Fibroblast growth factor, but not activin, is a potent activator of mitogen-activated protein kinase in Xenopus explants

A role for Syndecan-4 in neural induction involving ERK- and PKC-dependent pathways

Syndecan-4 (Syn4) is a heparan sulphate proteoglycan that is able to bind to some growth factors, including FGF, and can control cell migration. Here we describe a new role for Syn4 in neural induction in Xenopus. Syn4 is expressed in dorsal ectoderm and becomes restricted to the neural plate. Knockdown with antisense morpholino oligonucleotides reveals that Syn4 is required for the expression of neural markers in the neural plate and in neuralised animal caps. Injection of Syn4 mRNA induces the cell-autonomous expression of neural, but not mesodermal, markers. We show that two parallel pathways are involved in the neuralising activity of Syn4: FGF/ERK, which is sensitive to dominant-negative FGF receptor and to the inhibitors SU5402 and U0126, and a PKC pathway, which is dependent on the intracellular domain of Syn4. Neural induction by Syn4 through the PKC pathway requires inhibition of PKCdelta and activation of PKCalpha. We show that PKCalpha inhibits Rac GTPase and that c-Jun is a target of Rac. These findings might account for previous reports implicating PKC in neural induction and allow us to propose a link between FGF and PKC signalling pathways during neural induction.

Extracellular signal-regulated kinase 2 is necessary for mesoderm differentiation

The extracellular signal-regulated kinase (ERK) is a component of the mitogen-activated protein kinase cascade. Exon 2 of erk2 was deleted by homologous recombination and resulted in embryonic lethality at embryonic day 6.5. erk2 mutant embryos did not form mesoderm and showed increased apoptosis but comparable levels of BrdUrd incorporation, indicating a defect in differentiation. erk2 null embryonic stem (ES) cells exhibited reduced total ERK activity upon serum stimulation, augmented ERK1 phosphorylation, and decreased downstream p90Rsk phosphorylation and activity; yet ES cell proliferation was unaffected. Mutant ES cells were capable of forming mesoderm; however, treatment of mutant ES cells with the mitogen-activated protein kinase kinase inhibitor PD184352 decreased total ERK activity and expression of the mesodermal marker brachyury, suggesting that ERK1 can compensate for ERK2 in vitro. Normal embryos at embryonic day 6.5 expressed activated ERK1/2 in the extraembryonic ectoderm, whereas erk2 mutant embryos had no detectable activated ERK1/2 in this region, suggesting that activated ERK1 was not expressed, and therefore cannot compensate for loss of ERK2 in vivo. These data indicate that ERK2 plays an essential role in mesoderm differentiation during embryonic development.

Suppression of macho-1-directed muscle fate by FGF and BMP is required for formation of posterior endoderm in ascidian embryos

Specification of germ layers is a crucial event in early embryogenesis. In embryos of the ascidian, Halocynthia roretzi, endoderm cells originate from two distinct lineages in the vegetal hemisphere. Cell dissociation experiments suggest that cell interactions are required for posterior endoderm formation, which has hitherto been thought to be solely regulated by localized egg cytoplasmic factors. Without cell interaction, every descendant of posterior-vegetal blastomeres, including endoderm precursors, assumed muscle fate. Cell interactions are required for suppression of muscle fate and thereby promote endoderm differentiation in the posterior endoderm precursors. The cell interactions take place at the 16- to 32-cell stage. Inhibition of cell signaling by FGF receptor and MEK inhibitor also supported the requirement of cell interactions. Consistently, FGF was a potent signaling molecule, whose signaling is transduced by MEK-MAPK. By contrast, such cell interactions are not required for formation of the anterior endoderm. Our results suggest that another redundant signaling molecule is also involved in the posterior endoderm formation, which is likely to be mediated by BMP. Suppression of the function of macho-1, a muscle determinant in ascidian eggs, by antisense oligonucleotide was enough to allow autonomous endoderm specification. Therefore, the cell interactions induce endoderm formation by suppressing the function of macho-1, which is to promote muscle fate. These findings suggest the presence of novel mechanisms that suppress functions of inappropriately distributed maternal determinants via cell interactions after embryogenesis starts. Such cell interactions would restrict the regions where maternal determinants work, and play a key role in marking precise boundaries between precursor cells of different tissue types.

