Blastopore gating mechanism to regulate extracellular fluid excretion.

Fluid uptake and efflux play roles in early embryogenesis as well as in adult homeostasis. Multicellular organisms have two main pathways for fluid movement: cellular-level, such as transcellular and paracellular pathways, and tissue-level, involving muscle contraction. Interestingly, early Xenopus embryos with immature functional muscles excrete archenteron fluid via a tissue-level mechanism that opens the blastopore through a gating mechanism that is unclear. Using microelectrodes, we show that the archenteron has a constant fluid pressure and as development progress the blastopore pressure resistance decreases. Combining physical perturbations and imaging analyses, we found that the pushing force exerted by the circumblastoporal collars (CBCs) at the slit periphery regulates pressure resistance. We show that apical constriction at the blastopore dorsoventral ends contributes to this pushing force, and relaxation of ventral constriction causes fluid excretion. These results indicate that actomyosin contraction mediates temporal control of tissue-level blastopore opening and fluid excretion in early Xenopus embryos.

Rapalog-induced cell adhesion molecule inhibits mesoderm migration in Xenopus embryos by increasing frequency of adhesion to the ectoderm.

During the gastrula stage of Xenopus laevis, mesodermal cells migrate on the blastocoel roof (BCR) toward the animal pole. In this process, mesodermal cells directly adhere to the BCR via adhesion molecules, such as cadherins, which in turn trigger a repulsive reaction through factors such as Eph/ephrin. Therefore, the mesoderm and BCR repeatedly adhere to and detach from each other, and the frequency of this adhesion is thought to control mesoderm migration. Although knockdown of cadherin or Eph/ephrin causes severe gastrulation defects, these molecules have been reported to contribute not only to boundary formation but also to the internal function of each tissue. Therefore, it is possible that the defect caused by knockdown occurs due to tissue function abnormalities. To address this problem, we developed a method to specifically induce adhesion between different tissues using rapalog (an analog of rapamycin). When adhesion between the BCR and mesoderm was specifically enhanced by rapalog, mesoderm migration was strongly suppressed. Furthermore, we confirmed that rapalog significantly increased the frequency of adhesion between the two tissues. These results support the idea that the adhesion frequency controls mesoderm migration, and demonstrate that our method effectively enhances adhesion between specific tissues in vivo.

Scaling of pattern formations and morphogen gradients.

The concentration gradient of morphogens provides positional information for an embryo and plays a pivotal role in pattern formation of tissues during the developmental processes. Morphogen-dependent pattern formations show robustness despite various perturbations. Although tissues usually grow and dynamically change their size during histogenesis, proper patterns are formed without the influence of size variations. Furthermore, even when the blastula embryo of Xenopus laevis is bisected into dorsal and ventral halves, the dorsal half of the embryo leads to proportionally patterned half-sized embryos. This robustness of pattern formation despite size variations is termed as scaling. In this review, I focused on the morphogen-dependent dorsal-ventral axis formation in Xenopus and described how morphogens form a proper gradient shape according to the embryo size.

Ancestral mesodermal reorganization and evolution of the vertebrate head.

INTRODUCTION: The vertebrate head is characterized by unsegmented head mesoderm the evolutionary origin of which remains enigmatic. The head mesoderm is derived from the rostral part of the dorsal mesoderm, which is regionalized anteroposteriorly during gastrulation. The basal chordate amphioxus resembles vertebrates due to the presence of somites, but it lacks unsegmented head mesoderm. Gastrulation in amphioxus occurs by simple invagination with little mesodermal involution, whereas in vertebrates gastrulation is organized by massive cell movements, such as involution, convergence and extension, and cell migration. RESULTS: To identify key developmental events in the evolution of the vertebrate head mesoderm, we compared anterior/posterior (A/P) patterning mechanisms of the dorsal mesoderm in amphioxus and vertebrates. The dorsal mesodermal genes gsc, bra, and delta are expressed in similar patterns in early embryos of both animals, but later in development, these expression domains become anteroposteriorly segregated only in vertebrates. Suppression of mesodermal involution in vertebrate embryos by inhibition of convergence and extension recapitulates amphioxus-like dorsal mesoderm formation. CONCLUSIONS: Reorganization of ancient mesoderm was likely involved in the evolution of the vertebrate head.

