reported elsewhere, and these complicated
rela-tionships construct a signaling network Nonetheless,
specific interplay of these signal-transduction pathways
during this stage remains unclear There are limited
experimental data for some of these questions,
there-fore further efforts should be made to enhance our
knowledge about the detailed molecular procedures of
the embryo preimplantation stage
Acknowledgements
This work was supported by Key Program of National
Natural Scientific Foundation of China (No 30630012)
and sponsored by Shanghai Pujiang Program, China
(No 06PJ14058)
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