Spatio-temporal pattern of MAP kinase activation in embryos of the ascidian Halocynthia roretzi

To understand developmental mechanisms, it is important to know when and where signaling pathways are activated. The spatio-temporal pattern of activation of mitogen-activated protein kinase (MAPK/ERK) was investigated during embryogenesis of the ascidian Halocynthia roretzi, using an antibody specific to the activated form of MAPK. During cleavage stages, activated MAPK was transiently observed in nuclei of the precursor blastomeres of endoderm, notochord, mesenchyme, brain, secondary muscle, trunk lateral cells and trunk ventral cells. These sites of MAPK activation are consistent with results of previous studies that have analyzed the embryonic induction of various tissues, and with results of inhibition of MAPK kinase (MEK) in ascidians. Activation of MAPK in notochord and mesenchyme blastomeres was observed in a short period in a single cell cycle. In contrast, in brain and secondary muscle lineages, MAPK activation spanned two or three cell cycles, and upon each cleavage, MAPK was asymmetrically activated in only one of the two daughter cells that remained brain or secondary muscle lineages. During later stages, MAPK activation was predominantly observed in the central nervous system. A conspicuous feature at this stage was that activation appeared to alternate between positive and negative along the anterior-posterior axis of the neural tube. During the tail elongation stage, MAPK was quiescent.

Low-molecular-weight protein tyrosine phosphatase is a positive component of the fibroblast growth factor receptor signaling pathway

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) has been implicated in the regulation of cell growth and actin rearrangement mediated by several receptor tyrosine kinases, including platelet-derived growth factor and epidermal growth factor. Here we identify the Xenopus laevis homolog of LMW-PTP1 (XLPTP1) as an additional positive regulator in the fibroblast growth factor (FGF) signaling pathway during Xenopus development. XLPTP1 has an expression pattern that displays substantial overlap with FGF receptor 1 (FGFR1) during Xenopus development. Using morpholino antisense technology, we show that inhibition of endogenous XLPTP1 expression dramatically restricts anterior and posterior structure development and inhibits mesoderm formation. In ectodermal explants, loss of XLPTP1 expression dramatically blocks the induction of the early mesoderm gene, Xbrachyury (Xbra), by FGF and partially blocks Xbra induction by Activin. Moreover, FGF-induced activation of mitogen-activated protein (MAP) kinase is also inhibited by XLPTP1 morpholino antisense oligonucleotides; however, introduction of RNA encoding XLPTP1 is able to rescue morphological and biochemical effects of antisense inhibition. Inhibition of FGF-induced MAP kinase activity due to loss of XLPTP1 is also rescued by an active Ras, implying that XLPTP1 may act upstream of or parallel to Ras. Finally, XLPTP1 physically associates only with an activated FGFR1, and this interaction requires the presence of SNT1/FRS-2 (FGFR substrate 2). Although LMW-PTP1 has been shown to participate in other receptor systems, the data presented here also reveal XLPTP1 as a new and important component of the FGF signaling pathway.

Docking protein SNT1 is a critical mediator of fibroblast growth factor signaling during Xenopus embryonic development

The docking protein SNT1/FRS2 (fibroblast growth factor receptor substrate 2) is implicated in the transmission of extracellular signals from several growth factor receptors to the mitogen-activated protein (MAP) kinase signaling cascade, but its biological function during development is not well characterized. Here, we show that the Xenopus homolog of mammalian SNT1/FRS-2 (XSNT1) plays a critical role in the appropriate formation of mesoderm-derived tissue during embryogenesis. XSNT1 has an expression pattern that is quite similar to the fibroblast growth factor receptor-1 (FGFR1) during Xenopus development. Ectopic expression of XSNT1 markedly enhanced the embryonic defects induced by an activated FGF receptor, and increased the MAP kinase activity as well as the expression of a mesodermal marker in response to FGF receptor signaling. A loss-of-function study using antisense XSNT1 morpholino oligonucleotides (XSNT-AS) shows severe malformation of trunk and posterior structures. Moreover, XSNT-AS disrupts muscle and notochord formation, and inhibits FGFR-induced MAP kinase activation. In ectodermal explants, XSNT-AS blocks FGFR-mediated induction of mesoderm and the accompanying elongation movements. Our results indicate that XSNT1 is a critical mediator of FGF signaling and is required for early Xenopus development.