On the origin of vertebrate somites.

INTRODUCTION: Somites, blocks of mesoderm tissue located on either side of the neural tube in the developing vertebrate embryo, are derived from mesenchymal cells in the presomitic mesoderm (PSM) and are a defining characteristic of vertebrates. In vertebrates, the somite segmental boundary is determined by Notch signalling and the antagonistic relationship of the downstream targets of Notch, Lfng, and Delta1 in the anterior PSM. The presence of somites in the basal chordate amphioxus (Branchiostoma floridae) indicates that the last common ancestor of chordates also had somites. However, it remains unclear how the genetic mechanisms underlying somitogenesis in vertebrates evolved from those in ancestral chordates. RESULTS: We demonstrate that during the gastrula stages of amphioxus embryos, BfFringe expression in the endoderm of the archenteron is detected ventrally to the ventral limit of BfDelta expression in the presumptive rostral somites along the dorsal/ventral (D/V) body axis. Suppression of Notch signalling by DAPT (a γ-secretase inhibitor that indirectly inhibits Notch) treatment from the late blastula stage reduced late gastrula stage expression of BfFringe in the endodermal archenteron and somite markers BfDelta and BfHairy-b in the mesodermal archenteron. Later in development, somites in the DAPT-treated embryo did not separate completely from the dorsal roof of the archenteron. In addition, clear segmental boundaries between somites were not detected in DAPT-treated amphioxus embryos at the larva stage. Similarly, in vertebrates, DAPT treatment from the late blastula stage in Xenopus (Xenopus laevis) embryos resulted in disruption of somite XlDelta-2 expression at the late gastrula stage. At the tail bud stage, the segmental expression of XlMyoD in myotomes was diminished. CONCLUSIONS: We propose that Notch signalling and the Fringe/Delta cassette for dorso-ventral boundary formation in the archenteron that separates somites from the gut in an amphioxus-like ancestral chordate were co-opted for anteroposterior segmental boundary formation in the vertebrate anterior PSM during evolution.

Scaling of dorsal-ventral patterning by embryo size-dependent degradation of Spemann's organizer signals.

Spemann's organizer plays a key role in dorsal-ventral (DV) patterning in the amphibian embryo by secreting diffusible proteins such as Chordin, an antagonist to ventralizing bone morphogenetic proteins (BMPs). The DV patterning is so robust that an amphibian embryo with its ventral half surgically removed can develop into a smaller but proportionally patterned larva. Here, we show that this robust patterning depends on facilitated Chordin degradation and requires the expression of the Chordin-proteinase inhibitor Sizzled on the opposite side. Sizzled, which is stable and diffuses widely along the DV axis, stabilizes Chordin and expands its distribution in the ventral direction. This expanded Chordin distribution, in turn, limits BMP-dependent Sizzled production, forming an axis-wide feedback loop for shaping Chordin's activity. Using bisection assays, we demonstrate that Chordin degradation is dynamically controlled by embryo-size-coupled Sizzled accumulation. We propose a scaling model that enables the DV pattern to adjust proportionally to embryonic axis size.

Intrinsic transition of embryonic stem-cell differentiation into neural progenitors.

The neural fate is generally considered to be the intrinsic direction of embryonic stem (ES) cell differentiation. However, little is known about the intracellular mechanism that leads undifferentiated cells to adopt the neural fate in the absence of extrinsic inductive signals. Here we show that the zinc-finger nuclear protein Zfp521 is essential and sufficient for driving the intrinsic neural differentiation of mouse ES cells. In the absence of the neural differentiation inhibitor BMP4, strong Zfp521 expression is intrinsically induced in differentiating ES cells. Forced expression of Zfp521 enables the neural conversion of ES cells even in the presence of BMP4. Conversely, in differentiation culture, Zfp521-depleted ES cells do not undergo neural conversion but tend to halt at the epiblast state. Zfp521 directly activates early neural genes by working with the co-activator p300. Thus, the transition of ES cell differentiation from the epiblast state into neuroectodermal progenitors specifically depends on the cell-intrinsic expression and activator function of Zfp521.

