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PubMed2000

Xoom is required for epibolic movement of animal ectodermal cells in Xenopus laevis gastrulation.

Gastrulation is the most dynamic cell movement and initiates the body plan in amphibian development. In contrast to numerous molecular studies on mesodermal induction, the driving force of gastrulation is as yet poorly understood. A novel transmembrane protein, Xoom, was previously reported, which is required for Xenopus gastrulation. In the present study, the role of Xoom during Xenopus gastrulation was further examined in detail. Overexpression and misexpression of Xoom induced overproduction of Xoom protein, but not a changed phenotype. However, Xoom antisense ribonucleic acid (RNA) injection reduced the Xoom protein and caused gastrulation defects without any influence on the involution and translation levels of mesodermal marker genes. Normal migrating activity of dorsal mesodermal cells was recognized in the antisense RNA-injected explant. Morphological examination using artificial exogastrulation showed that convergent extension of mesodermal cells occurred normally, but the ectodermal cell layer significantly shrank in the antisense RNA-injected embryo. Comparison of cell shape among various experimental conditions showed that inhibition of cell spreading occurs specifically in the outer ectodermal layer of the antisense RNA-injected embryo. Cytochemical examination indicated disorganization of F-actin in the ectodermal cells of the antisense RNA-injected embryo. These results suggest that Xoom plays an important role in the epibolic movement of ectodermal cells through some regulation of actin filament organization.

Actins↗
PubMed2001

Involvement of a urethane-sensitive system in timing the onset of gastrulation in Xenopus laevis embryos.

This paper describes success in delaying the onset of gastrulation in Xenopus laevis embryos without damage to their subsequent development by temporarily arresting cleavage with urethane. Exposure of X. laevis embryos to 150 mM urethane before gastrulation resulted in cleavage arrest and its removal led to cleavage resumption. During cleavage arrest, cyclic activities including nuclear replication and the M-phase-promoting factor cycle continued, although their duration was lengthened to nearly 1.8-fold that of the controls. Because of a 30-min time lag from removal of urethane to resumption of cleavage, as well as the retardation of cyclic activities during cleavage arrest, the development of embryos after a 60-min exposure to urethane lagged two cell cycles behind that of control embryos. Here, the two cell cycle delay is equivalent to 50 min at 22-23 degrees C. The start of gastrulation in exposed embryos was accordingly delayed about 50 min, although the delay in mid-blastula transition was as little as 20-25 min. Consistent results were obtained in embryos exposed to urethane for 90 or 120 min and those exposed to procaine or NH4Cl for 60 min. Although these results imply that delay in the start of gastrulation in exposed embryos is ascribed simply to delay in their development raised by cleavage arrest, at the same time they suggest that the onset of gastrulation is timed by systems sensitive to urethane, procaine and NH4Cl in X. laevis embryos.

Ammonium Chloride↗
PubMed2001

Inhibitors of procollagen C-terminal proteinase block gastrulation and spicule elongation in the sea urchin embryo.

In the sea urchin embryo, inhibition of collagen processing and deposition affects both gastrulation and embryonic skeleton (spicule) formation. It has been found that cell-free extracts of gastrula-stage embryos of Strongylocentrotus purpuratus contain a procollagen C-terminal proteinase (PCP) activity. A rationally designed non-peptidic organic hydroxamate, which is a potent and specific inhibitor of human recombinant PCP (FG-HL1), inhibited both the sea urchin PCP as well as purified chick embryo tendon PCP. In the sea urchin embryo, FG-HL1 inhibited gastrulation and blocked spicule elongation, but not spicule nucleation. A related compound with a terminal carboxylate rather than a hydroxamate (FG-HL2) did not inhibit either chick PCP or sea urchin PCP activity in a procollagen-cleavage assay. However, FG-HL2 did block spicule elongation without affecting spicule nucleation or gastrulation. Neither compound was toxic, because their effects were reversible on removal. It was shown that the inhibition of gastrulation and spicule elongation were independent of tissue specification events, because both the endoderm specific marker Endo1 and the primary mesenchyme cell specific marker SM50 were expressed in embryos treated with FG-HL1 and FG-HL2. These results suggest that disruption of the fibrillar collagen deposition in the blastocoele blocks the cell movements of gastrulation and may disrupt the positional information contained within the extracellular matrix, which is necessary for spicule formation.