Role of the FGF and MEK signaling pathway in the ascidian embryo

In the ascidian embryo, a fibroblast growth factor (FGF)-like signal from presumptive endoderm blastomeres between the 32-cell and early 64-cell stages induces the formation of notochord and mesenchyme cells. However, it has not been known whether endogenous FGF signaling is involved in the process. Here it is shown that 64-cell embryos exhibit a marked increase in endogenous extracellular signal-regulated kinase (ERK/MAPK) activity. The increase in ERK activity was reduced by treatment with an FGF receptor 1 inhibitor, SU5402, and a MEK (ERK kinase/MAPKK) inhibitor, U0126. Both drugs blocked the formation of notochord and mesenchyme when embryos were treated at the 32-cell stage, but not at the 2- or 110-cell stages. The dominant-negative form of Ras also suppressed notochord and mesenchyme formation. Both inhibitors suppressed induction by exogenous basic FGF. These results suggest that the FGF signaling cascade is indeed necessary for the formation of notochord and mesenchyme cells during ascidian embryogenesis. It is also shown that FGF signaling is required for formation of the secondary notochord, secondary muscle and neural tissues, and at least ERK activity is necessary for the formation of trunk lateral cells and posterior endoderm. Therefore, FGF and MEK signaling are required for the formation of various tissues in the ascidian embryo.

Expression of activated MAP kinase in Xenopus laevis embryos: evaluating the roles of FGF and other signaling pathways in early induction and patterning

FGF signaling has been implicated in germ layer formation and axial determination. An antibody specific for the activated form of mitogen-activated protein kinase (MAPK) was used to monitor FGF signaling in vivo during early Xenopus development. Activation of MAPK in young embryos is abolished by injection of a dominant negative FGF receptor (XFD) RNA, suggesting that MAPK is activated primarily by FGF in this context. A transition from cytoplasmic to nuclear localization of activated MAPK occurs in morula/blastula stage embryo animal and marginal zones coinciding with the proposed onset of mesodermal competence. Activated MAPK delineates the region of the dorsal marginal zone before blastopore formation and persists in this region during gastrulation, indicating an early role for FGF signaling in dorsal mesoderm. Activated MAPK was also found in posterior neural tissue from late gastrulation onward. Inhibition of FGF signaling does not block posterior neural gene expression (HoxB9) or activation of MAPK; however, inhibition of FGF signaling does cause a statistically significant decrease in the level of activated MAPK. These results point toward the involvement of other receptor tyrosine kinase signaling pathways in posterior neural patterning.

Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The amphibian body plan is established as the result of a series of inductive interactions. During early cleavage stages cells in the vegetal hemisphere induce overlying animal hemisphere cells to form mesoderm. The interaction represents the first major body-patterning event and is mediated by peptide growth factors. Various peptide growth factors have been implicated in mesoderm development, including most notably members of the transforming growth factor-beta superfamily. Identification of the so-called "natural" inducer from among the several candidate peptide growth factors is being achieved by employing several experimental strategies, including the use of a tissue explant assay for testing potential inducers, cloning of marker genes as indices of early induction events, and microinjection of altered peptide growth factor receptors to disrupt normal embryonic inductions. Activin emerges as the most likely choice for assignment of the role of endogenous mesoderm inducer, because it currently best fulfills the rigorous set of criteria expected of such an important embryonic signaling molecule. Activin, however, may not act alone in mesoderm induction. Other peptide growth factors such as fibroblast growth factor might be involved, especially in the regional patterning of the mesoderm. In addition, several genes (e.g., Wnt and noggin), which are expressed after the mesoderm is initially induced, probably assist in further definition of the mesoderm pattern. Following mesoderm induction, the primary embryonic organizer tissue (first described in 1924 by Spemann) develops and contributes further to body patterning by its action as a neural inducer. Peptide growth factors such as activin may also be involved in the inductive event, either directly (by facilitating gene expression) or indirectly (by serving to constrain pathways).

Protein kinase A is involved in the induction of early mesodermal marker genes by activin

In this study we have investigated the role of cAMP-dependent protein kinase A (PKA) in the induction of the early mesodermal marker genes goosecoid and no tail by activin in zebrafish embryos. We show that upon treatment with activin, zebrafish blastula cells exhibit a rapid and transient increase in PKA activity. In these cells, activin rapidly induces the expression of the immediate early response genes goosecoid and no tail. Stimulation and inhibition of PKA by activin, respectively, enhances and reduces the induction of goosecoid and no tail mRNA expression. Similar effects of PKA stimulation and inhibition on the induction by activin of a 1.8 kb zebrafish goosecoid promoter construct were observed. The induction by activin of a fragment of the zebrafish goosecoid promoter that mediates an immediate early response to activin is blocked by inhibition of PKA. Activation of PKA alone has no effect in these experiments. Finally, inhibition of PKA in whole embryos by overexpression of a dominant negative regulatory subunit of PKA reduces the expression of no tail and goosecoid, whereas the expression of even-skippedl remains unaltered. Overexpression of the catalytic subunit of PKA in embryos does not affect expression of goosecoid, no tail or even-skippedl. These data show that in dissociated blastulae, PKA is required, but not sufficient for activin signalling towards induction of goosecoid and no tail. In intact zebrafish embryos, PKA contributes to induction of goosecoid and no tail, although it is not required or sufficient.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