Anterior neural development requires Del1, a matrix-associated protein that attenuates canonical Wnt signaling via the Ror2 pathway.

During early embryogenesis, the neural plate is specified along the anterior-posterior (AP) axis by the action of graded patterning signals. In particular, the attenuation of canonical Wnt signals plays a central role in the determination of the anterior brain region. Here, we show that the extracellular matrix (ECM) protein Del1, expressed in the anterior neural plate, is essential for forebrain development in the Xenopus embryo. Overexpression of Del1 expands the forebrain domain and promotes the formation of head structures, such as the eye, in a Chordin-induced secondary axis. Conversely, the inhibition of Del1 function by a morpholino oligonucleotide (MO) represses forebrain development. Del1 also augments the expression of forebrain markers in neuralized animal cap cells, whereas Del1-MO suppresses them. We previously reported that Del1 interferes with BMP signaling in the dorsal-ventral patterning of the gastrula marginal zone. By contrast, we demonstrate here that Del1 function in AP neural patterning is mediated mainly by the inhibition of canonical Wnt signaling. Wnt-induced posteriorization of the neural plate is counteracted by Del1, and the Del1-MO phenotype (posteriorization) is reversed by Dkk1. Topflash reporter assays show that Del1 suppresses luciferase activities induced by Wnt1 and beta-catenin. This inhibitory effect of Del1 on canonical Wnt signaling, but not on BMP signaling, requires the Ror2 pathway, which is implicated in non-canonical Wnt signaling. These findings indicate that the ECM protein Del1 promotes forebrain development by creating a local environment that attenuates the cellular response to posteriorizing Wnt signals via a unique pathway.

Robust stability of the embryonic axial pattern requires a secreted scaffold for chordin degradation.

Dorsal axial formation during vertebrate embryogenesis exhibits robust resistance to perturbations in patterning signals. However, how such stability is supported at the molecular level remains largely elusive. Here we show that Xenopus ONT1, an Olfactomedin-class secreted protein, stabilizes axial formation by restricting Chordin activity on the dorsal side. When ONT1 function is attenuated, the embryo becomes hyperdorsalized by a normally subeffective dose of Chordin. ONT1 binds Chordin and BMP1/Tolloid-class proteinases (B1TP) via distinct domains and acts as a secreted scaffold that enhances B1TP-mediated Chordin degradation by facilitating enzyme-substrate association. ONT1 is indispensable for fine-tuning BMP signaling in the axial tissue, and a similar role has been suggested for dorsally expressed BMPs such as ADMP. Simultaneous inhibition of ONT1 and dorsally expressed BMPs (ADMP and BMP2) synergistically caused drastic dorsalization. These results indicate that stable axial formation depends on two compensatory regulatory pathways involving ONT1/B1TP and dorsally expressed BMPs.

The secreted EGF-Discoidin factor xDel1 is essential for dorsal development of the Xenopus embryo.

We show here that a secreted EGF-Discoidin-domain protein, Xenopus Del1 (xDel1), is an essential factor for dorsal development in the early Xenopus embryo. Knockdown of the xDel1 function causes obvious ventralization of the embryo. Conversely, overexpression of xDel1 expands dorsal-marker expression and suppresses ventral-marker expression in the gastrula embryo. Forced expression of xDel1 dorsalizes ventral marginal zone explants, whereas it weakly induces neural differentiation but not mesodermal differentiation in animal caps. The dorsalizing activity of xDel1 is dependent on the Discoidin domains and not on the RGD motif (which is implicated in its angiogenic activity) or EGF repeats. Luciferase assays show that xDel1 attenuates BMP-signaling reporter activity by interfering with the pathway downstream of the BMP receptor. Thus, xDel1 functions as a unique extracellular regulatory factor of DV patterning in early vertebrate embryogenesis.