Animals↗
PubMed1999

Morphogenetic movements at gastrulation require the SH2 tyrosine phosphatase Shp2.

The SH2 domain-containing tyrosine phosphatase Shp2 plays a pivotal role during the gastrulation of vertebrate embryos. However, because of the complex phenotype observed in mouse mutant embryos, the precise role of Shp2 during development is unclear. To define the specific functions of this phosphatase, Shp2 homozygous mutant embryonic stem cells bearing the Rosa-26 LacZ transgene were isolated and used to perform a chimeric analysis. Here, we show that Shp2 mutant cells amass in the tail bud of embryonic day 10.5 chimeric mouse embryos and that this accumulation begins at the onset of gastrulation. At this early stage, Shp2 mutant cells collect in the primitive streak of the epiblast and thus show deficiencies in their contribution to the mesoderm lineage. In high-contribution chimeras, we show that overaccumulation of Shp2 mutant cells at the posterior end of the embryo results in two abnormal phenotypes: spina bifida and secondary neural tubes. Consistent with a failure to undergo morphogenic movements at gastrulation, Shp2 is required for embryo fibroblast cells to mount a positive chemotactic response to acidic fibroblast growth factor in vitro. Our results demonstrate that Shp2 is required at the initial steps of gastrulation, as nascent mesodermal cells form and migrate away from the primitive streak. The aberrant behavior of Shp2 mutant cells at gastrulation may result from their inability to properly respond to signals initiated by fibroblast growth factors.

Animals↗
PubMed2000

Xwnt11 and the regulation of gastrulation in Xenopus.

The molecular basis of gastrulation is poorly understood. In this paper we address this problem by taking advantage of the observation that the transcription activator Brachyury is essential for gastrulation movements in Xenopus and mouse embryos. We infer from this observation that amongst the target genes of Brachyury are some that are involved in the regulation of gastrulation. In the course of a screen for Brachyury targets we identified Xwnt11. Use of a dominant-negative Xwntll construct confirms that signalling by this class of Wnts is essential for normal gastrulation movements, and further investigation suggests that Xwntll signals not through the canonical Wnt signalling pathway involving GSK-3 and beta-catenin but through another route, which may require small GTPases such as Rho and Rac. Future work will concentrate on elucidating the Xwnt11 signal transduction pathway and on investigating its influence on cell shape and polarity during Xenopus gastrulation.

Animals↗
PubMed1995

Identification of a developmental timer regulating the stability of embryonic cyclin A and a new somatic A-type cyclin at gastrulation.

We have identified a second Xenopus cyclin A, called cyclin A2. Cyclin A2 is a 46.6-kD protein that shows a greater homology to human cyclin A than to the previously identified Xenopus cyclin A1. It is present throughout embryonic development (up to stage 46 at least) and is found in adult tissues as well as in Xenopus tissue culture cell lines. In contrast, cyclin A1 is present in eggs and early embryos but cannot be detected in late embryos or in tissue culture cells. We have found that the maternally stored pools of mRNAs encoding both of these cyclin A proteins are stable until the onset of gastrulation and then are degraded abruptly. At this time, new transcription replaces cyclin A2 mRNA. Interestingly, we have also observed a dramatic change in the stability of the cyclin A proteins at this time. Prior to the onset of gastrulation, cyclin A1 protein is stable during interphase of the cell cycle. At gastrulation, however, both A1 and A2 proteins turn over rapidly during interphase of the cell cycle. Together, these results indicate that developmental programs controlling cyclin A protein and mRNA stability are activated at gastrulation. We have shown that this program is independent of new transcription beginning at the mid-blastula transition. Furthermore, treatment of early stage embryos with cycloheximide demonstrates that activation of this degradative program is independent of cell division and translation. Collectively, our observations suggest that a previously uncharacterized timing mechanism activates new degradative pathways at the onset of gastrulation, which could play an essential role in releasing cells from maternal programming.