Crosstalk between the phosphatidylinositol cycle and MAP kinase signaling pathways in Xenopus mesoderm induction

Recent studies have established a role for the phosphoinositide (PI) cycle in the early patterning of Xenopus mesoderm. In explants, stimulation of this pathway in the absence of growth factors does not induce mesoderm, but when accompanied by growth factor treatment, simultaneous PI cycle stimulation results in profound morphological and molecular changes in the mesoderm induced by the growth factor. This suggests the possibility that the PI cycle exerts its influence via crosstalk, by modulating some primary mesoderm-inducing pathway. Given recent identification of mitogen-activated protein kinase (MAPK) as an intracellular mediator of some mesoderm-inducing signals, the present study explores MAPK as a potential site of PI cycle-mediated crosstalk. We report that MAPK activity, like PI cycle activity, increases in intact embryos during mesoderm induction. Phosphoinositide cycle stimulation during treatment of explants with basic fibroblast growth factor (bFGF) synergistically increases late-phase MAPK activity and potentiates bFGF-induced expression of Xbra, a MAPK-dependent mesodermal marker.

A POU protein regulates mesodermal competence to FGF in Xenopus

XLPOU91, a POU-homeobox gene is expressed in a narrow window during early Xenopus development. We show that ectopic expression of XLPOU91 RNA causes severe posterior truncations in embryos without inhibiting the formation of Spemann's organizer. Ectopic XLPOU91 expression also inhibits mesoderm induction by fibroblast growth factor (FGF) and activin in animal cap explants. Using antisense RNA, we depleted endogenous XLPOU91 protein in animal caps. Gastrula-stage animal caps expressing XLPOU91 antisense RNA do not lose competence to FGF, unlike controls, these animal caps express XBra after FGF treatment. Endogenous XLPOU91 levels are peaking when FGF mesoderm-inducing competence is lost in animal caps. Thus XLPOU91 protein may act as a competence switch during early development, as XLPOU91 levels increase in the embryo, the mesoderm response to FGF is lost.

Mesoderm induction by heterodimeric AP-1 (c-Jun and c-Fos) and its involvement in mesoderm formation through the embryonic fibroblast growth factor/Xbra autocatalytic loop during the early development of Xenopus embryos

We have previously demonstrated the involvement of AP-1/Jun in fibroblast growth factor (FGF) signaling by loss-of-function assay (Dong, Z., Xu, R.-H., Kim, J., Zhan, S.-N., Ma, W.-Y., Colburn, N. H., and Kung, H. (1996) J. Biol. Chem. 271, 9942-9946). Further investigations by gain-of-function are reported in this study. AP-1 transactivation activity was increased by the treatment of animal cap explants with FGF. Ectopic overexpression of two components of AP-1 (c-jun and c-fos together, but not alone) produced posteriorized embryos and induced mesoderm formation in animal cap explants, indicating that both AP-1 heterodimers are required for mesoderm induction. Since Ras/AP-1 functions downstream of FGF signaling, we then tested the involvement of Ras/AP-1 in mesoderm maintenance mediated by embryonic FGF/Xbra using dominant-negative mutants. Mesoderm maintenance mediated by embryonic FGF/Xbra was blocked by dominant-negative mutants of Ras/AP-1, and AP-1 enhanced the expression of Xbra. Further studies demonstrated the inhibition of Ras/AP-1-mediated mesoderm formation by dominant-negative mutants of the FGF receptor and Xbra. These results indicate that Ras/AP-1 and FGF/Xbra signals are involved in the mesoderm maintenance machinery and mesoderm formation through the synergistic action of the diversified signal pathways derived from the FGF/Xbra autocatalytic loop.