XTsh3 is an essential enhancing factor of canonical Wnt signaling in Xenopus axial determination.

In Xenopus, an asymmetric distribution of Wnt activity that follows cortical rotation in the fertilized egg leads to the dorsal-ventral (DV) axis establishment. However, how a clear DV polarity develops from the initial difference in Wnt activity still remains elusive. We report here that the Teashirt-class Zn-finger factor XTsh3 plays an essential role in dorsal determination by enhancing canonical Wnt signaling. Knockdown of the XTsh3 function causes ventralization in the Xenopus embryo. Both in vivo and in vitro studies show that XTsh3 substantially enhances Wnt signaling activity in a beta-catenin-dependent manner. XTsh3 cooperatively promotes the formation of a secondary axis on the ventral side when combined with weak Wnt activity, whereas XTsh3 alone has little axis-inducing ability. Furthermore, Wnt1 requires XTsh3 for its dorsalizing activity in vivo. Immunostaining and protein analyses indicate that XTsh3 is a nuclear protein that physically associates with beta-catenin and efficiently increases the level of beta-catenin in the nucleus. We discuss the role of XTsh3 as an essential amplifying factor of canonical Wnt signaling in embryonic dorsal determination.

Enhanced JNK activation by NESK without kinase activity upon caspase-mediated cleavage during apoptosis.

Nck-interacting kinase-like embryo-specific kinase (NESK) is a protein kinase that is predominantly expressed in skeletal muscle during the late stages of mouse embryogenesis. NESK belongs to the germinal center kinase (GCK) family and selectively activates the c-Jun N-terminal kinase (JNK) pathway when overexpressed in cultured cells. Some members of the GCK family have been shown to be proteolytically cleaved and activated during apoptosis. Here, we report that NESK is also proteolytically cleaved during apoptosis. Treatment of NESK-transfected HeLa cells with TNF-alpha in the presence of cycloheximide or with staurosporine induced proteolytic cleavage of NESK. The cleavage of NESK occurred at two sites, generating three fragments: an N-terminal fragment containing a kinase domain, an intermediate fragment and a C-terminal fragment containing a regulatory CNH domain. These two cleavages occurred in a stepwise manner and were dependent on a caspase activity. The cleavage sites were identified as aspartic acid residues at 868 and 1091. The N-terminal fragment had less kinase activity than the full-length NESK and did not activate the JNK pathway. In contrast, the C-terminal fragment activated the JNK pathway more strongly than the full-length NESK and promoted TNF-alpha-induced apoptotic cell death. These results implicate NESK in the JNK pathway-mediated promotion of apoptosis through its C-terminal regulatory domain generated by proteolytic cleavage during apoptosis, in a unique manner different from other GCK family kinases.

A scaffold protein JIP-1b enhances amyloid precursor protein phosphorylation by JNK and its association with kinesin light chain 1.

Amyloid precursor protein (APP) is the precursor molecule of beta-amyloid peptides, the major components of amyloid plaque in patients with Alzheimer's disease. In this study, we isolated JIP-1b, a JNK signaling scaffold protein, as a binding protein of APP, and analyzed the roles of JIP-1b in APP phosphorylation by JNK and the association of kinesin light chain 1 with APP. APP phosphorylation at threonine 668 by JNK was enhanced on the JIP-1b scaffold in vitro and in cultured cells exogenously expressing APP. APP phosphorylation in nerve growth factor-differentiated PC12 cells was mediated by activation of JNK signaling. JIP-1b also enhanced the association of kinesin light chain 1 with APP. Our results suggest that JIP-1b may function as a protein linking the kinesin-I motor protein to the cargo receptor, APP, and that the JNK signaling pathway may regulate the phosphorylation of this cargo protein through the JIP-1b scaffold.