Amino Acid Sequence↗
PubMed2004

Pigment cells trigger the onset of gastrulation in tropical sea urchin Echinometra mathaei.

In the tropical sea urchin Echinometra mathaei, pigment cells are just detectable before the onset of gastrulation, owing to an early accumulation of red pigment granules. Taking advantage of this feature, behavior of pigment cells was studied in relation to the processes of gastrulation. Before the initiation of primary invagination, pigment cells were arranged in a hemi-circle in the dorsal half of the vegetal plate. Inward bending of the vegetal plate first occurred at the position occupied by pigment cells, while the bending was not conspicuous in the ventral half of the blastopore. Rhodamine-phalloidin staining showed that actin filaments were abundant at the apical corticies of pigment cells. It was also found that the onset of gastrulation was considerably delayed in the NiCl2-treated embryos, in which pigment cells were drastically reduced in number. It is notable that the NiCl2-treated embryos began to gastrulate on schedule if they contained a number of pigment cells in spite of treatment. This shows that pigment cells are the bottle cells that trigger the onset of gastrulation. In the embryos devoid of pigment cells, a short stub-like gut rudiment formed in a delayed fashion, and several secondary mesenchyme cells (SMC) appeared at the tip of the rudiment and elongated gradually until its tip reached the apical plate. This observation suggests that the SMC that pull the gut rudiment upward are not pigment cells but blastocoelar cells, because pigment cells change their fate to blastocoelar cells upon NiCl2-treatment.

Animals↗
PubMed2004

Morphogenesis during Xenopus gastrulation requires Wee1-mediated inhibition of cell proliferation.

Major developmental events in early Xenopus embryogenesis coincide with changes in the length and composition of the cell cycle. These changes are mediated in part through the regulation of CyclinB/Cdc2 and they occur at the first mitotic cell cycle, the mid-blastula transition (MBT) and at gastrulation. In this report, we investigate the contribution of maternal Wee1, a kinase inhibitor of CyclinB/Cdc2, to these crucial developmental transitions. By depleting Wee1 protein levels using antisense morpholino oligonucleotides, we show that Wee1 regulates M-phase entry and Cdc2 tyrosine phosphorylation in early gastrula embryos. Moreover, we find that Wee1 is required for key morphogenetic movements involved in gastrulation, but is not needed for the induction of zygotic transcription. In addition, Wee1 is positively regulated by tyrosine autophosphorylation in early gastrula embryos and this upregulation of Wee1 activity is required for normal gastrulation. We also show that overexpression of Cdc25C, a phosphatase that activates the CyclinB/Cdc2 complex, induces gastrulation defects that can be rescued by Wee1, providing additional evidence that cell cycle inhibition is crucial for the gastrulation process. Together, these findings further elucidate the developmental function of Wee1 and demonstrate the importance of cell cycle regulation in vertebrate morphogenesis.

Animals↗
PubMed1991

mRNA localization studies suggest that murine FGF-5 plays a role in gastrulation.

During gastrulation in the mouse, the pluripotent embryonic ectoderm cells form the three primary germ layers, ectoderm, mesoderm and endoderm. Little is known about the mechanisms responsible for these processes, but evidence from previous studies in amphibians, as well as expression studies in mammals, suggest that signalling molecules of the Fibroblast Growth Factor (FGF) family may play a role in gastrulation. To determine whether this might be the case for FGF-5 in the mouse embryo, we carried out RNA in situ hybridization studies to determine when and where in the early postimplantation embryo the Fgf-5 gene is expressed. We chose to study this particular member of the FGF gene family because we had previously observed that its pattern of expression in cultures of teratocarcinoma cell aggregates is consistent with the proposal that Fgf-5 plays a role in gastrulation in vivo. The results reported here show that Fgf-5 expression increases dramatically in the pluripotent embryonic ectoderm just prior to gastrulation, is restricted to the cells forming the three primary germ layers during gastrulation, and is not detectable in any cells in the embryo once formation of the primary germ layers is virtually complete. Based on this provocative expression pattern and in light of what is known about the functions in vitro of other members of the FGF family, we hypothesize that in the mouse embryo Fgf-5 functions in an autocrine manner to stimulate the mobility of the cells that contribute to the embryonic germ layers or to render them competent to respond to other inductive or positional signals.