Role of Mitf in differentiation and transdifferentiation of chicken pigmented epithelial cell

Mitf encodes a basic helix-loop-helix-leucine-zipper (bHLHzip) protein that is known to function in the development of melanocytes, pigmented epithelial cells (PECs), osteoclasts, and mast cells. In this paper, we report on the isolation, expression, and overexpression of the chicken Mitf and discuss the role of its protein product in the differentiation and transdifferentiation of PECs. Northern blotting showed that chicken Mitf is predominantly expressed in embryonic retinal pigmented epithelium (PE), but is expressed at low levels in other tissues. A 5' RACE analysis revealed differences in the 5' region Mitf nRNA in PE and other tissues. Immunological analysis revealed that Mitf, the protein encoded by Mitf, is first detected in the nuclei of the optic vesicle cells at embryonic stage 13 in a restricted region covered with mesenchymal cells. From stage 14 to 24, the specific staining is observable in the PE and precursor of the PE, the outer layer of the optic cup. In embryos at stages later than stage 29 the signals for Mitf in the future iris, ciliary body, and posterior retinal regions become faint. These results show that expression of Mitf starts at the optic vesicle stage at which no other marker genes for PECs such as mmp115 and tyrosinase are expressed. Dedifferentiation of cultured retinal PECs (rPECs) was induced by phenylthiourea and testicular hyaluronidase, bFGF, or TGF-beta. Mitf expression was inhibited by these factors and reactivated during redifferentiation of the dedifferentiated cells into rPECs, showing the correlation between Mitf expression and rPEC differentiation. Retrovirus-mediated overexpression of Mtif inhibited bFGF-induced dedifferentiation and transdifferentiation of rPECs to both lens and neural cells. These findings showed that downregulation of Mitf expression is essential for the transdifferentiation of rPEC. Mitf overexpression caused hyperpigmentation in cultured rPECs and suppressed the changes in gene expression induced by bFGF. Mitf overexpression promoted expression of mmp115 and tyrosinase in bFGF-treated rPECs suggesting a critical role for Mitf in rPEC differentiation. Mitf overexpression, however, did not promote expression of another rPEC-specific gene, pP344, in bFGF-treated rPECs. This result suggests the presence of other regulatory genes promoting rPEC differentiation. The expression patterns of pax6 and Mitf are complementary both in vivo in vitro. Overexpression of Mitf inhibited expression of pax6 in cultured rPECs. These observations suggest that Mitf regulates pax6 expression negatively.

Ras is an essential component for notochord formation during ascidian embryogenesis

In ascidian embryos, inductive interactions are necessary for the fate specification of notochord cells. Previous studies have shown that notochord induction occurs at the 32-cell stage and that basic fibroblast growth factor (bFGF) has notochord-inducing activity in ascidian embryos. In vertebrate, it is known that bFGF receptors have tyrosine kinase domain and the signaling pathway is mediated by the small-GTP binding protein, Ras. To study the role of Ras in ascidian embryos, we injected dominant negative Ras (RasN17) into fertilized eggs. RasN17 inhibited the formation of notochord, suggesting that the Ras signaling pathway is involved in signal transduction in the induction of notochord cells. When the presumptive-notochord (A6.2) blastomere was co-isolated with the inducer (A6.1) blastomere and then RasN17 was injected into the A6.2 blastomere, notochord differentiation was suppressed. The presumptive-notochord blastomeres injected with RasN17 were treated with bFGF. Many of them failed to develop notochord-specific features. Next, we examined the effect of injecting constitutively active Ras (RasV12) into the A6.2 blastomeres. However, microinjection of RasV12 into these cells did not bypass notochord induction. These results suggest that the Ras signaling pathway is essential for the formation of notochord and that another signaling pathway also must be activated simultaneously in notochord formation during ascidian embryogenesis.

xnf7 functions in dorsal-ventral patterning of the Xenopus embryo

Xenopus nuclear factor 7 (xnf7) is a maternally expressed nuclear protein that is retained in the cytoplasm from oocyte maturation until the midblastula transition (MBT). Mutations of the xnf7 phosphorylation sites to glutamic acids (dnxnf7) resulted in the retention of the endogenous protein in the cytoplasm past the MBT, indicating that cytoplasmic retention is a phosphorylation dependent process. In addition, dnxnf7 acted as a dominant negative mutant by keeping the endogenous xnf7 protein in the cytoplasm past the MBT. Overexpression of dnxnf7 in future dorsal blastomeres resulted in a ventralized or posteriorized phenotype in which the embryos lacked anterior structures, while overexpression in ventral blastomeres resulted in dorsalized embryos. dnxnf7 also affected the expression of both dorsal and ventral mesodermal markers. These data suggest that xnf7 functions in dorsal/ventral patterning and that the movement of the protein from the cytoplasm to the nucleus at the MBT is critical for the execution of a genetic program conferring a dorsal or ventral identity to the mesoderm.

Localization of MAP kinase activity in early Xenopus embryos: implications for endogenous FGF signaling

We have used a sensitive assay for MAP kinase activity to investigate the role of endogenous fibroblast growth factor (FGF)-activated MAP kinase in early Xenopus embryonic patterning. MAP kinase activity is low during cleavage stages and increases significantly during gastrulation. The temporal profile of this activity correlates well with the expression pattern of Xenopus eFGF. Spatially, MAP kinase activity is lowest in animal pole tissue and higher in vegetal pole cells and the marginal zone. Endogenous MAP kinase activity is FGF receptor-dependent, demonstrating that FGF signaling is active in all three germ layers of the early embryo. This activity is necessary for normal expression of Mix.1, a mesoendodermal marker, in the endoderm as well as in the mesoderm, indicating that MAP kinase plays a functional role in patterning of both of these germ layers. Spatial and temporal changes in MAP kinase activation during gastrulation also suggest a role for FGF signaling in this process. In addition, we find that embryonic wounding during dissection results in significant stimulation of this pathway, providing a possible explanation for earlier observations of effects of surgical manipulation on cell fate in early embryos.

Regulation of embryonic cell division by a Xenopus gastrula-specific protein kinase

We describe a novel protein kinase, Pk9.7, and its role in cell division in the Xenopus embryo. Pk9.7 is transcribed only during blastula and gastrula stages. Expression of Pk9.7 in Xenopus oocytes induces meiotic maturation, while overexpression in embryos blocks blastomere cleavage in a MAP kinase-independent fashion. In both Pk9.7-injected oocytes and mitotic cells of cleavage-blocked embryos, chromosomes appear detached from abnormal spindles, and in oocytes additional microtubule structures are formed, suggesting that one function of Pk9.7 is to regulate formation of, and chromosome attachment to, the spindle. Consistent with this, Pk9.7 co-immunoprecipitates tubulin and phosphorylates it in vitro. Pk9.7 expression coincides with the switch from maternal to zygotic control of the cell cycle, and with the switch from microtubule independence to microtubule dependence. Our results suggest that Pk9.7 plays a role in these processes.

Modulation of Xenopus embryo mesoderm-specific gene expression and dorsoanterior patterning by receptors that activate the phosphatidylinositol cycle signal transduction pathway

A role for the phosphatidylinositol (PI) cycle signal transduction pathway in Xenopus mesoderm induction has been revealed by observations of PI cycle activation coincident with this process, combined with the demonstration that Li+ (a PI cycle inhibitor) blocks this response and hyperdorsalizes mesoderm induction in intact embryos or augments growth factor-mediated induction in animal caps. It has been suggested that spatially restricted PI cycle activity in the marginal zone might modulate (but not, itself, activate) mesoderm induction. To better characterize the ability of PI cycle activity to modulate the pattern of mesoderm-specific gene expression elicited by mesoderm-inducing growth factors we have expressed in the embryo exogenous 5-hydroxytryptamine receptors that activate the PI cycle. In embryos, ventral expression and activation of these receptors during mesoderm induction are without obvious effect, whereas dorsal expression and activation yield dorsoanterior-deficient tadpoles. In animal caps induced with activin, simultaneous activation of exogenous 5-hydroxytryptamine receptors inhibits both convergent extension movements associated with dorsal mesoderm induction and the expression of goosecoid, a dorsal-specific gene, but is without effect on expression of a 149 generic mesodermal marker, Xbra. All of these effects of a 149 PI cycle-stimulating receptor are the opposites of those previously reported for the PI cycle inhibitor, Li+. PI cycle activity thus proves able to modulate the dorsal/ventral character of early mesodermal gene expression elicited by growth factor, suggesting a model for mesodermal patterning.