Animals↗
PubMed1995

The T gene is necessary for normal mesodermal morphogenetic cell movements during gastrulation.

The T (Brachyury) deletion in mouse is responsible for defective primitive streak and notochord morphogenesis, leading to a failure of the axis to elongate properly posterior to the forelimb bud. T/T embryonic stem (ES) cells colonise wild-type embryos, but in chimeras at 10.5 days post coitum (dpc) onwards they are found predominantly in the distal tail, while trunk paraxial and lateral mesoderm are deficient in T/T cells (Wilson, V., Rashbass, P. and Beddington, R. S. P. (1992) Development 117, 1321-1331). To determine the origin of this abnormal tissue distribution, we have isolated T/T and control T/+ ES cell clones which express lacZ constitutively using a gene trap strategy. Visualisation of T/T cell distribution in chimeric embryos throughout gastrulation up to 10.5 dpc shows that a progressive buildup of T/T cells in the primitive streak during gastrulation leads to their incorporation into the tailbud. These observations make it likely that one role of the T gene product is to act during gastrulation to alter cell surface (probably adhesion) properties as cells pass through the primitive streak. As the chimeric tail elongates at 10.5 dpc, abnormal morphology in the most distal portion becomes apparent. Comparison of T expression in the developing tailbud with the sites of accumulation of T/T cells in chimeras shows that T/T cells collect in sites where T would normally be expressed. T expression becomes internalised in the tailbud following posterior neuropore closure while, in abnormal chimeric tails, T/T cells remain on the surface of the distal tail. We conclude that prevention of posterior neuropore closure by the wedge of T/T cells remaining in the primitive streak after gastrulation is one source of the abnormal tail phenotypes observed. Accumulation of T/T cells in the node and anterior streak during gastrulation results in the preferential incorporation of T/T cells into the ventral portion of the neural tube and axial mesoderm. The latter forms compact blocks which are often fused with the ventral neural tube, reminiscent of the notochordal defects seen in intact mutants. Such fusions may be attributed to cell-autonomous changes in cell adhesion, possibly related to those observed at earlier stages in the primitive streak.

Animals↗
PubMed1996

An extracellular matrix molecule that is selectively expressed during development is important for gastrulation in the sea urchin embryo.

The extracellular matrix is important in the regulation of many cellular events of early development including migration, shape change, proliferation and gene expression. In the sea urchin embryo, disruption of the extracellular matrix results in selective defects in each of these events during gastrulation. Here we describe a new molecule of the extracellular matrix in Lytechinus variegatus, referred to as ECM 18, that has several important features. First, antibody interference of ECM 18 results in a profound but reversible inhibition of primary mesenchyme cell organization and endoderm morphogenesis during gastrulation. Second, during gastrulation, ECM 18 mRNA accumulates to highest levels in the invaginating endoderm and the ECM 18 protein deposited in the basal lamina surrounding the archenteron as well as in other areas of the blastocoel wall. Immunolocalization by fluorescence and electron microscopy demonstrates the selective accumulation of ECM 18 in the extracellular matrix. Third, although the mRNA encoding ECM 18 is present throughout development, the protein accumulates only during gastrulation. ECM 18 protein is not detected in eggs or early embryos and analysis of polysome-associated mRNA suggests that at least part of the translational regulation of ECM 18 is at the level of ECM 18 mRNA-polysome formation. Finally, sequence analysis of ECM 18 shows that the protein contains a repeat sequence with a conserved cysteine motif, suggestive of involvement in protein-protein interactions. Thus, ECM 18 appears to be important in mediating select morphogenetic changes during gastrulation and the pattern of its expression in the embryo is unique among the extracellular matrix molecules known in this embryo.