Activation mechanism and function of the MAP kinase cascade

MAP kinase (MAPK) and its activator, MAP kinase kinase (MAPKK), are commonly activated by a variety of extracellular stimuli in mammalian cells and in the process of Xenopus oocyte maturation. In order to investigate the function of the MAPK cascade in oocyte maturation, we produced an anti-Xenopus MAPKK which specifically reacts with MAPKK in vitro. When this antibody was microinjected into immature oocytes, MAPK activation induced by progesterone was prevented. Surprisingly, H1 kinase activation and germinal vesicle breakdown were also inhibited in the oocytes injected with this antibody. These results suggest that the MAPK cascade plays an important role in the maturation promoting factor (MPF) activation during the oocyte maturation process. When this antibody together with Mos was microinjected into Xenopus two-cell embryos, the Mos-induced metaphase arrest (CSF arrest) was prevented. Thus, the MAPK cascade may mediate CSF arrest. During Xenopus early embryogenesis, a low but significant level of MAPK remains active. Injection of mRNA encoding a constitutively active MAPKK resulted in mesoderm induction in animal cap explants. In addition, fibroblast growth-factor (FGF)-induced mesoderm induction was inhibited by expressing CL100 (a MAP kinase phosphatase) in animal cap explants. Thus the MAPK cascade may be involved in the mesoderm induction of Xenopus embryos. The activation pathways and roles of the MAPKK/MAPK cascade in various signaling processes will be discussed.

Regulation of Raf-1-dependent signaling during early Xenopus development

The Raf-1 gene product is activated in response to cellular stimulation by a variety of growth factors and hormones. Raf-1 activity has been implicated in both cellular differentiation and proliferation. We have examined the regulation of the Raf-1/MEK/MAP kinase (MAPK) pathway during embryonic development in the frog Xenopus laevis. We report that Raf-1, MEK, and MAPK activities are turned off following fertilization and remain undetectable up until blastula stages (stage 8), some 4 h later. Tight regulation of the Raf-1/MEK/MAPK pathway following fertilization is crucial for embryonic cell cycle progression. Inappropriate reactivation of MAPK activity by microinjection of oncogenic Raf-1 RNA results in metaphase cell cycle arrest and, consequently, embryonic lethality. Our findings demonstrate an absolute requirement, in vivo, for inactivation of the MAPK signaling pathway to allow normal cell cycle progression during the period of synchronous cell divisions which occur following fertilization. Further, we show that cytostatic factor effects are mediated through MEK and MAPK.

Mesoderm induction in Xenopus caused by activation of MAP kinase

Mesoderm induction is a critical early step in vertebrate development, involving changes in gene expression and morphogenesis. In Xenopus, normal mesoderm formation depends on signalling through the fibroblast growth factor (FGF) tyrosine kinase receptor. One important signalling pathway from receptor tyrosine kinases involves p21ras (ref. 5). Ras associates with the serine kinase c-Raf-1 in a GTP-dependent manner, and this complex phosphorylates and activates MAPK/ERK kinase (MEK), a protein kinase with dual specificity. MEK then activates p42mapk and (at least in mammals) p44mapk, members of the mitogen-activated protein (MAP) kinase family. FGF activates MAP kinase during mesoderm induction, and the use of dominant-negative constructs suggests that mesoderm induction by FGF requires both Ras and Raf. However, these experiments do not reveal whether Ras and Raf do act through MAP kinase to induce mesoderm or whether another pathway, such as the phosphatidylinositol 3-kinase cascade, is involved. Here we show that expression of active forms of MEK or of MAP kinase induces ventral mesoderm of the kind elicited by FGF. Overexpression of a Xenopus MAP kinase phosphatase blocks mesoderm induction by FGF, and causes characteristic defects in mesoderm formation in intact embryos, whereas inhibition of the P13 kinase and p70 S6 kinase pathways has no effect on mesoderm induction by FGF. FGF induces different types of mesoderm in a dose-dependent manner; strikingly, this is mimicked by expressing different levels of activated MEK. Together, these experiments demonstrate that activation of MAP kinases is necessary and sufficient for mesoderm formation.

Role of MAP kinase in mesoderm induction and axial patterning during Xenopus development

We have examined the role of MAP kinase during mesoderm induction and axial patterning in Xenopus embryos. MAP Kinase Phosphatase (MKP-1) was used to inactivate endogenous MAP kinase and was found to prevent the induction of early and late mesodermal markers by both FGF and activin. In whole embryos, MKP-1 was found to disrupt posterior axial patterning, generating a phenotype similar to that obtained with a dominant inhibitory FGF receptor. Overexpression of either constitutively active MAP kinase or constitutively active MAP kinase (MEK) was sufficient to induce Xbra expression, while only constitutively active MEK was able to significantly induce expression of muscle actin. When MAP kinase phosphorylation was used as a sensitive marker of FGF receptor activity in vivo, this activity was found to persist at a low and relatively uniform level throughout blastula stage embryos. The finding that a low level of MAP kinase phosphorylation exists in unstimulated animal caps and is absent in caps overexpressing a dominant inhibitory FGF receptor provides a basis for our previous observation that overexpression of this receptor inhibits activin induction. These results indicate that FGF-dependent MAP kinase activity plays a critical role in establishing the responsiveness of embryonic tissues to mesoderm inducers.