Amino Acid Sequence↗
PubMed1997

A forkhead gene related to HNF-3beta is required for gastrulation and axis formation in the ascidian embryo.

We have isolated a member of the HNF-3/forkhead gene family in ascidians as a means to determine the role of winged-helix genes in chordate development. The MocuFH1 gene, isolated from a Molgula oculata cDNA library, exhibits a forkhead DNA-binding domain most similar to zebrafish axial and rodent HNF-3beta. MocuFH1 is a single copy gene but there is at least one other related forkhead gene in the M. oculata genome. The MocuFH1 gene is expressed in the presumptive endoderm, mesenchyme and notochord cells beginning during the late cleavage stages. During gastrulation, MocuFH1 expression occurs in the prospective endoderm cells, which invaginate at the vegetal pole, and in the presumptive notochord and mesenchyme cells, which involute over the anterior and lateral lips of the blastopore, respectively. However, this gene is not expressed in the presumptive muscle cells, which involute over the posterior lip of the blastopore. MocuFH1 expression continues in the same cell lineages during neurulation and axis formation, however, during the tailbud stage, MocuFH1 is also expressed in ventral cells of the brain and spinal cord. The functional role of the MocuFH1 gene was studied using antisense oligodeoxynucleotides (ODNs), which transiently reduce MocuFH1 transcript levels during gastrulation. Embryos treated with antisense ODNs cleave normally and initiate gastrulation. However, gastrulation is incomplete, some of the endoderm and notochord cells do not enter the embryo and undergo subsequent movements, and axis formation is abnormal. In contrast, the prospective muscle cells, which do not express MocuFH1, undergo involution and later express muscle actin and acetylcholinesterase, markers of muscle cell differentiation. The results suggest that MocuFH1 is required for morphogenetic movements of the endoderm and notochord precursor cells during gastrulation and axis formation. The effects of inhibiting MocuFH1 expression on embryonic axis formation in ascidians are similar to those reported for knockout mutations of HNF-3beta in the mouse, suggesting that HNF-3/forkhead genes have an ancient and fundamental role in organizing the body plan in chordates.

Amino Acid Sequence↗
PubMed1997

Evidence for the presence and participation of 85-75 KDa extracellular matrix components in cell interactions of Bufo arenarum gastrulation.

We studied the presence and distribution of the extracellular materials (ECM), obtained by mild embryonic dissociation through nondenaturing and denaturing PAGE, immunoblotting and immunocytochemical wholemount in the gastrulation of anuran amphibian Bufo arenarum. The SDS-PAGE, under reducing conditions, revealed the protein profile of the ECM which comprised six bands. The Western immunoblotting effected with antibodies against fibronectins (FN) of Xenopus laevis, Ambystoma mexicanum and Bufo arenarum revealed that the 210 and 190 KDa bands (EP1-EP2) present in the ECM were identified as FN. Polyclonal antibodies against the 85-75 KDa polypeptides (EP3-EP4) were obtained and used throughout this study. The distribution of FN and EP3-EP4 was comparatively studied in the blastocoelic roof (BCR) of stage 10.5 Bufo arenarum, Xenopus laevis and Ambystoma mexicanum embryos. In the anurans, FN appeared as a network of fine fibrils apparently oriented at random, while in Ambystoma, FN appeared as a complex anastomosing network of oriented fibrils. EP3-EP4 were found in Bufo and in Xenopus both in the intercellular contact zones and in the cellular periphery. No linear arrangements of these proteins were observed. Few, if any, EP3-EP4 were found on the BCR of Ambystoma mexicanum. At stage 11, EP3-EP4, which showed a dramatic increase at the chordomesoderm-neuroectoderm junction in Bufo arenarum embryos, appeared as an amorphous material. For the purpose of analyzing the role of EP3-EP4 during Bufo arenarum gastrulation, anti-EP3-EP4 antibodies and anti-EP3-EP4 Fab fragments were microinjected into the blastocoel cavity of stage 9 embryos, an event that cause severe alterations in the gastrulation process. Convergent extension of the dorsal marginal zone and the epiboly of the BCR were the most strongly affected events. Results show that EP3-EP4 are required for normal Bufo arenarum gastrulation.