Fibroblast growth factor (FGF) 3 from Xenopus laevis (XFGF3) binds with high affinity to FGF receptor 2

We demonstrate that purified fibroblast growth factor (FGF) 3 from Xenopus laevis (XFGF3) activates the mitogen-activated protein kinase pathway and induces DNA synthesis in quiescent cells. To characterize the high affinity cell surface receptors that mediate these responses, the ligand binding domains of different FGF receptors (FGFR) were expressed on COS-1 cells, and their affinity for XFGF3 was determined. Unlabeled XFGF3 efficiently competed with 125I-FGF1 for binding to the IIIb and IIIc isoforms of FGFR2, giving 50% displacement (ID50) at 0.3-0.8 nM. Higher XFGF3 concentrations were needed to displace 125I-FGF1 from FGFR3 and FGFR1 (ID50 approximately 4 and 21 nM, respectively), indicating that XFGF3 has a lower affinity for these receptors. No association of XFGF3 with FGFR4 was found using this assay. FGFR2 isoforms isolated from both mouse and Xenopus showed similar high affinity binding of XFGF3 as determined by direct binding assays (Kd values in the range of 0.2-0.6 nM). These results indicate that the binding specificity of XFGF3 is different from that of other FGFs, and identifies FGFR2 as its high affinity receptor.

Regulation of Spemann organizer formation by the intracellular kinase Xgsk-3

Dorsal axis formation in the Xenopus embryo can be induced by the ectopic expression of several Wnt family members. In Drosophila, the protein encoded by the Wnt family gene, wingless, signals through a pathway that antagonizes the effects of the serine/threonine kinase zeste-white 3/shaggy. We describe the isolation and characterization of a Xenopus homolog of zeste-white 3/shaggy, Xgsk-3. A kinase-dead mutant of Xgsk-3, Xgsk-3K-->R, has a dominant negative effect and mimics the ability of Wnt to induce a secondary axis by induction of an ectopic Spemann organizer. Xgsk-3K-->R, like Wnt, induces dorsal axis formation when expressed in the deep vegetal cells, which do not contribute to the axis. These results indicate that the dorsal fate is actively repressed by Xgsk-3, which must be inactivated for dorsal axis formation to occur. Furthermore, our work suggests that the effects of Xgsk-3K-->R are mediated by an additional intercellular signal.

The MAP kinase cascade: its role in Xenopus oocytes, eggs and embryos

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine kinase that is activated by mitogens. Now MAPK and its activator, MAPK kinase (MAPKK), are thought to function in a wide variety of intracellular signalling pathways from yeast to vertebrate. We describe here a brief summary of the dissection of the MAPK cascade and its possible functions, especially in Xenopus oocytes and embryos.

Heparan sulfate proteoglycans are required for mesoderm formation in Xenopus embryos

Mesoderm forms in the vertebrate embryo as a result of inductive interactions involving secreted growth factors and cell surface molecules. Proteoglycans have recently been implicated in the control of cell adhesion, migration and growth factor responsiveness. We have found that removal of glycosaminoglycan chains of proteoglycans from Xenopus ectodermal explants by heparinase, but not by chondroitinase, results in inhibition of elongation and mesodermal differentiation in response to signaling factors: activin, FGF and Wnt. Heparinase treatment differentially affected expression of early general and region-specific mesodermal markers, suggesting that mesodermal cell fates become specified in the early embryo via at least two signaling pathways which differ in their requirements for heparan sulfate proteoglycans. Addition of soluble heparan sulfate restored activin-mediated induction of muscle-specific actin gene in heparinase-treated explants. Finally, heparinase inhibited autonomous morphogenetic movements and mesodermal, but not neural, differentiation in dorsal marginal zone explants, which normally give rise to mesoderm in the embryo. These results directly demonstrate that heparan sulfate proteoglycans participate in gastrulation and mesoderm formation in the early embryo.