Animals↗
PubMed2000

Participation of the GM1 ganglioside in the gastrulation of anuran amphibian Bufo arenarum.

In the present paper we established the ganglioside composition of the blastula and gastrula stages of the anuran amphibian Bufo arenarum, two relevant stages characterized by dynamic changes in morphology and cellular rearrangements. Densitometric studies evidenced that GD1a and GT1b were the more abundant gangliosides of the blastula embryos whereas GM1 and GM2 were the predominant species in gastrula embryos. Analysis of ganglioside abundance indicates that the "a" and "b" synthesis pathways perform similar biosynthetic activities in the blastula stage, in contrast to the gastrula stage in which a marked predominance of the "a" pathway occurred. The spatio-temporal expression of GM1 and of polygangliotetraosyl ceramides (pGTC) was investigated by wholemount immunocytochemistry using cholera toxin B subunit (CTB) and an affinity purified human anti-GM1 antibody. The pGTC were detected as GM1 after treatment with neuraminidase. Blastomeres from the inner surface of the blastocoelic roof (BCR) of blastula embryos were GM1 and pGTC positive. At midgastrula stage, embryos showed an increased labeling on the inner surface of BCR. To establish whether the GM1 ganglioside was involved in the gastrulation processes, CTB, anti-GM1 antibodies and anti-GM1 Fab' fragments were microinjected into the blastocoel cavity of blastula embryos. Treatment with the probes blocked gastrulation. Scanning electron microscopy analysis of blocked embryos revealed that mesodermal cell migration, radial interdigitation, and convergent extension movements were affected. The blocking of gastrulation was correlated with the absence of fibronectin and EP3/EP4 on the inner surface of blastocoelic roof of CTB- or anti-GM1 treated embryos. Results show that the GM1 ganglioside is differentially expressed by embryonic cells and participates in the morphogenetic processes of amphibian gastrulation. J. Exp. Zool. 286:457-472, 2000.

Animals↗
PubMed1996

Genetic evidence that glycolysis is necessary for gastrulation in the mouse.

Mouse embryos homozygous for the Gpi1-sa-m1H null allele (abbreviated to m) of glucose phosphate isomerase (GPI) die early in development. A histological study was undertaken to identify the earliest abnormalities attributable to the absence of this glycolytic enzyme. Two groups of embryos were produced and examined histologically from E6.5 to E9.5 days. Experimental embryos were produced by crossing heterozygous Gpi1-sa/m females with heterozygous Gpi1-sb/m males and compared with control embryos produced by crossing heterozygous Gpi1-sb/Gpi1-sb males. The first sign of abnormality attributable to homozygous m/m embryos appeared at 7.5 days when 32.2% of the embryos in the experimental group were histologically abnormal or retarded, compared to 8.3% in the control group. The putative homozygous m/m embryos had a range of abnormalities, but consistently the egg cylinder failed to be divided into the three cavities characteristic of normal 7.5-day embryos. This suggests that a deficiency in extraembryonic mesoderm formation resulted in the failure to form the amnion or chorionic mesoderm. At 8.5 and 9.5 days the abnormal embryos from the experimental cross had progressed little further. It is suggested that in the absence of GPI, energy production is impaired so that the embryo fails to develop beyond the egg cylinder stage and gastrulation has begun completed. Developmental failure may occur before gastrulation or once gastrulation has begun and produced some mesoderm. It is concluded that glucose phosphate isomerase and presumably, therefore, glycolysis is needed for normal gastrulation of mouse embryos.

Animals↗
PubMed2000

Modeling the interplay of generic and genetic mechanisms in cleavage, blastulation, and gastrulation.

Early development of multicellular organisms is marked by a rapid initial increase in their cell numbers, accompanied by spectacular morphogenetic processes leading to the gradual formation of organs of characteristic shapes. During morphogenesis, through differentiation under strict genetic control, cells become more and more specialized. Morphogenesis also requires coordinated cell movement and elaborate interactions between cells, governed by fundamental physical or generic principles. As a consequence, early development must rely on an intricate interplay of generic and genetic mechanisms. We present the results of computer simulations of the first nontrivial morphogenetic transformations in the life of multicellular organisms: initial cleavages, blastula formation, and gastrulation. The same model, which is based on the physical properties of individual cells and their interactions, describes all these processes. The genetic code determines the values of the model parameters. The model accurately reproduces the major steps of early development. It predicts that physical constraints strongly influence the timing of gastrulation. Gastrulation must occur prior to the appearance of dynamical instability, which would destabilize and eventually derail normal development. Within our model, to avoid the instability, we suddenly change the values of some of the model parameters. We interpret this change as a consequence of specific gene activity. After changing the physical characteristics of some cells, normal development resumes, and gastrulation proceeds.

Algorithms↗
PubMed2003

Local activation of protein kinase A inhibits morphogenetic movements during Xenopus gastrulation.

cAMP-dependent protein kinase (PKA) has various biological roles in many organisms. However, little is known about its role in the developmental processes of vertebrates. In this study, we describe the functional analysis of PKA during gastrulation movements in Xenopus laevis. Overexpression of constitutively active PKA (cPKA) in the dorsal equatorial region of the embryo affects morphogenetic movement during gastrulation. We also show that intrinsic differences of PKA activities along the dorsoventral axis are set up and the level of PKA activity on the dorsal region is lower than that on the ventral region from late blastula to gastrula stages. In addition, PKA activation in animal explants inhibits activin-induced elongation. In cPKA-injected embryos, there were no changes in the expressions of markers involved in mesoderm specification, although the correct expression domains of these genes were altered. The effects of PKA activation can be restored by coexpression of PKI, a pseudosubstrate of PKA. We further analyzed the effects of PKA activation on the behavior of migratory gastrulating cells in vitro. Expression of cPKA in head mesoderm cells causes less polarized and/or randomized migration as demonstrated by a directional cell migration assay. Finally, we show that RhoA GTPase lies downstream of PKA, affecting activin-induced convergent extension movements. Taken together, these results suggest that overexpressed PKA can modulate a pathway responsible for morphogenetic movements during Xenopus gastrulation.

Animals↗
PubMed2007

BMP type I receptor ALK2 is essential for proper patterning at late gastrulation during mouse embryogenesis.

Bone morphogenetic proteins (BMPs) have multiple functions during vertebrate development. Previously, it was shown that BMP type I receptor ALK2 (also known as ACVRI, ActRI, or ActRIA) was important for normal mouse gastrulation by deleting exon 4 or exon 5 of Alk2. Recently, flanking exon 7 by loxP sites generated a conditional allele for Alk2. To assess whether the deletion of exon 7 causes functional null of ALK2, and does not produce a dominant negative form or a partially functional form of ALK2, we performed a comparative analysis between Alk2 homozygous mutant embryos with an exon 5 deletion (Alk2(Delta5/Delta5)) and embryos with an exon 7 deletion (Alk2(Delta7/Delta7)). Both Alk2(Delta5/Delta5) and Alk2(Delta7/Delta7) mutants showed identical morphological gastrulation defects. Histological examinations and molecular marker analyses revealed identical abnormal gastrulation phenotypes in Alk2(Delta5/Delta5) and Alk2(Delta7/Delta7) mutants. Although Fgf8 was expressed in the primitive streak of Alk2(Delta5/Delta5) and Alk2(Delta7/Delta7) mutants, Brachyury, Wnt3a, and Tbx6 were dramatically downregulated in Alk2(Delta5/Delta5) and Alk2(Delta7/Delta7) mutants. These results indicate that deletion of exon 7 for Alk2 leads to a functionally null mutation in vivo, and Alk2 is crucial for sustaining the proper gastrulation events in early mouse embryogenesis.

Activin Receptors, Type I↗